## ui/src/vbt/VBT.i3

Copyright (C) 1992, Digital Equipment Corporation
See the file COPYRIGHT for a full description.

by Steve Glassman, Mark Manasse and Greg Nelson
modified on Thu Oct  7 17:57:18 PDT 1993 by sfreeman
modified on Sat Oct  3 11:17:12 PDT 1992 by msm
modified on Mon Feb 24 13:58:30 PST 1992 by muller
modified on Sun Dec 22 14:33:44 PST 1991 by gnelson
modified on Fri Aug  3 10:13:01 PDT 1990 by steveg
<*PRAGMA LL*>

A VBT.T (or simply a VBT) is the basic window abstraction of the Trestle system.

INTERFACE VBT;

IMPORT Word, Axis, Point, Rect, Region, Trapezoid,
Path, Pixmap, Cursor, Font, PaintOp, ScrnPixmap;

\subsection{The public methods}

A VBT is represented as an object with a private prefix and twelve public methods, which define the way the VBT responds to events. Here are the type declarations that reveal the public methods, while concealing the private prefix:

TYPE
T <: Public;
Public = Prefix OBJECT
METHODS
<* LL.sup = mu *>
redisplay();
<* LL.sup = mu.SELF *>
shape(ax: Axis.T; n: CARDINAL): SizeRange;
<* LL.sup <= mu *>
RAISES {Error};
write(sel: Selection; val: Value; tc: CARDINAL)
RAISES {Error};
END;
Prefix <: ROOT;

For example, if the user reshapes a window, Trestle will call the window's reshape method; if the user exposes some part of the window, Trestle will call the window's repaint method. The remainder of the VBT interface specifies the methods in detail. The pragmas about LL are explained in the section on locking level, below.

You should never call a VBT's methods directly. The VBTClass interface provides wrapper procedures that call the methods indirectly.

\subsection{Screens and domains}

Every VBT has a {\it screen} that associates a pixel value with each integer lattice point. We write v[p] to denote the value of the pixel at point p of the screen of the VBT v. Changing the pixel values in a VBT's screen is called {\it painting}.

The part of a VBT's screen that is visible to the user---or that would be visible if other windows weren't in the way---is called the {\it domain} of the VBT:

PROCEDURE Domain(v: T): Rect.T; <* LL.sup < v *>

Return the rectangular extent of the visible part of v's screen.

(* The domain is an arbitrary rectangle: it can be empty, the
coordinate origin can be anywhere inside or outside it, and it does
not necessarily correspond to the position of the window on the
physical display screen.

When "v" is reshaped, "Domain(v)" changes from one rectangle to
another.  During this transformation Trestle tries to save the old
screen until the new screen is fully repainted: thus in the midst
of reshaping, "v[p]" can be useful for some points "p" outside
"Domain(v)".  At other times, Trestle keeps track of "v[p]" only
for points "p" inside "Domain(v)".

The pragma "LL.sup < v" is explained in the next section.  *)

\subsection{Locking level}

The global mutex mu serializes operations that affect the tree of VBTs: \index{LL (Locking Level)@{\protect\tt LL} (Locking Level)}

VAR mu: MUTEX;

In addition, every VBT includes a private mutex that serializes operations on the VBT itself. The private mutex of a VBT is revealed in the VBTClass interface, not in this interface.

The order in which a thread is allowed to acquire these locks is called the locking order''. It is defined by these two rules:

\medskip\bulletitem The global mu precedes every VBT.

\medskip\bulletitem Every VBT precedes its parent.

\medskip\noindent The locking level'' of a thread, or LL for short, is the set of locks that the thread has acquired. The expression LL.sup denotes the maximum of the locks in LL. (The locking order is partial, but LL.sup will be defined for any thread in a correct program, since threads acquire locks in ascending order.)

Each procedure declaration in the Trestle system includes a pragma specifying the locking level at which a thread can legally call the procedure. For example, the pragma LL.sup < v on the Domain procedure allows a thread to call Domain with no locks, or with mu locked, or with descendants of v locked, but forbids calling it with any other VBTs locked.

Similarly, each public data field and method of an object has a locking level. In both cases, a locking level pragma applies to all the fields or methods between it and the next pragma. These pragmas may contain the special identifier SELF, which refers to the object itself.

The locking level for a method is identical to the locking level for a procedure: it specifies the locking level at which a thread can legally call the method. For example, whenever the mouse, position, redisplay, misc, key, or discard methods of a VBT are called, the locking level satisfies LL.sup = mu.

The locking level for a writable data field is of the form

      LL >= {mu1, ..., muN}.

This specifies that in order to write the field, a thread must hold all of the locks mu1 through muN. As a consequence, a thread can read the field if it holds any of the locks.

(In a locking level pragma, the ordering symbols >=, <=, <, and > are overloaded to denote either set containment or lock order, depending on context. For example, LL >= {mu, v} indicates that the thread has both mu and v locked, while LL.sup <= mu indicates that all locks held by the thread precede mu in the locking order.)

A data field may also be commented CONST, meaning that it is readonly after initialization and therefore can be read with no locks at all.

There is one more special notation related to locking levels: a VBT v can hold a share'' of the global lock mu; its share is denoted by mu.v. This is explained in the section of this interface that specifies the reshape method.

All the procedures in the Trestle system restore the caller's locking level when they return. For example, calling Domain(v) has no net effect on a thread's locking level.

\subsection{ScreenTypes}

Pixel values are integers. The color associated with a pixel value is determined in some manner that depends on the {\it screentype} of the VBT. A value st of type VBT.ScreenType represents a screentype:

TYPE
ScreenType <: ScreenTypePublic;
ScreenTypePublic = OBJECT (*CONST*)
depth: INTEGER;
color: BOOLEAN;
res: ARRAY Axis.T OF REAL
END;

The integer st.depth is the number of bits per pixel in screens of type st. The boolean st.color is TRUE if the pixels are colored, FALSE if they are black and white or gray-scale. The array st.res gives the horizontal and vertical resolution of the screen in pixels per millimeter for desk-top displays, or in visually equivalent units for other displays.

The screentype of a newly-allocated VBT is NIL; it becomes non-NIL only when the VBT is connected to a window system.

Here are two procedures for reading the screentype of a VBT and for converting distances to screen coordinates:

PROCEDURE ScreenTypeOf(v: T): ScreenType;
<* LL.sup < v *>

Return the screentype of v.

PROCEDURE MMToPixels(v: T; mm: REAL; ax: Axis.T)
: REAL; <* LL.sup < v *>

Return the number of pixels that correspond to mm millimeters on v's screentype in the axis ax; or return 0 if v's screentype is NIL.
The ScreenType interface reveals more details, for example,

\subsection{Splits and leaves}

User interfaces are usually constructed from a tree of VBTs whose root is the top-level window'' known to the window manager. VBTs are classified into two main subtypes based on their positions in the tree:

TYPE
Split <: T;
Leaf <: T;

PROCEDURE Parent(v: T): Split; <* LL.sup < v *>

Return v's parent, or NIL if v has no parent.
A Split (also called a parent VBT) divides its screen up
among its children according to some layout policy that depends on
the class of split.  Each pixel of the parent screen represents
a pixel of one of the child VBTs, which is said to control that
pixel.  For example, overlapping windows are provided by a class of
split called a ZSplit, for which the children are ordered bottom to
top, and each pixel v[p] of the parent domain is controlled by the
top-most child whose domain includes p.

See the Split interface for common operations on splits (e.g., enumerating children).

