<*PRAGMA LL*>

The domain of the child of a JoinedVBT.T is the union of the parent domains; you should wrap the child in a filter if you want to make the child domain have some other shape. Since the domains of the parents may not always be in the most convenient coordinate system, you may want to use TranslateVBT or something similar to change domains.

Event-time operations on the child are performed on the event-time parent (the parent that last received a mouse click, or as set by method calls on the VBT. When the event-time parent changes, all selections held by the old event-time parent are released (if the parents are on different Trestles). These event-time operations are handled by making the parent an ETAgent and inserting a filter between the parent and the child. When a parent receives a VBT.Deleted code, it removes itself (and the filter). Other miscellaneous codes are handled as an ETAgent.

The value of v.parent is NIL if the JoinedVBT v has no parents; otherwise it is an arbitrary parent (or rather the filter below the parent---these will not be explicitly mentioned hereafter).

Those who are familiar with the rules for using concurrency while reformatting splits will note that two parents of a Joined may be reformatted concurrently. These reformat procedures may wish to call Reshape, or Redisplay. Therefore, an internal lock is used by the JoinedVBT to serialize these operations; their locking level is LL <= VBT.mu instead of LL = VBT.mu.

INTERFACE JoinedVBT;

IMPORT Filter, VBT;

TYPE
  T <: Public;
  Public = Filter.T OBJECT
           METHODS
             <* LL.sup <= VBT.mu *>
             init (ch: VBT.T): T
           END;

PROCEDURE New (ch: VBT.T): T;
<* LL.sup <= VBT.mu *>

New(...) is equivalent to NEW(T).init(...).

END JoinedVBT.