ui/src/vbt/Palette.i3

 Copyright (C) 1992, Digital Equipment Corporation                         
 All rights reserved.                                                      
 See the file COPYRIGHT for a full description.                            
                                                                           
 by Steve Glassman, Mark Manasse and Greg Nelson           
 Last modified on Tue Mar 10 18:59:24 1992 by steveg   
      modified on Mon Feb 24 13:57:27 PST 1992 by muller   
      modified on Wed Dec 11 18:45:20 PST 1991 by gnelson  


<*PRAGMA LL*>

 The Palette interface allows you to implement your own
   screen-independent resources by registering a closure to
   produce an appropriate screen-dependent resource for any given
   screentype.  


INTERFACE Palette;

IMPORT VBT, PaintOp, Cursor, Pixmap, Font,
  ScrnPaintOp, ScrnCursor, ScrnPixmap, ScrnFont;

 Translating a screen-independent resource into its screen-dependent
   form is called {\it resolving} the resource.  Here are the closure
   types for resolving resources: 


TYPE
  OpClosure = OBJECT METHODS
    <* LL.sup <= VBT.mu *>
    apply(st: VBT.ScreenType): ScrnPaintOp.T;
  END;

  CursorClosure = OBJECT METHODS
    <* LL.sup <= VBT.mu *>
    apply(st: VBT.ScreenType): ScrnCursor.T;
  END;

  PixmapClosure = OBJECT METHODS
    <* LL.sup <= VBT.mu *>
    apply(st: VBT.ScreenType): ScrnPixmap.T;
  END;

  FontClosure = OBJECT METHODS
    <* LL.sup <= VBT.mu *>
    apply(st: VBT.ScreenType): ScrnFont.T;
  END;

 When an apply method is called, st # NIL.
  If the method returns NIL, then some default screen-dependent
  resource will be used; for example, the built-in font or the
  transparent painting operation.


  The following procedures produce screen-independent resources
  from closures: 


PROCEDURE FromOpClosure(cl: OpClosure): PaintOp.T;
<* LL.sup <= VBT.mu *>

 Return a PaintOp.T that behaves like cl.apply(st) on st. 

PROCEDURE FromCursorClosure
  (cl: CursorClosure): Cursor.T; <* LL.sup <= VBT.mu *>

 Return a Cursor.T that behaves like cl.apply(st) on st. 

PROCEDURE FromPixmapClosure
  (cl: PixmapClosure): Pixmap.T; <* LL.sup <= VBT.mu *>

 Return a Pixmap.T that behaves like cl.apply(st) on st. 

PROCEDURE FromFontClosure(cl: FontClosure): Font.T;
<* LL.sup <= VBT.mu *>

 Return a Font.T that behaves like cl.apply(st) on st. 
 If your apply method that resolves a resource needs to resolve some
   other resource, you should use one of the following procedures to
   do so.  In all cases, st must be non-NIL. 


PROCEDURE ResolveOp(st: VBT.ScreenType; op: PaintOp.T)
  : ScrnPaintOp.T;

 Resolve op for st. 

PROCEDURE ResolveCursor(st: VBT.ScreenType;
  cursor: Cursor.T): ScrnCursor.T;

 Resolve cursor for st. 

PROCEDURE ResolvePixmap(st: VBT.ScreenType;
  pixmap: Pixmap.T): ScrnPixmap.T;

 Resolve pixmap for st. 

PROCEDURE ResolveFont(st: VBT.ScreenType; font: Font.T)
  : ScrnFont.T;

 Resolve font for st. 
 If you create a cycle of screen-independent resources each of which
   tries to resolve the next resource in the cycle, then the program
   will deadlock.


   To implement screen-independent resources, every screentype includes
   a {\it palette}, which is a table of screen-dependent resources
   appropriate for that screentype.  Most clients don't need to worry
   about the palette, but if you are implementing a VBT class that
   translates to some other window system---like X or Microsoft
   Windows---here is the procedure for building the palette in the
   screentype for a top-level window: \index{palette}


PROCEDURE Init(st: VBT.ScreenType);
<* LL.sup = VBT.mu.v *>

 Initialize st's palette, if it is not already initialized,
   by resolving all screen-independent resources for st and
   storing the results.  

END Palette.