A Leaf is a VBT in which the twelve public methods make the Leaf ignore all events, be indifferent about its shape, and do nothing when discarded. It is provided as a starting point: you can define a useful subtype of Leaf by overriding the methods that are relevant to the new class.

Almost all subtypes of VBT are subtypes of either Split or Leaf.

\subsection{Timestamps, modifiers, mouse buttons, and cursor positions}

The following types are used in several of the event methods:

TYPE
TimeStamp = Word.T;

Modifier =
{Shift, Lock, Control, Option,
Mod0, Mod1, Mod2, Mod3,
MouseL, MouseM, MouseR,
Mouse0, Mouse1, Mouse2, Mouse3, Mouse4};

Button = [Modifier.MouseL..Modifier.Mouse4];

Modifiers = SET OF Modifier;

ScreenID = INTEGER;

CursorPosition = RECORD
pt: Point.T;
screen: ScreenID;
gone, offScreen: BOOLEAN;
END;

CONST
Buttons = Modifiers{FIRST(Button)..LAST(Button)};

Trestle has an internal unsigned clock register that is incremented every few milliseconds. When Trestle reports a mouse or keyboard event to a VBT, it also reports the value of the clock register when the event occurred, which is called the {\it timestamp} of the event. Timestamps serve as unique identifiers for the associated events. Also, the absolute time interval between two events can be computed by subtracting their timestamps with Word.Minus and multiplying by Trestle.TickTime(), which is the absolute interval between clock ticks. \index{time interval between events}

A few keys on the keyboard are defined to be {\it modifiers}, like Shift, Control, and Option. When Trestle reports a mouse or keyboard event to a VBT, it also reports the set of modifier keys and buttons that were down when the event occurred. Thus the application can distinguish shifted mouse clicks from unshifted mouse clicks, for example.

The modifier Shift is reported if either of the keyboard's shift keys is down; similarly for Control and Option. The modifier Lock is reported if the lock key is locked down. If the keyboard has a key labelled lock but this key does not have mechanical alternate action, then the modifier Lock reflects the simulated state of the lock key (that is, alternate presses of the lock key turn the modifier on or off). Trestle does not define whether it reports up and down transitions for lock keys while the modifier is set.

Some Trestle servers interpret other keys as modifiers: the type definition accommodates up to four additional modifiers, Mod0 through Mod3.

The mouse buttons are reported as modifiers. The naming of the first three buttons assumes a three-button mouse; in general it is assumed that there are at most eight buttons.

When Trestle reports a mouse position event to a VBT v, it also reports a value cp of type CursorPosition. The point cp.pt is the position of the cursor; the integer cp.screen identifies the screen of the window system where the event occurred; and cp.offScreen is TRUE if the position is on a different screen than v, and FALSE otherwise. If cp.offScreen is FALSE, then cp.pt is in v's coordinate system, otherwise cp.pt is in the coordinate system of cp.screen. The boolean cp.gone is TRUE if v doesn't control the position cp.pt, and FALSE if it does. If cp.offScreen is TRUE, then so is cp.gone. A position is controlled by a VBT w if a mouse-click at that position would ordinarily be delivered to w. All positions controlled by a VBT are in its domain; every pixel in the domain of a split is controlled by at most one child of that split. You should think of the positions controlled by a VBT as the visible positions in its domain.

\subsection{The mouse method}

Trestle calls a VBT's mouse method to report mouse clicks. The method will be called with LL.sup = mu, and takes an argument of type MouseRec.

TYPE MouseRec = RECORD
whatChanged: Button;
time: TimeStamp;
cp: CursorPosition;
modifiers: Modifiers;
clickType: ClickType;
clickCount: INTEGER;
END;

ClickType =
{FirstDown, OtherDown, OtherUp, LastUp};

The method call v.mouse(cd) indicates that the mouse button cd.whatChanged went down or up at time cd.time and cursor position cd.cp.

The field cd.clickType is FirstDown if the button went down when no other buttons were down, OtherDown if it went down when some other button(s) were already down, LastUp if it went up when all other buttons were up, and OtherUp if it went up when some other button(s) were still down.

The field cd.modifiers reflects the state of the modifiers (either just before or just after the button transition; it is not specified which).

If cd.clickType is FirstDown, then cd.cp.gone will be FALSE.

The field cd.clickCount is the number of preceding transitions of the button that were near in time and space. For example, clickCount=3 on the final up transition of a double click. Some Trestle implementations have auxilliary interfaces that allow you to set the amount of time and mouse motion allowed.

\subsection{The mouse focus rule}

A split relays mouse clicks to whichever child of the split controls the pixel at the position of the click---more or less. If this rule were applied blindly, a child could receive a down-click and never receive the corresponding up-click, which would make it impossible to program many user interfaces that involve dragging. Therefore the actual rule is more complicated. \index{mouse~focus}

Each split sp contains a variable mouseFocus(sp), which records the child of the split that has received a transition of type FirstDown but not yet received a subsequent transition of type LastUp. If there is no such child, mouseFocus(sp) is NIL. The split sp relays the MouseRec cd by the mouse focus rule'':

      IF some child ch controls cd.cp THEN
w := ch;
w.mouse(cd)
ELSE
w := NIL
END;
IF cd.clickType = ClickType.FirstDown THEN
mouseFocus(sp) := w
ELSE
IF mouseFocus(sp) # NIL AND mouseFocus(sp) # w THEN
cd.cp.gone := TRUE;
mouseFocus(sp).mouse(cd)
END;
IF cd.clickType = ClickType.LastUp THEN
mouseFocus(sp) := NIL
END
END

The mouse focus is guaranteed to receive all button transitions until the last button comes up, no matter where it occurs.

\subsection{The position method}

Trestle calls a VBT's position method to report cursor positions. The method will be called with LL.sup = mu, and takes an argument of type PositionRec.

TYPE PositionRec = RECORD
cp: CursorPosition;
time: TimeStamp;
modifiers: Modifiers;
END;

The method call v.position(cd) indicates that at the time cd.time the cursor position was cd.cp and the set of modifiers keys that were down was cd.modifiers.

The next section explains how to control the delivery of cursor positions.

\subsection{Tracking the cursor by setting cages}

Every VBT v contains a field cage(v), which represents a set of cursor positions. As long as the cursor's position is inside v's cage, Trestle won't report the position to v. As soon as the cursor's position moves outside cage(v), Trestle reports the position to v, after first resetting v's cage to contain all cursor positions. Resetting the cage inhibits further reporting of cursor positions: to continue tracking, the position method must set a new cage. \index{cursor~tracking} \index{cages~(for~cursor~tracking)}

TYPE
Cage = RECORD
rect: Rect.T;
inOut: InOut;
screen: ScreenID;
END;
InOut = SET OF BOOLEAN;

CONST
AllScreens: ScreenID = -1;

The cage cg contains the cursor position cp if

\medskip\bulletitem cp.pt is in cg.rect,

\medskip\bulletitem cp.gone is in cg.inOut, and

\medskip\bulletitem either cg.screen = AllScreens or cg.screen = cp.screen.

