Make painting app list items more efficient by caching the labels.

ui/app_list/views/cached_label.cc

+// Copyright 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ui/app_list/views/cached_label.h"
+
+#include "base/utf_string_conversions.h"
+#include "ui/base/resource/resource_bundle.h"
+#include "ui/gfx/canvas.h"
+
+namespace app_list {
+
+CachedLabel::CachedLabel()
+    : needs_repaint_(true) {
+}
+
+void CachedLabel::PaintToBackingImage() {
+  if (bitmap_.width() != width() || bitmap_.height() != height()) {
+    bitmap_.setConfig(SkBitmap::kARGB_8888_Config, width(), height());
+    bitmap_.allocPixels();
+    image_ = gfx::ImageSkia::CreateFrom1xBitmap(bitmap_);
+  }
+
+  SkCanvas sk_canvas(bitmap_);
+  scoped_ptr<gfx::Canvas> canvas(gfx::Canvas::CreateCanvasWithoutScaling(
+      &sk_canvas, ui::SCALE_FACTOR_100P));
+
+  canvas->FillRect(gfx::Rect(0, 0, width(), height()),
+                   SkColorSetARGB(0x0, 0, 0, 0), SkXfermode::kSrc_Mode);

[xiyuan, 2013/03/13 04:14:50]

Transparency might break ClearType painting and have artifacts. Suggest to paint
on opaque app list background #f5f5f5.

[koz (OOO until 15th September), 2013/03/13 04:43:28]

The difficulty I found with opaque backgrounds is that you need to make
everything beneath it opaque as well but currently the colour is determined by
various semi-transparent tones, some of which have the potential to animate
(though we have set the animation time to zero now).

I'm hoping this will suffice as a temporary fix until win/aura is the default.

+  Label::OnPaint(canvas.get());

[xiyuan, 2013/03/13 04:14:50]

Are you using a SkBitmap to save some allocation? If not, think we can get rid
of SkBitmap etc (line 18-26) and simplify as:

gfx::Canvas canvas(size(), ui::SCALE_FACTOR_100P,
                   true /* is_opaque */);
canvas->FillRect(GetLocalBounds(), SkColorSetRGB(0xF5, 0xF5, 0xF5));
Label::OnPaint(&canvas);
image_ = gfx::ImageSkia(canvas.ExtractImageRep());

[koz (OOO until 15th September), 2013/03/13 04:43:28]

Everything I do here is out of ignorance of a better way :-) Yours is much
nicer! I've removed the SkBitmap.

+}
+
+#if defined(OS_WIN)
+void CachedLabel::OnPaint(gfx::Canvas* canvas) {
+  if (needs_repaint_)
+    PaintToBackingImage();
+  needs_repaint_ = false;
+  canvas->DrawImageInt(image_, 0, 0);
+}
+#endif
+
+}  // namespace app_list