Use SkLerpXfermode to blend LabelButton images.

ui/views/controls/button/label_button_border.cc

Index: ui/views/controls/button/label_button_border.cc
diff --git a/ui/views/controls/button/label_button_border.cc b/ui/views/controls/button/label_button_border.cc
index efb80df3a1b5b672855f1151b26902216cd3ac0d..6dbf1e1858cc8a6f7b9f67322f15d9309c3d2bb0 100644
--- a/ui/views/controls/button/label_button_border.cc
+++ b/ui/views/controls/button/label_button_border.cc
@@ -6,6 +6,8 @@
 
 #include "base/logging.h"
 #include "grit/ui_resources.h"
+#include "third_party/skia/include/core/SkPaint.h"
+#include "third_party/skia/include/effects/SkLerpXfermode.h"
 #include "ui/base/animation/animation.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/gfx/canvas.h"
@@ -119,17 +121,19 @@ void LabelButtonBorder::Paint(const View& view, gfx::Canvas* canvas) {
   ui::NativeTheme::State state = native_theme_delegate->GetThemeState(&extra);
 
   if (animation && animation->is_animating()) {
-    // Composite the background and foreground painters during state animations.
-    int alpha = animation->CurrentValueBetween(0, 0xff);
+    // Linearly interpolate background and foreground painters during animation.
+    canvas->sk_canvas()->saveLayer(NULL, NULL);

[reed1, 2013/08/14 13:33:56]

Not specific to this CL, but..

Skia definitely takes advantage of the optional rect parameter in saveLayer. If
you pass it, it will limit the size of the offscreen, which saves time both for
initialization and during the restore (when we 'compose' the layer).

Do we have this rect (or a conservative version of it?)

[msw, 2013/08/14 16:17:11]

Done.

     state = native_theme_delegate->GetBackgroundThemeState(&extra);
-    canvas->SaveLayerAlpha(static_cast<uint8>(0xff - alpha));
     PaintHelper(this, canvas, theme, part, state, rect, extra);
-    canvas->Restore();
 
+    SkPaint paint;
+    paint.setXfermode(SkLerpXfermode::Create(animation->GetCurrentValue()));

[reed1, 2013/08/14 13:33:56]

Create returns an object of reference-count 1
setXfermode will increment that reference-count (to 2)
the paint's destructor will decrement it back to 1 .... LEAK

The fix is to call unref somehow on the xfermode itself.

either

paint.setXfermode(Create(...))->unref()

or

SkAutoTUnref<SkXfermode> mode(Create(...));
paint.setXfermode(mode);

will do the trick

[msw, 2013/08/14 16:17:11]

Done. I too easily forgot how tricky using skia in chrome can be.

+    canvas->sk_canvas()->saveLayer(NULL, &paint);

[reed1, 2013/08/14 13:33:56]

Same comment here about the value of passing in a bounds-rect to saveLayer

[msw, 2013/08/14 16:17:11]

Done.

     state = native_theme_delegate->GetForegroundThemeState(&extra);
-    canvas->SaveLayerAlpha(static_cast<uint8>(alpha));
     PaintHelper(this, canvas, theme, part, state, rect, extra);
-    canvas->Restore();
+    canvas->sk_canvas()->restore();
+
+    canvas->sk_canvas()->restore();
   } else {
     PaintHelper(this, canvas, theme, part, state, rect, extra);
   }