\medskip\noindent Trestle imposes the restriction on cages that if cg.screen = AllScreens, then cg.rect must be Rect.Full or Rect.Empty, and if cg contains no cursor positions, then it must be equal as a record to EmptyCage (which is declared below). For example, here are some useful cages:

CONST
GoneCage =
Cage{Rect.Full, InOut{TRUE}, AllScreens};
InsideCage =
Cage{Rect.Full, InOut{FALSE}, AllScreens};
EverywhereCage =
Cage{Rect.Full, InOut{FALSE, TRUE}, AllScreens};
EmptyCage =
Cage{Rect.Empty, InOut{}, AllScreens};

GoneCage contains all cursor positions that are gone''; set it on a VBT to wait for the cursor to be over a position controlled by the VBT. The cage InsideCage is the complement of GoneCage: it contains all positions that the VBT controls. The cage EverywhereCage contains all cursor positions, and EmptyCage contains none.

Here is the procedure for setting the cage of a VBT:

PROCEDURE SetCage(v: T; READONLY cg: Cage);
<* LL.sup < v *>

Set cage(v) to the intersection of cage(v) with cg.
In the usual case, SetCage is called from v's position method,
at which point v's cage is EverywhereCage and therefore the
intersection just comes out to cg.  In unusual cases, it will be found
that intersecting the new cage with the old is what is required.

The procedure CageFromPosition is helpful for tracking the cursor continuously. By setting CageFromPosition(cp) in response to each cursor position cp, you can track the cursor as long as it moves within your VBT. There are two additional optional boolean arguments: setting trackOutside allows you to track the cursor over the whole screen containing the VBT; setting trackOffScreen allows you to track the cursor even onto other screens:

PROCEDURE CageFromPosition(
trackOutside, trackOffScreen: BOOLEAN := FALSE)
: Cage; <* LL arbitrary *>

CageFromPosition(cp) returns the cage that contains only the position cp; or GoneCage if either cp.gone or cp.offScreen is TRUE and the corresponding argument is not.
More precisely, CageFromPosition is equivalent to:

      IF NOT cp.gone OR
trackOutside AND NOT cp.offScreen OR
trackOffScreen
THEN
RETURN the cage containing only the position cp
ELSIF cp.offScreen AND trackOutside THEN
RETURN Cage{Rect.Full, InOut{FALSE,TRUE}, cp.screen}
ELSE
RETURN GoneCage
END

Finally, the following two procedures are occasionally useful:

PROCEDURE Outside(
: BOOLEAN; <* LL arbitrary *>

Return whether the position cp is outside the cage cg.

READONLY cp: CursorPosition): Cage; <* LL arbitrary *>

Return Cage{r, InOut{cp.gone}, cp.screen}.
The effect of SetCage(v, CageFromRect(r, cp)) is to suspend cursor
positions as long as the cursor stays inside the rectangle r and
has the same value of gone as cp does.  This is useful when
sweeping text selections, for example.

Splits relay cursor positions to their children. If several of the children are tracking the cursor at the same time, the order in which positions are relayed to the different children can be important. The order is determined by the following rule, which specifies the way a split sp forwards a PositionRec cd to its children (the variable current(sp) is the child that controls the last cursor position seen by sp):

      IF some child ch controls cd.cp THEN
w := ch
ELSE
w := NIL
END;
goneCd := cd;
goneCd.cp.gone := TRUE;
IF w # current(sp) THEN
Deliver(current(sp), goneCd)
END;
FOR all ch other than w and current(sp) DO
Deliver(ch, goneCd)
END;
IF w # NIL THEN Deliver(w, cd) END;
current(sp) := w

where

      Deliver(v, cd) =
IF Outside(cd.cp, cage(v)) THEN
cage(v) := EverywhereCage;
v.position(cd)
END

A split maintains its cage to be a subset of the intersection of its children's cages, so that it will receive any cursor positions that it owes its children.

\subsection{The key method}

Trestle calls a VBT's key method to report keystrokes. The method will be called with LL.sup = mu, and takes an argument of type KeyRec. \index{key method}

TYPE
KeyRec = RECORD
whatChanged: KeySym;
time: TimeStamp;
wentDown: BOOLEAN;
modifiers: Modifiers;
END;

KeySym = INTEGER;

CONST
NoKey: KeySym = 0;

The method call v.key(cd) indicates that the key cd.whatChanged went up or down at time cd.time. The boolean cd.wentDown is true if the key went down; false if it went up. The set cd.modifiers reflects the state of the modifiers (either just before or just after the transition; it is not specified which).

A KeySym represents a symbol on a key of the keyboard. For example, there are separate KeySyms for upper and lower case letters. The interfaces Latin1Key and KeyboardKey specify the KeySym codes for many symbols that occur on standard keyboards. These interfaces are shipped with SRC Trestle but are not included in the printed version of the reference manual. The codes are chosen to agree with the X Keysym codes (see X Window System, Scheifler et al., \cite{XSpec} Appendix E).

If the keyboard, like most keyboards, has two symbols on some of the keys, then the KeySym for the down transition and later up transition might be different. For example, if the user pushes the left shift key, then the z/Z key, and then releases the keys in the same order, Trestle would report these four transitions:

      left shift down, modifiers = {} or {Shift}
Z down, modifiers = {Shift}
left shift up, modifiers = {} or {Shift}
z up, modifiers = {}

Although the same physical Z/z key went down and up, the down transition is reported for the Z KeySym and the up transition is reported for the z KeySym.

The constant NoKey is simply an unused KeySym code.

To get Trestle to deliver keystrokes to a VBT, you make the VBT the owner of the keyboard focus by calling the procedure VBT.Acquire.

\subsection{The redisplay method}

A typical VBT has a display invariant'' that defines what its screen looks like as a function of its state. When the state changes, the display invariant is reestablished by updating the screen. \index{redisplay~method} \index{marking~for~redisplay}

When a series of changes are made, each of which invalidates the display invariant, it is undesirable to update the screen after every change. For example, if the border width and the border texture of a BorderedVBT both change, it is better not to paint the intermediate state.

Therefore, Trestle keeps track of a set of VBTs that have been marked for redisplay''. Procedures that invalidate a VBT's display invariant mark the VBT instead of updating the screen directly. Trestle automatically schedules a call to the redisplay method of every marked window (unless the window's screentype is NIL). The method takes no arguments: the call v.redisplay() must reestablish v's display invariant. It will be called with LL.sup = mu.

The default redisplay method for a Leaf calls the reshape method with an empty saved rectangle.

There are several procedures related to redisplay:

PROCEDURE Mark(v: T); <* LL.sup < v *>

Mark v for redisplay.

PROCEDURE IsMarked(v: T): BOOLEAN; <* LL.sup < v *>

Return TRUE if v is marked for redisplay.

PROCEDURE Unmark(v: T); <* LL.sup < v *>

If v is marked for redisplay, unmark it.
A marked window is automatically unmarked when it is redisplayed,
reshaped, or rescreened.  Thus the Unmark procedure is rarely
needed.

\subsection{The reshape method}

Trestle calls a VBT's reshape method to report changes in its domain. The method will be called with LL.sup = mu.v (as explained below), and takes an argument of type ReshapeRec. \index{reshape method}

TYPE ReshapeRec = RECORD
new, prev, saved: Rect.T;
marked: BOOLEAN
END;

The method call v.reshape(cd) indicates that the domain of v has changed from cd.prev to cd.new. The rectangle cd.saved is the subset of the previous domain that Trestle has preserved for the client in case it is of use in painting the new domain. This is the only case in which Trestle tries to save portions of a VBT's screen outside its domain. After the reshape method returns, Trestle will generally forget the old parts of the screen. The boolean cd.marked indicates whether v was marked when it was reshaped; in any case, v is automatically unmarked as it is reshaped.

If new = Rect.Empty then the window is no longer visible (for example, this happens when the window is iconized). Any background threads that are painting should be stopped, since their efforts are useless.

