have canvas send discard instead of retain if the draw would overwrite everything

include/core/SkCanvas.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCanvas_DEFINED
 #define SkCanvas_DEFINED
 
@@ skipping 1337 matching lines @@
     virtual SkCanvas* canvasForDrawIter();
 
     // Clip rectangle bounds. Called internally by saveLayer.
     // returns false if the entire rectangle is entirely clipped out
     // If non-NULL, The imageFilter parameter will be used to expand the clip
     // and offscreen bounds for any margin required by the filter DAG.
     bool clipRectBounds(const SkRect* bounds, SaveFlags flags,
                         SkIRect* intersection,
                         const SkImageFilter* imageFilter = NULL);
 
+private:
+    enum ShaderOverrideOpacity {
+        kNone_ShaderOverrideOpacity,        //!< there is no overriding shader (bitmap or image)
+        kOpaque_ShaderOverrideOpacity,      //!< the overriding shader is opaque
+        kNotOpaque_ShaderOverrideOpacity,   //!< the overriding shader may not be opaque
+    };
+
     // notify our surface (if we have one) that we are about to draw, so it
     // can perform copy-on-write or invalidate any cached images

[robertphillips, 2015/07/20 15:15:29]

willOverwriteEntireSurface (no extra 's') ?

-    void predrawNotify();
+    void predrawNotify(bool willOverwritesEntireSurface = false);
+    void predrawNotify(const SkRect* rect, const SkPaint* paint, ShaderOverrideOpacity);
+    void predrawNotify(const SkRect* rect, const SkPaint* paint, bool shaderOverrideIsOpaque) {
+        this->predrawNotify(rect, paint, shaderOverrideIsOpaque ? kOpaque_ShaderOverrideOpacity
+                                                                : kNotOpaque_ShaderOverrideOpacity);
+    }
 
-private:
     class MCRec;
 
     SkAutoTUnref<SkClipStack> fClipStack;
     SkDeque     fMCStack;
     // points to top of stack
     MCRec*      fMCRec;
     // the first N recs that can fit here mean we won't call malloc
     enum {
         kMCRecSize      = 128,  // most recent measurement
         kMCRecCount     = 8,    // common depth for save/restores
@@ skipping 24 matching lines @@
 
     friend class SkDrawIter;        // needs setupDrawForLayerDevice()
     friend class AutoDrawLooper;
     friend class SkLua;             // needs top layer size and offset
     friend class SkDebugCanvas;     // needs experimental fAllowSimplifyClip
     friend class SkDeferredDevice;  // needs getTopDevice()
     friend class SkSurface_Raster;  // needs getDevice()
     friend class SkRecorder;        // InitFlags
     friend class SkNoSaveLayerCanvas;   // InitFlags
     friend class SkPictureImageFilter;  // SkCanvas(SkBaseDevice*, SkSurfaceProps*, InitFlags)
+    friend class SkPictureRecord;   // predrawNotify (why does it need it? <reed>)
 
     enum InitFlags {
         kDefault_InitFlags                  = 0,
         kConservativeRasterClip_InitFlag    = 1 << 0,
     };
     SkCanvas(const SkIRect& bounds, InitFlags);
     SkCanvas(SkBaseDevice* device, InitFlags);
 
     void resetForNextPicture(const SkIRect& bounds);
 
@@ skipping 29 matching lines @@
     void internalRestore();
     static void DrawRect(const SkDraw& draw, const SkPaint& paint,
                          const SkRect& r, SkScalar textSize);
     static void DrawTextDecorations(const SkDraw& draw, const SkPaint& paint,
                                     const char text[], size_t byteLength,
                                     SkScalar x, SkScalar y);
 
     // only for canvasutils
     const SkRegion& internal_private_getTotalClip() const;
 
+    /*
+     *  Returns true if drawing the specified rect (or all if it is null) with the specified
+     *  paint (or default if null) would overwrite the entire root device of the canvas
+     *  (i.e. the canvas' surface if it had one).
+     */
+    bool wouldOverwriteEntireSurface(const SkRect*, const SkPaint*, ShaderOverrideOpacity) const;
+
+
     /*  These maintain a cache of the clip bounds in local coordinates,
         (converted to 2s-compliment if floats are slow).
      */
     mutable SkRect fCachedLocalClipBounds;
     mutable bool   fCachedLocalClipBoundsDirty;
     bool fAllowSoftClip;
     bool fAllowSimplifyClip;
     bool fConservativeRasterClip;
 
     const SkRect& getLocalClipBounds() const {
@@ skipping 119 matching lines @@
 
 class SkCanvasClipVisitor {
 public:
     virtual ~SkCanvasClipVisitor();
     virtual void clipRect(const SkRect&, SkRegion::Op, bool antialias) = 0;
     virtual void clipRRect(const SkRRect&, SkRegion::Op, bool antialias) = 0;
     virtual void clipPath(const SkPath&, SkRegion::Op, bool antialias) = 0;
 };
 
 #endif