Goal: ensure we always balance lock/unlock pixels calls.

A big caller of lockPixels is setContext in the bitmapshader.

This change replaces beginSession/endSession with adding endContext(), and
adds debugging code to ensure that
1. setContext calls are never nested
2. endContext is always called after each setContext call.
Review URL: https://codereview.appspot.com/6937046

git-svn-id: http://skia.googlecode.com/svn/trunk@6798 2bbb7eff-a529-9590-31e7-b0007b416f81

2 files changed, 23 insertions, 15 deletions

diff --git a/include/core/SkComposeShader.h b/include/core/SkComposeShader.h
index a8a8e0bb60..b0790bf393 100644
--- a/include/core/SkComposeShader.h
+++ b/include/core/SkComposeShader.h
@@ -34,11 +34,10 @@ public:
     SkComposeShader(SkShader* sA, SkShader* sB, SkXfermode* mode = NULL);
     virtual ~SkComposeShader();
 
-    // override
-    virtual bool setContext(const SkBitmap& device, const SkPaint& paint, const SkMatrix& matrix);
-    virtual void shadeSpan(int x, int y, SkPMColor result[], int count);
-    virtual void beginSession();
-    virtual void endSession();
+    virtual bool setContext(const SkBitmap&, const SkPaint&,
+                            const SkMatrix&) SK_OVERRIDE;
+    virtual void endContext() SK_OVERRIDE;
+    virtual void shadeSpan(int x, int y, SkPMColor[], int count) SK_OVERRIDE;
 
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkComposeShader)
 
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 1286177271..7edbe6fd4b 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -139,12 +139,29 @@ public:
     /**
      *  Called once before drawing, with the current paint and device matrix.
      *  Return true if your shader supports these parameters, or false if not.
-     *  If false is returned, nothing will be drawn.
+     *  If false is returned, nothing will be drawn. If true is returned, then
+     *  a balancing call to endContext() will be made before the next call to
+     *  setContext.
+     *
+     *  Subclasses should be sure to call their INHERITED::setContext() if they
+     *  override this method.
      */
     virtual bool setContext(const SkBitmap& device, const SkPaint& paint,
                             const SkMatrix& matrix);
 
     /**
+     *  Assuming setContext returned true, endContext() will be called when
+     *  the draw using the shader has completed. It is an error for setContext
+     *  to be called twice w/o an intervening call to endContext().
+     *
+     *  Subclasses should be sure to call their INHERITED::endContext() if they
+     *  override this method.
+     */
+    virtual void endContext();
+
+    SkDEBUGCODE(bool setContextHasBeenCalled() const { return fInSetContext; })
+
+    /**
      *  Called for each span of the object being drawn. Your subclass should
      *  set the appropriate colors (with premultiplied alpha) that correspond
      *  to the specified device coordinates.
@@ -183,14 +200,6 @@ public:
     }
 
     /**
-     *  Called before a session using the shader begins. Some shaders override
-     *  this to defer some of their work (like calling bitmap.lockPixels()).
-     *  Must be balanced by a call to endSession.
-     */
-    virtual void beginSession();
-    virtual void endSession();
-
-    /**
      Gives method bitmap should be read to implement a shader.
      Also determines number and interpretation of "extra" parameters returned
      by asABitmap
@@ -355,7 +364,7 @@ private:
     uint8_t             fPaintAlpha;
     uint8_t             fDeviceConfig;
     uint8_t             fTotalInverseClass;
-    SkDEBUGCODE(SkBool8 fInSession;)
+    SkDEBUGCODE(SkBool8 fInSetContext;)
 
     static SkShader* CreateBitmapShader(const SkBitmap& src,
                                         TileMode, TileMode,