The default reshape method for a Leaf calls the repaint method to repaint the whole new domain.

When the reshape method is called, mu is locked, and it will remain locked until the method returns. However, Trestle may lock mu and then reshape, repaint, or rescreen several VBTs concurrently, so you can't assume that an activation of your reshape method excludes the activation of another VBT's reshape, repaint, or rescreen method.

This locking level will be referred to as v's share of mu, and written mu.v. Holding mu is logically equivalent to holding mu.v for every v. Consequently, mu.v < mu in the locking order. Holding mu.v does not suffice to call a procedure that requires mu to be locked; on the other hand you cannot lock mu while holding mu.v, since this would deadlock.

\subsection{The rescreen method}

Trestle calls a VBT's rescreen method to report changes to its screentype. The method will be called with LL.sup = mu.v, and takes an argument of type RescreenRec. \index{rescreen method}

TYPE RescreenRec = RECORD
prev: Rect.T;
st: ScreenType;
marked: BOOLEAN;
END;

The method call v.rescreen(cd) indicates that the screentype of v has changed to cd.st and that its domain has changed from cd.prev to Rect.Empty. (Typically the VBT will be reshaped to a non-empty domain on the new screentype.) It is possible that cd.st=NIL. The boolean cd.marked indicates whether v was marked when it was rescreened; in any case, v is automatically unmarked as it is rescreened. VBT.Leaf.rescreen reshapes v to empty.

\subsection{The repaint method}

Trestle calls a VBT's repaint method to report that part of its screen has been exposed and must be repainted. The method will be called with LL.sup = mu.v, and takes an argument of type Region.T. \index{repaint method}

There are some subtleties if you are scrolling (that is, copying bits from one part of the screen to another) at the same time that Trestle is activating your repaint method. To explain them we will become more formal and precise.

Every VBT v has a bad region'' bad(v). For each point p that is in Domain(v) and not in bad(v), the pixel v[p] is displayed to the user; that is, if vis[p] denotes what is actually visible at pixel p, then we have the basic invariant \index{bad region} \index{exposed region}

      vis[p] = v[p] for all p controlled by v and outside bad(v)

Trestle can expand bad(v) at any time, as though cosmic rays had damaged the pixels.

Whenever bad(v) contains pixels that are controlled by v, Trestle will call v's repaint method by setting exposed(v) (the exposed region'' of v) to include all such pixels, and then executing the following code:

      < bad(v) := the set difference bad(v) - exposed(v);
FOR p in exposed(v) DO v[p] := vis[p] END >;
v.repaint(exposed(v));
exposed(v) := the empty set

That is, as a pixel p is removed from bad(v) and added to exposed(v), the screen v[p] is changed to vis[p], so that the basic invariant is maintained. You can imagine that the cosmic ray's damage has now reached v[p], not just vis[p]. The angle brackets indicate that the shrinking of bad(v) and the damaging of v[p] occur atomically, so that the basic invariant is maintained. (In particular, the basic invariant is true whenever you call the procedure VBT.Scroll, where you can find more about the bad region and the exposed region.)

Sometimes it is convenient to do all painting from the repaint method; in which case the following procedure is useful:

PROCEDURE ForceRepaint(v: T; READONLY rgn: Region.T);
<* LL.sup < v *>

Set bad(v) := Region.Join(rgn, bad(v)). If the resulting bad(v) is non-empty, schedule an activation of v's repaint method.

Trestle's painting procedures all follow the same pattern. The arguments to the procedure specify:

\medskip\bulletitem a {\it destination}, which is a set of pixels in a VBT's screen. For example, the destination could be a rectangle, a trapezoid, a shape bounded by a curved path, or a region.

\medskip\bulletitem a {\it source}, which is conceptually an infinite array of pixels, not necessarily of the same depth as those on the screen. For example, the source could be a texture, a text string in some font, an explicit bitmap or image, or the VBT's screen itself.

\medskip\bulletitem an {\it operation}, which is a function that takes a destination pixel value and a source pixel value and produces a destination pixel value. For example, the operation could be planewise XOR.

\medskip\noindent The effect of the painting procedure is to apply the operation to each pixel in the destination region. That is, if v is the VBT, the effect of the painting procedure is to set v[p] := op(v[p], s[p]) for each point p in the destination, where op is the operation, v[p] is the pixel at point p of v's screen, and s[p] is the source pixel at point p.

Two useful operations are PaintOp.Bg and PaintOp.Fg, defined by

      PaintOp.Bg(d, s) = the screen's background pixel
PaintOp.Fg(d, s) = the screen's foreground pixel

These operations ignore their arguments; they set each destination pixel to a constant value, regardless of its previous value or the source value. The actual background and foreground pixels vary from screentype to screentype; you can think of Bg as white and Fg as black (unless you prefer video-reversed screens).

Another useful operation is PaintOp.Copy, defined by

      PaintOp.Copy(d, s) = s

For example, PaintOp.Copy can be used to paint an eight-bit pixmap source on an eight-bit pixmap screen. It would be an error to use PaintOp.Copy with a one-bit source and an eight-bit screen---the system wouldn't crash, but anything could happen to the destination pixels.s

For more painting operations, see the PaintOp interface.

\subsection{Scrolling (copying one part of the screen to another)}

PROCEDURE Scroll(
v: Leaf;
op: PaintOp.T := PaintOp.Copy); <* LL.sup < v *>

Translate a rectangle of v's screen by delta and use it as a source for the operation op applied to each destination pixel in the clipping rectangle clip.
The Scroll procedure uses v's screen as source.  It can
therefore be used to copy pixels from one part of v's screen
to another.  Any operation can be used for combining the
translated pixels with the destination pixels, but the operation
defaults to PaintOp.Copy.

The source rectangle can be computed from clip by subtracting delta. More precisely, Scroll(v, clip, delta, op) is equivalent to:

      for each pair of points p, q such that
p is in clip,
p = q + delta, and
q is in Domain(v)
simultaneously assign
v[p] := op(v[p], v[q]);
if q is in exposed(v) and p is not,
or if q is in bad(v)

By simultaneously'' it is meant that the pairs p, q are enumerated in an order so that no destination pixel of an early pair corresponds to a source pixel of any later pair. \index{bad region} \index{exposed region}

Recall the bad region and exposed region bad(v) and exposed(v) from the description of the repaint method.

If you do all your painting from within the repaint, reshape, and redisplay methods, then you can ignore the subtleties involving the bad(v) and exposed(v). But if you have any asynchronous threads that call Scroll, you have to be careful. For example, suppose you do all your painting from a concurrent worker thread, and arrange for your repaint and reshape methods to simply add entries to the worker thread's queue recording the painting that must be done. Then you must be careful to avoid the following sequence of events:

\medskip\bulletitem The worker thread removes from its work queue an item indicating that it must repaint some region A, and determines that the best way to do this is to scroll some other region B.

\medskip\bulletitem The repaint method is activated with exposed region B; it adds B to the work queue and returns. As it returns, the system sets the VBT's bad and exposed regions to be empty. (See the description of the repaint method.)

\medskip\bulletitem The worker thread copies the garbage from B into A.

\medskip Eventually the worker thread will get around to repainting B, but the damage to A will never be repaired.

To avoid this race condition, the repaint method should convey the bad region to the worker thread by a separate communication path, rather than simply put it the ordinary work queue. The worker thread can thus avoid using bad bits as the source of scroll operations.

