Copyright 1992 Digital Equipment Corporation. 
 Distributed only by permission. 
 Last modified on Sun Jul 11 11:22:41 PDT 1993 by mhb   
      modified on Thu Feb 11 16:05:59 PST 1993 by johnh 


IMPORT ReactivityVBT, VBT, ZeusClass;
A View.T is a subclass of a ZeusClass.T with four additional methods:

init(ch) inserts the ch argument as the child of the View.T (which is a ReactivityVBT.T), sets the reactivity to Passive, and returns the View.T. If ch is NIL, then a child is created that displays the background. The init() method should be called just after the View.T is created. Any user-supplied override to the init method must invoke the init method of the supertype.

isCompat(alg) is called by ZeusPanel to determine whether the view is compatible with a particular algorithm. Compatibility means that the view is prepared to display the events that the algorithm generates. This method generally just calls ISTYPE; the user should not have to override it except under unusual circumstances.

startrun(v) is called by ZeusPanel just after the user issues the command to start running the algorithm, and before the algorithm is actually run. Many subclasses will override the startrun method to erase anything in the window left over from the previous execution of the algorithm. If a view's startrun procedure is called, ZeusPanel guarantees that its endrun procedure will eventually be called.

endrun(v) is called by ZeusPanel just after the algorithm finishes running, either because it was aborted by the user, it crashed, or it came to a normal completion. Typically, it is used by views to clean up after themselves -- to kill any active threads, etc. An endrun method will only be called if its startrun method had been called previously.

The default ZeusClass config method is a noop. The other ZeusClass methods have the following defaults:

install tells Trestle to insert self into the window system, but not to put self on the screen; restore will be called immediately after install, and will do that.

delete tells Trestle to delete self from the window system

snapshot records location of Trestle window

restore installs and moves self to a previously recorded location

reactivity calls ReactivityVBT.Set to make the view Passive or Active. The view is made Dormant whenever it is incompatible with the current algorithm, and in this case the reactivity method has no effect.

Subclasses of View.T that are created by zume will be extended with methods for each OUTPUT event and UPDATE event in the .evt file. The output methods are invoked with LL < VBT.mu. Update methods (which are called in response to a feedback event) are invoked with LL = VBT.mu. User-supplied overrides for OUTPUT and UPDATE methods MUST NOT invoke the corresponding supertype methods.

   ZeusClass.Private <: ReactivityVBT.T;

  T <: Public;
  Public = ZeusClass.T OBJECT
             (* LL = VBT.mu *)
             init (ch: VBT.T): T;
             isCompat (alg: ZeusClass.T): BOOLEAN;
             (* LL < VBT.mu *)
             startrun ();
             endrun   ();

END View.