tabstrip: Use a PaintRecorder to access the Canvas for painting.

ui/compositor/compositing_recorder.cc

Index: ui/compositor/compositing_recorder.cc
diff --git a/ui/compositor/compositing_recorder.cc b/ui/compositor/compositing_recorder.cc
index 18a8fa2efd6f11a59551cc093c0dd1f7c479e55e..6fdcf0e50329a84d839351e953932d1ceaa6f1fa 100644
--- a/ui/compositor/compositing_recorder.cc
+++ b/ui/compositor/compositing_recorder.cc
@@ -9,14 +9,17 @@
 
 namespace ui {
 
-CompositingRecorder::CompositingRecorder(const PaintContext& context,
-                                         float opacity)
-    : canvas_(context.canvas()) {
-  canvas_->SaveLayerAlpha(0xff * opacity);
+CompositingRecorder::CompositingRecorder(const PaintContext& context, int alpha)
+    : canvas_(context.canvas()), saved_(alpha < 255) {
+  DCHECK_GE(alpha, 0);
+  DCHECK_LE(alpha, 255);
+  if (saved_)
+    canvas_->SaveLayerAlpha(alpha);

[Peter Kasting, 2015/04/07 21:32:39]

Because SaveLayerAlpha() takes a uint8, I think |alpha| should also be a uint8
(well, probably a uint8_t).  This eliminates the need to do DCHECKs as well.

[danakj, 2015/04/07 21:36:26]

I did this at first (which is why the extra git cl format changes). But the
style guide pretty clearly says to use int not unsigned, and use DCHECKs for
bounds checking instead:

"Do not use unsigned types to mean "this value should never be < 0". For that,
use assertions or run-time checks (as appropriate)."

That feels like it probably applies to "should never be > 255" just the same?
And the existing code is all ints.

WDYT?

[Peter Kasting, 2015/04/07 21:45:59]

I would prefer to be consistent with the existing API there.  If someone later
wants to change all levels of the API to use int at once they can.

Even in that case, I actually think this is a case where use of uint8_t is
arguably correct.  It's not just "can't be less than zero", it's a very specific
range of values that corresponds exactly to a particular type.  The specific
sized-ness of the type is relevant.

 }
 
 CompositingRecorder::~CompositingRecorder() {
-  canvas_->Restore();
+  if (saved_)
+    canvas_->Restore();
 }
 
 }  // namespace ui