Of course it is possible for the scrolling to happen after the repaint method is called but before the method has conveyed the bad region to the worker thread. There is no way to prevent this sequence of events, but there is no need to, either: in this case the source of the scroll operation will be in the exposed region (since the repaint method has not yet returned), and therefore (by the specification above) the call to Scroll will expand the bad region. This will eventually lead to the repaint method being activated a second time, repairing the damage.

In short, in order to allow concurrent painting, we do not clear the exposed region until the repaint method returns, and we specify that a scroll from a q in bad(v) or exposed(v) to a p that is not in bad(v) invalidates the destination.

Notice that a scroll from exposed(v) to exposed(v) does not invalidate the destination. This allows the repaint method to paint a portion of exposed(v) and then scroll that portion to other parts of exposed(v)---unusual, but legal.

\subsection{Painting textures}

This section describes procedures for texturing rectangles, regions, and trapezoids.

PROCEDURE PaintTexture(
v: Leaf;
op: PaintOp.T := PaintOp.BgFg;
src: Pixmap.T;
READONLY delta := Point.Origin); <* LL.sup < v *>

Paint the rectangle clip with the texture src+delta using the operation op.
A {\it texture} is an infinite periodic pixmap.  A texture txt
is represented by a pixmap src with a finite non-empty rectangular
domain Domain(src); the rule is that txt is the result of tiling
the plane with translates of the pixmap src.  Using the convenient
procedure Rect.Mod we can state this rule as: txt[p] = src[Rect.Mod(p,
Domain(src))].

The texture src+delta is the translation of the texture src by the vector delta.

Putting this all together, PaintTexture(v, clip, op, src, delta) is equivalent to:

      for each pair of points p, q such that
p is in clip and
p = q + delta
assign
v[p] := op(v[p], src[Rect.Mod(q, Domain(src))]).

Note that setting delta to Point.Origin causes the texture to be aligned in an absolute coordinate system independent of the domain of the window (which helps to make textures in different windows match), while setting it to the northwest corner of v's domain causes the texture to be aligned in the window's coordinate system (which allows a window to be reshaped by scrolling the old domain into the new).

If src's domain is empty, the effect is undefined but limited to the clipping region.

The default paint operation for PaintTexture is BgFg, defined by

      PaintOp.BgFg(d, 0) = the screen's background pixel
PaintOp.BgFg(d, 1) = the screen's foreground pixel

This paint operation is only appropriate if src is one-bit deep; the effect is to copy the source to the destination, interpreting 0 as background and 1 as foreground.

PROCEDURE PaintTint(
v: Leaf;
op: PaintOp.T); <* LL.sup < v *>

Paint the rectangle clip with the texture Pixmap.Solid using the operation op.
For example, PaintTint(v, clip, PaintOp.Bg) paints clip with
the background color, and PaintTint(v, clip, PaintOp.Fg) paints
clip with the foreground color.

PROCEDURE PolyTint(
v: Leaf;
op: PaintOp.T); <* LL.sup < v *>

Paint each rectangle clip[i] in order with the texture Pixmap.Solid using the operation op.

PROCEDURE PolyTexture(
v: Leaf;
op: PaintOp.T := PaintOp.BgFg;
src: Pixmap.T;
READONLY delta := Point.Origin); <* LL.sup < v *>

Paint each rectangle clip[i] in order with the texture src+delta using the operation op.

PROCEDURE PaintRegion(
v: Leaf;
op: PaintOp.T := PaintOp.BgFg;
src: Pixmap.T := Pixmap.Solid;
READONLY delta := Point.Origin); <* LL.sup < v *>

Paint the region rgn with the texture src+delta using the operation op.

PROCEDURE PaintTrapezoid(
v: Leaf;
op: PaintOp.T := PaintOp.BgFg;
src: Pixmap.T := Pixmap.Solid;
READONLY delta := Point.Origin); <* LL.sup < v *>

Paint the intersection of clip and trap with the texture src+delta using the operation op.
\subsection{Filling and stroking paths}

Trestle also supports PostScript-like graphics operations \cite{PostScript}:

TYPE
WindingCondition = {Odd, NonZero};
EndStyle = {Round, Butt, Square};
JoinStyle = {Round, Bevel, Miter};

PROCEDURE Fill(
v: Leaf;
path: Path.T;
wind := WindingCondition.NonZero;
op: PaintOp.T := PaintOp.BgFg;
src: Pixmap.T := Pixmap.Solid;
READONLY delta := Point.Origin); <* LL.sup < v *>

Paint the intersection of clip and the region entwined by path with the texture src+delta using the operation op.
The point p is entwined by path if the winding number of path
around p satisfies the winding condition wind.  To ensure that
the winding number is defined even for the points on the path, the
path is regarded as translated north by $\epsilon$ and west by
$\epsilon^2$, where $\epsilon$ is infinitesimal.

PROCEDURE Stroke(
v: Leaf;
path: Path.T;
width: CARDINAL := 0;
end := EndStyle.Round;
join := JoinStyle.Round;
op: PaintOp.T := PaintOp.BgFg;
src: Pixmap.T := Pixmap.Solid;
READONLY delta := Point.Origin); <* LL.sup < v *>

Paint the intersection of clip and the stroke determined by path, end, and join with the texture src+delta using the operation op.
The exact results of Stroke are different on different Trestle
implementations.  The approximate specification is like PostScript:

If end = Round and join = Round, the path is drawn by a circular brush of diameter width that traverses the path.

If end = Butt, then the ends of unclosed trails in the path are stroked by a line segment of length width centered and perpendicular to the path in the neighborhood of the endpoint. If end = Square, the path is extended at the endpoint by a straight line segment of length width/2 tangent to the path and a butt end is drawn.

If join = Bevel, the joint between two patches is constructed by using Butt endstyles for them and then filling the triangular notch that remains. If join = Miter, then instead of just filling the triangular notch, the outer edges of the two lines are extended to meet at a point, and the resulting quadrilateral is filled.

If width = 0, join is ignored and end determines whether the final endpoint of an open subpath should be drawn: if end is Butt, the final endpoint is omitted, otherwise it is drawn.

If join = Miter, width > 0, and the angle formed by the two segments meeting at some joint is small, then the tip of the miter may extend quite far from the joint point. Trestle implementations are free to bevel those joints whose angle is smaller than some implementation-dependent {\it miter limit}. The miter limit will be made available to clients in some to-be-determined interface. On X, the miter limit is 11 degrees.

Finally, there is a convenience procedure for stroking a path containing a single straight line segment:

PROCEDURE Line(
v: Leaf;
p, q: Point.T;
width: CARDINAL := 0;
end := EndStyle.Round;
op: PaintOp.T := PaintOp.BgFg;
src: Pixmap.T := Pixmap.Solid;
READONLY delta := Point.Origin); <* LL.sup < v *>

Like Stroke applied to the path containing the segment (p,q).
\subsection{Painting pixmaps}

The following procedure paints a pixmap without replicating it into an infinite texture:

PROCEDURE PaintPixmap(
v: Leaf;
op: PaintOp.T := PaintOp.BgFg;
src: Pixmap.T;
READONLY delta: Point.T); <* LL.sup < v *>

Translate the pixmap src by delta and paint it on the screen of v, using the operation op and clipping to the rectangle clip.
More precisely, PaintPixmap(v, clip, op, src, delta) is
equivalent to

      for each pair of points p, q such that
p is in clip,
q is in Domain(src), and
p = q + delta,
assign
v[p] := op(v[p], src[q])

Since a Pixmap.T is a screen-independent resource, you can't read its domain without specifying the VBT it is to be used on:

PROCEDURE PixmapDomain(v: T; pix: Pixmap.T): Rect.T;
<* LL.sup < v *>

Return the domain of pix on the screentype of v.
It is also possible to paint screen-dependent pixmaps:

PROCEDURE PaintScrnPixmap(
v: Leaf;
op: PaintOp.T := PaintOp.Copy;
src: ScrnPixmap.T;
READONLY delta: Point.T); <* LL.sup < v *>

Like PaintPixmap, but with a screen-dependent pixmap instead of a screen-independent pixmap.
If src does not have an appropriate screentype for v, the effect
of the procedure is undefined but limited to the clipping
region.

