ui/src/split/OverlayVBT.i3

 Copyright (C) 1993, Digital Equipment Corporation                         
 All rights reserved.                                                      
 See the file COPYRIGHT for a full description.                            
                                                                           
 Last modified on Tue Apr  4 11:36:49 PDT 1995 by msm                      

<* PRAGMA LL *>

 An OverlayVBT is a filter that can paint on top of its child. 


INTERFACE OverlayVBT;

IMPORT VBT, Region, Filter;

TYPE
  T <: Public;
  Public =
    Filter.T OBJECT
    METHODS
      <* LL.sup <= VBT.mu *>
      init (ch: VBT.T): T;
      <* LL = VBT.mu.v *>
      set (READONLY rgn: Region.T);
      (* client-supplied method: *)
      <* LL.sup = ch *>
      paint (READONLY rgn: Region.T);
    END;

 The call v.init(..) initializes v as an OverlayVBT
   with child ch and an empty overlay region.  By setting the
   region to be non-empty, the client's paint method will be called
   as needed to maintain the client overlay; the region supplied
   will be the subset of the region that needs to be repaired.  Clients
   may want to override the reshape method to set an appropriate region. 


REVEAL VBT.Split <: VBT.Leaf;

 So that painting on a T is known to be legit. 

END OverlayVBT.