The module

<*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 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 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: 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.