Because Trestle batches painting operations, the pixmap src must be regarded as still in use after PaintScrnPixmap returns. If you wish to free the pixmap by calling src.free(), you should first call VBT.Sync(v).

\subsection{Painting text}

The text painting procedures take an optional array of displacements, whose entries have the following type:

TYPE
DeltaH = [-512 .. 511];
Displacement =
RECORD index: CARDINAL; dh: DeltaH END;

A displacement d causes all characters whose index in the text is d.index or greater to be displaced d.dh pixels to the right. The first character has index 0. The d.index values in an array of displacements must be non-decreasing.

PROCEDURE PaintText(
v: Leaf;
fnt: Font.T := Font.BuiltIn;
t: TEXT;
op: PaintOp.T := PaintOp.TransparentFg;
READONLY dl := ARRAY OF Displacement{});
<* LL.sup < v *>

Paint the text t onto the screen of v, starting at position pt, using the font fnt, the operation op, and the displacement list dl.
The arguments to PaintText must satisfy at least one of the
following two conditions:

\medskip\bulletitem the background operation is transparent; that is, op(p, 0) = p for any pixel p, or

\medskip\bulletitem the font is self-clearing (see below) and dl is empty.

\medskip\noindent If neither condition is true, the effect of PaintText is implementation-dependent, but is confined to the clipping rectangle.

The ScrnFont interface defines the properties of fonts. Here we introduce names for the properties needed to explain PaintText. If f is a font and ch is a character, then

\medskip\bulletitem printWidth(ch, f) is the printing width of ch; that is, the amount to increment the reference point when ch is printed in font f;

\medskip\bulletitem bits(ch, f) is the bitmap for ch in f, which is positioned with ch's reference point at the origin;

\medskip\bulletitem height(ch, f) is the height of ch above the baseline; that is, the number of rows of bits(ch, f) whose v-coordinate is at most zero; and depth(ch, fnt) is the number of rows of bits(ch, f) whose v-coordinate exceeds zero;

\medskip\bulletitem ascent(f) and descent(f) are the logical extent of f above and below the baseline. Some characters may extend higher or lower.

\medskip\noindent A font is {\it self-clearing} if

\medskip\bulletitem each character's height and depth equal the font's ascent and descent, and

\medskip\bulletitem each character's printWidth equals the width of its bitmap and each character's reference point is at the west boundary of its bitmap (or each character's printWidth equals the negative of the width of its bitmap and each character's reference point is at the east boundary of its bitmap).

\medskip\noindent The call to PaintText is equivalent to the following loop:

      rp := pt;
i := 0;
LOOP
IF dl # NIL THEN
FOR j := 0 TO HIGH(dl^) DO
IF dl[j].index = i THEN INC(rp.h, dl[j].dh) END
END
END;
IF i = Text.Length(t) THEN EXIT END;
PaintPixmap(v, clip, op, bits(t[i], fnt), rp);
rp.h := rp.h + PrintWidth(t[i], fnt);
i := i + 1
END


The following two procedures are useful for computing the sizes of texts. Since fonts are screen-independent, they take the VBT whose screentype is to be used:

PROCEDURE BoundingBox
(v: Leaf; txt: TEXT; fnt: Font.T): Rect.T;
<* LL.sup < v *>

Return the bounding box of the text txt if it were painted at the origin on the screen of v.
More precisely, let r be the smallest rectangle that contains the
bounding boxes of the characters of txt if txt were painted on
v in the font fnt with txt's reference point at the origin.
Then BoundingBox returns a rectangle with the same horizontal extent
as r, but whose height and depth are the maximum height and depth
of any character in the font.

PROCEDURE TextWidth
(v: Leaf; txt: TEXT; fnt: Font.T): INTEGER;
<* LL.sup < v *>

Return the sum of the printing widths of the characters in txt in the font fnt.
TextWidth returns the displacement of the reference point
that would occur if t were painted on v in font fnt.  It may
differ from the width of BoundingBox(txt, fnt), since the printing
width of the last character can be different from the width of its
bounding box, and the reference point for the first character might
not be at the left edge of txt's bounding box.

You can paint characters out of an array instead of a TEXT:

PROCEDURE PaintSub(
v: Leaf;
fnt: Font.T := Font.BuiltIn;
op: PaintOp.T := PaintOp.TransparentFg;
READONLY dl :=  ARRAY OF Displacement{});
<* LL.sup < v *>

Like PaintText applied to the characters in chars.
\subsection{Synchronization of painting requests}

To improve painting performance, Trestle combines painting commands into batches, and sends them to the server a batch at a time.

Most applications can ignore the batching, but the procedures in this section can be of use in applications where the timing of paint operations is critical.

For example, when replacing one line of text with another in a non-self-clearing font, the old text must be erased before the new text is painted. If the painting command that erases the old text happens to fall at the end of a batch, there may be a delay of several milliseconds between the time it affects the screen and the time the following paint text command affects the screen, which can produce an undesirable flickering effect. The chances of this happening can be greatly reduced by enclosing the two commands in a {\it group}, using the following two procedures: \index{paint batch} \index{batch (of painting commands)}

PROCEDURE BeginGroup(v: Leaf; sizeHint: INTEGER := 0);
<* LL.sup < v *>

Begin a group of painting commands.

PROCEDURE EndGroup(v: Leaf); <* LL.sup < v *>

End the current group of painting commands.
If a group of painting commands are bracketed by BeginGroup and
EndGroup, Trestle will try to avoid introducing delays between
the commands, such as might otherwise be introduced by batching.
Trestle assumes that you will generate the painting commands and
the EndGroup in rapid succession.

Increasing the value of sizeHint may improve atomicity, at the cost of throughput. The maximum useful value of sizeHint is the total size in bytes of the painting commands in the group, which you can compute using the interface PaintPrivate.

PROCEDURE Sync(v: Leaf; wait := TRUE); <* LL.sup < v *>

Force all painting commands issued to v prior to the call to be executed. If wait = FALSE then Sync just flushes the output queue and returns. Otherwise, Sync waits until it believes the commands in the output queue have been completed.
\subsection{Screen capture}

PROCEDURE Capture(
v: T;
VAR (*out*) br: Region.T)
: ScrnPixmap.T; <* LL.sup < v *>

Return a pixmap containing the part of v's screen in the rectangle rect.
The screentype of the result will be the same as the screentype of
v.  Because a VBT's screen is forgetful, it may be impossible
to read the requested region.  In this case br is set to contain
all positions of pixels that were not copied.  Naturally, Trestle
makes br as small as it can.  If none of the bits are available,
the result may be NIL.

