Add GM that exercises SkCanvas::discard()

gm/discard.cpp

Index: gm/discard.cpp
diff --git a/gm/discard.cpp b/gm/discard.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..ae3947dfe414b0ffc5ab547bad8e033d36ea8ddf
--- /dev/null
+++ b/gm/discard.cpp
@@ -0,0 +1,95 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#if SK_SUPPORT_GPU
+
+#include "gm.h"
+#include "SkCanvas.h"
+#include "SkColorShader.h"
+#include "SkPaint.h"
+#include "SkSurface.h"
+
+namespace skiagm {
+
+/*
+ * This GM exercises SkCanvas::discard() by creating an offscreen SkSurface and repeatedly
+ * discarding it, drawing to it, and then drawing it to the main canvas.
+ */
+class DiscardGM : public GM {
+
+public:
+    DiscardGM() {
+    }
+
+    virtual uint32_t onGetFlags() const SK_OVERRIDE { return kGPUOnly_Flag; }
+
+protected:
+    virtual SkString onShortName() SK_OVERRIDE {
+        return SkString("discard");
+    }
+
+    virtual SkISize onISize() SK_OVERRIDE {
+        return SkISize::Make(100, 100);
+    }
+
+    virtual void onDraw(SkCanvas* canvas) SK_OVERRIDE {
+        GrContext* context = canvas->getGrContext();
+        if (NULL == context) {
+            return;
+        }
+
+        SkISize size = this->getISize();
+        size.fWidth /= 10;
+        size.fHeight /= 10;
+        SkImageInfo info = SkImageInfo::MakeN32Premul(size);
+        SkSurface* surface = SkSurface::NewRenderTarget(context, info);
+
+        if (NULL == surface) {
+            return;
+        }
+
+        canvas->clear(SK_ColorBLACK);
+
+        SkRandom rand;
+        for (int x = 0; x < 10; ++x) {
+            for (int y = 0; y < 10; ++y) {
+              surface->getCanvas()->discard();
+              // Make something that isn't too close to the background color, black.
+              SkColor color = rand.nextU() | 0xFF404040;
+              switch (rand.nextULessThan(3)) {
+                  case 0:
+                      surface->getCanvas()->drawColor(color);
+                      break;
+                  case 1:
+                      surface->getCanvas()->clear(color);
+                      break;
+                  case 2:
+                      SkColorShader shader(color);
+                      SkPaint paint;
+                      paint.setShader(&shader);
+                      surface->getCanvas()->drawPaint(paint);
+                      break;
+              }
+              surface->draw(canvas, 10*x, 10*y, NULL);
+            }
+        }
+
+        surface->getCanvas()->discard();
+        surface->unref();
+    }
+
+private:
+    typedef GM INHERITED;
+};
+
+//////////////////////////////////////////////////////////////////////////////
+
+DEF_GM( return SkNEW(DiscardGM); )
+
+} // end namespace
+
+#endif