\subsection{Controlling the cursor shape}

Every VBT v contains a field cursor(v), which is set with the following procedure:

PROCEDURE SetCursor(v: T; cs: Cursor.T);
<* LL.sup < v *>

Set cursor(v) to cs.
A split displays the cursor of its mouse focus, or of its current
child if its mouse focus is NIL.  Only if the cursor of the relevant
child is Cursor.DontCare or if there is no relevant child does the
split display its own cursor.
\index{cursor shape, how to change}

To be more precise, the shape of the cursor over the top level window v is determined by the following recursive procedure:

      GetCursor(v) =
IF NOT ISTYPE(v, Split) THEN
RETURN cursor(v)
ELSE
IF mouseFocus(v) # NIL THEN
cs := GetCursor(mouseFocus(v))
ELSIF current(v) # NIL THEN
cs := GetCursor(current(v))
ELSE
cs := Cursor.DontCare
END;
IF cs = Cursor.DontCare THEN
RETURN cursor(v)
ELSE
RETURN cs
END
END


\subsection{Selections}

Trestle maintains an internal table of named selections, which initially contains several selections of general use, and which can be extended by users:

TYPE Selection = RECORD sel: CARDINAL END;

PROCEDURE GetSelection(name: TEXT): Selection;
<* LL arbitrary *>

Return the selection with the given name, creating it if necessary.

PROCEDURE SelectionName(s: Selection): TEXT;
<* LL arbitrary *>

Return the name used to create s, or NIL if s is unknown.

VAR (*CONST*)
NilSel:  Selection (* := GetSelection("NilSel") *);
Forgery: Selection (* := GetSelection("Forgery") *);
KBFocus: Selection (* := GetSelection("KBFocus") *);
Target:  Selection (* := GetSelection("Target") *);
Source:  Selection (* := GetSelection("Source") *);

NilSel and Forgery are reserved for Trestle's internal use. The owner of KBFocus (the keyboard focus) is the VBT that receives keystrokes. \index{input or keyboard focus} \index{keyboard focus}

We offer the following suggestions for the use of target and source selections: \index{target selection} \index{source selection}

\medskip\bulletitem The target selection. If text, this should be underlined black or reverse video. The selection gesture should not require modifiers like shift or control.

\medskip\bulletitem The source selection. If text, this should be underlined gray. The source gesture should be a modified version of the gesture for making the target selection.

\medskip An operation like copy'' should replace the target selection with the value of the source selection.

The following exception declaration provides for the errors that can occur in dealing with selections.

EXCEPTION Error(ErrorCode);

TYPE ErrorCode =
Unwritable, UnownedSelection, WrongType};

Explanation of error codes:

\medskip\bulletitem EventNotCurrent: Raised by attempts to access a selection with an event time that is not current.

\medskip\bulletitem TimeOut: If you attempt to read or write a selection, and the selection owner's method does not return for an unreasonably long time, then Trestle stops waiting and raises this exception.

\medskip\bulletitem Uninstalled: Raised by event-time operations on uninstalled VBTs; that is, on VBTs none of whose ancestors have been connected to a window system by one of the installation procedures in the Trestle interface.

\medskip\bulletitem Unreadable, Unwritable: Raised by attempts to read an unreadable selection, or write an unwritable selection.

\medskip\bulletitem UnownedSelection: Raised by attempts to read, write, or deliver miscellaneous codes to the owner of an unowned selection.

\medskip\bulletitem WrongType: Raised by attempts to read or write a selection with a type not supported by the selection owner.

\subsection{Acquiring and releasing selection ownership}

PROCEDURE Acquire(
v: T;
s: Selection;
t: TimeStamp)
RAISES {Error}; <* LL.sup < v *>

Make v the owner of selection s, provided that t is the current event.
If Acquire(v, s, t) is successful, the previous owner of the
selection will receive a miscellaneous code of type Lost (even
if the owner is v).  The window system affected is the one
to which v is connected.  The possible error codes are
EventNotCurrent and Uninstalled.

PROCEDURE Release(v: T; s: Selection);
<* LL.sup < v *>

If the current owner of s is v, then a Lost code is queued for delivery to v and the owner of s becomes NIL
The window system affected is the one to which v is connected.
Release is a no-op if the current owner is not v or if v is
not installed.

\subsection{The miscellaneous method}

Trestle calls a VBT's misc method to deliver miscellaneous codes. The method will be called with LL.sup = mu, and takes an argument of type MiscRec. \index{misc method}

Trestle maintains an internal table of named miscellaneous code types, which initially contains several types of general interest, and which can be extended by users.

TYPE MiscRec = RECORD
type: MiscCodeType;
detail: MiscCodeDetail;
time: TimeStamp;
selection: Selection;
END;

MiscCodeType = RECORD typ: CARDINAL END;
MiscCodeDetail = ARRAY [0 .. 1] OF INTEGER;

PROCEDURE GetMiscCodeType(name: TEXT): MiscCodeType;
<* LL arbitrary *>

Return the MiscCodeType with the given name, creating it if necessary.

PROCEDURE MiscCodeTypeName(type: MiscCodeType): TEXT;
<* LL arbitrary *>

Return the name used to create s, or NIL if s is unknown.

CONST
NullDetail = MiscCodeDetail {0, ..};

The interface MiscDetail provides some convenient procedures for encoding a REF as an integer for internal miscellaneous codes.

VAR (*CONST*)
Deleted: MiscCodeType;
Disconnected: MiscCodeType;
TakeSelection: MiscCodeType;
Lost: MiscCodeType;
TrestleInternal: MiscCodeType;
Moved: MiscCodeType;

These variables'' are really constants for the following codes:

      GetMiscCodeType("Deleted")
GetMiscCodeType("Disconnected")
GetMiscCodeType("TakeSelection")
GetMiscCodeType("Lost")
GetMiscCodeType("TrestleInternal")
GetMiscCodeType("Moved")

The method call v.misc(cd) sends v the misc code relevant to cd.selection as part of the event cd.time. The meaning of the type and detail fields is up to the application, except for the following.

A Deleted code is delivered to a top-level window when it is explicitly deleted from its server, either by a user command to the window manager or under program control. A Disconnected code is delivered to a top-level window when it is disconnected from its server, either because the server crashed or because the network connection was lost. A TakeSelection code is delivered to a top-level window when the user has gestured that it would like the window to acquire the indicated selection; most often the keyboard focus. (The nature of the gesture is between the user and the window manager. Many applications also acquire the keyboard focus in response to mouse clicks.) A Lost code with selection = s will be delivered to a window when it loses ownership of s. TrestleInternal codes are reserved for the implementation. A Moved code is delivered to a top-level window when it is moved in a way that does not cause a rescreen or a reshape to be delivered.

The timestamp in a TakeSelection code is the timestamp for the current event and is therefore valid for event-time operations. The timestamps in Deleted, Disconnected, Lost, Moved codes are not. The selection field is relevant in Lost and TakeSelection codes; it is irrelevant in Deleted, Disconnected, Moved codes.

\subsection{Sending miscellaneous codes}

You can send a miscellanous code to the owner of a selection by using the following procedure:

PROCEDURE Put(
v: T;
s: Selection;
t: TimeStamp;
type: MiscCodeType;
RAISES {Error}; <* LL.sup < v *>

Create a MiscRec with the given fields and enqueue it for delivery to the owner of selection s, if t is the current event-time.
The window system affected is the one to which v is connected.  The
possible error codes are EventNotCurrent, Uninstalled, and
UnownedSelection.  If the selection is unowned it is possible that
the Put will be silently ignored.

\subsection{Circumventing event-time}

The following procedure offers an escape from the event-time protocol. For example, a long-running thread that has no idea what the current event time is can forge a miscellaneous code to itself and use its timestamp to acquire the keyboard focus. (Your users may not like it if you do this.)

PROCEDURE Forge(
v: T;
type: MiscCodeType;
RAISES {Error}; <* LL.sup < v *>

Create a MiscRec with the given type and detail fields, with selection field Forgery, and with a newly created timestamp and enqueue it for delivery to v.
The timestamp will be valid for event-time operations (provided that
it is used promptly).  Forging codes that have meaning to the window
manager (e.g., a Deleted code) could have unexpected effects if
they are delivered to installed windows or their descendants.  The
only possible error code is Uninstalled.

\subsection{Communicating selection values}

When you read the value of a Trestle selection you get a result of type Value:

TYPE
Value <: Value_Public;
Value_Public =
OBJECT METHODS toRef(): REFANY RAISES {Error} END;

Call the toRef method to convert the Value into a REFANY.

The simplest way to construct a Value is with the following procedure:

PROCEDURE FromRef(r: REFANY): Value;
<* LL.sup <= mu *>

Return a Value v such that v.toRef() is equal to the result of pickling and unpickling r.
On a system without pickles, the value r must have type TEXT.
If r does not have type TEXT, any exceptions raised by pickling

Using FromRef leads to synchronous transmission of selection values---that is, the value is transferred as part of the call to Read or Write. To get asynchronous behavior, allocate your own Values and override the toRef method. Trestle will transmit the Value to the other application, and only when that application calls the toRef method will your toRef method be called.

The toRef method in a Value will be called with LL.sup <= mu. The toRef method can raise the error Unreadable if, for example, the address space of the selection owner has been destroyed. It can also raise the error WrongType if the underlying REFANY cannot be represented in the address space calling the method; this can only happen with non-TEXT selections.

The procedure Ready tests whether a value is synchronous or asynchronous:

PROCEDURE Ready(v: Value): BOOLEAN; <* LL.sup <= mu *>

Return TRUE if calling v.toRef() will return quickly; return FALSE if calling v.toRef() might be slow or block.
Finally, here are the procedures for reading and writing selections:

PROCEDURE Read(
v: T;
s: Selection;
t: TimeStamp;
tc: INTEGER := -1)
: Value
RAISES {Error}; <* LL.sup <= mu *>

Return the value of selection s as a reference of type tc, if t is the current event-time.
If tc = -1, Read uses the typecode for TEXT.  The window system
affected is the one to which v is connected.  The KBFocus
is erroneous, calling the toRef method of the returned Value
may produce a reference with a typecode other than tc.  The
possible error codes are EventNotCurrent, Uninstalled,
Unreadable, WrongType, TimeOut, and UnownedSelection.

PROCEDURE Write(
v: T;
s: Selection;
t: TimeStamp;
val: Value;
tc: INTEGER := -1)
RAISES {Error}; <* LL.sup <= mu *>

Replace the selection s with the value val, which encodes a reference with typecode tc, assuming t is the current event-time.
If tc = -1, Write uses the typecode for TEXT.  The window
system affected is the one to which v is connected.  The KBFocus
selection is always unwritable.  The possible error codes are
EventNotCurrent, Uninstalled, Unwritable, TimeOut, and
WrongType.

Trestle calls a VBT's read and write methods to access any selections that it owns. The method will be called with LL.sup <= mu (see below). \index{read method}

The signature of the read method is

      (s: Selection; tc: CARDINAL): Value RAISES {Error}

Trestle calls v.read(s, tc) whenever v is the owner of selection s and some application passes s and tc to Read. The method should return the value of the selection, or raise Error(Unreadable) if for some reason the value cannot be delivered, or Error(WrongType) if the selection cannot be converted to the requested type. The methods will be called with LL.sup <= mu; in fact, if the caller of Read is in the same address space, LL for the method call is the same as LL for the caller of Read, else LL for the method call is {}.

The signature of the write method is

      (s: Selection; val: Value; tc: CARDINAL)
RAISES {Error}

Trestle calls v.write(s, val, tc) whenever v is the owner of selection s and some application passes s, val, and tc to Write. The method should replace the selection with the value of val, or raise the exception with error code Unwritable if for some reason the selection is not writable, or with error code WrongType if the selection cannot be written with the requested type. Trestle does not enforce any consistency between tc and the typecode of the reference val.toRef(). For example, if val.toRef() is NIL, the meaning could be determined by tc. The locking level is the same as for the read method. \index{write method}

While a read or write method is active in a descendant of an installed window, Trestle will block the delivery to that window of any mouse or key events, misc codes, or cursor positions. If the computations are long, it is therefore preferable to do them asynchronously, to avoid blocking the user.

\subsection{Controlling the shape of a VBT}

The preferred shape of a VBT is represented by a pair of records of type SizeRange, one for each axis:

TYPE SizeRange = RECORD lo, pref, hi: CARDINAL END;

CONST DefaultShape =
SizeRange{lo := 0, pref := 0, hi := 99999};

If a VBT's preferred shape in the axis ax is the SizeRange sh, then the desirable sizes for the VBT in axis ax range from sh.lo to sh.hi-1, and its preferred size is sh.pref.

A SizeRange sh is illegal unless sh.lo <= sh.pref < sh.hi.

When a parent VBT divides its screen up between its children, it tries to satisfy its children's shape requirements, which it finds by calling the children's shape method.

The signature of the shape method is

      (ax: Axis.T; n: CARDINAL): SizeRange

The behavior of the shape method depends on whether n is zero. The call v.shape(ax, 0) returns the preferred shape for v in the ax axis, assuming nothing is known about its size in the other axis. If n#0, the call sh := v.shape(ax, n) returns the preferred shape for v in the ax axis assuming that v's size in the other axis is n. When the method is called, LL.sup = mu.v.

It is a checked runtime error for a shape method to return an illegal size range. A common error is to return an illegal size range with sh.lo = sh.hi.

The child must not assume that its shape requirement is satisfied, since, for example, the requirements of a split's children can be inconsistent.

The default shape method for a Leaf returns DefaultShape.

When the preferred shape of a VBT changes, you should call NewShape:

PROCEDURE NewShape(v: T);
<* LL.sup >= mu.v AND LL.sup < v *>

Notify v's parent that its preferred size range may have changed.
Typically, the parent will mark itself, and any change will take effect
at the time of the next redisplay.  Notice that the locking level
allows NewShape to be called from a reshape or rescreen method;
it can also be called from a thread that has mu locked.

\subsection{Putting properties on a VBT}

Associated with each window is a property set'', which is a set of non-nil traced references. \index{property set, of window}

PROCEDURE PutProp(v: T; ref: REFANY); <* LL.sup < v *>

Add ref to v's property set, replacing any existing reference of the same type as ref. This is a checked runtime error if ref is NIL.

PROCEDURE GetProp(v: T; tc: INTEGER): REFANY;
<* LL.sup < v *>

Return the element of v's property set with typecode tc, or NIL if no such element exists.

PROCEDURE RemProp(v: T; tc: INTEGER); <* LL.sup < v *>

Remove the element with typecode tc from v's property set, if one exists.

It is good form to call VBT.Discard(v) when v is about to be garbage-collected: \index{discard method}

PROCEDURE Discard(v: T); <* LL.sup = mu *>

Prepare for and call v.discard().
The discard method will be called with LL.sup = mu, and takes
no argument.  The method should perform any class-dependent cleanup
that is needed.  The default discard method is a no-op.

END VBT.