cc: Make PrepareToDraw return an enum for why it aborts

cc/trees/thread_proxy.cc

Index: cc/trees/thread_proxy.cc
diff --git a/cc/trees/thread_proxy.cc b/cc/trees/thread_proxy.cc
index 0b7d7ef9ebe1519c734d49005591378d4359841f..b5f455556cf7b01c807ed45537e04d76fd1ae6ae 100644
--- a/cc/trees/thread_proxy.cc
+++ b/cc/trees/thread_proxy.cc
@@ -1088,17 +1088,21 @@ DrawSwapReadbackResult ThreadProxy::DrawSwapReadbackInternal(
     bool readback_requested) {
   TRACE_EVENT_SYNTHETIC_DELAY("cc.DrawAndSwap");
   DrawSwapReadbackResult result;
-  result.did_draw = false;
   result.did_swap = false;
   result.did_readback = false;
+
   DCHECK(IsImplThread());
   DCHECK(layer_tree_host_impl_.get());
-  if (!layer_tree_host_impl_)
+  if (!layer_tree_host_impl_) {
+    result.draw_result = DrawSwapReadbackResult::DRAW_ABORTED_NO_TREE;
     return result;
+  }
 
   DCHECK(layer_tree_host_impl_->renderer());
-  if (!layer_tree_host_impl_->renderer())
+  if (!layer_tree_host_impl_->renderer()) {
+    result.draw_result = DrawSwapReadbackResult::DRAW_ABORTED_NO_RENDERER;
     return result;
+  }
 
   base::TimeTicks start_time = base::TimeTicks::HighResNow();
   base::TimeDelta draw_duration_estimate = DrawDurationEstimate();
@@ -1139,16 +1143,19 @@ DrawSwapReadbackResult ThreadProxy::DrawSwapReadbackInternal(
     if (drawing_for_readback)
       readback_rect = readback_request_on_impl_thread_->rect;
 
-    if (layer_tree_host_impl_->PrepareToDraw(&frame, readback_rect) ||
-        forced_draw)
-      draw_frame = true;
+    result.draw_result =
+        layer_tree_host_impl_->PrepareToDraw(&frame, readback_rect);

[brianderson, 2014/01/29 01:31:22]

It feels funny to have PrepareToDraw return DID_DRAW when the draw hasn't
occurred yet, but I don't see how to make it any better without needlessly
complicating the code.

[enne (OOO), 2014/01/29 01:40:19]

Yeah, I thought that too.  I could change DID_DRAW to DRAW_SUCCESS, and then it
might make sense before drawing and after drawing?

[danakj, 2014/01/29 01:42:35]

WILL_DRAW?

[enne (OOO), 2014/01/29 01:47:20]

WILL_DRAW doesn't make sense as a result after DrawAndSwapIfPossible.  This
seems like the reverse of why DID_DRAW doesn't make sense for PrepareToDraw.

+    draw_frame =
+        forced_draw || result.draw_result == DrawSwapReadbackResult::DID_DRAW;
   }
 
   if (draw_frame) {
     layer_tree_host_impl_->DrawLayers(
         &frame,
         scheduler_on_impl_thread_->LastBeginImplFrameTime());
-    result.did_draw = true;
+    result.draw_result = DrawSwapReadbackResult::DID_DRAW;
+  } else {
+    DCHECK_NE(DrawSwapReadbackResult::DID_DRAW, result.draw_result);
   }
   layer_tree_host_impl_->DidDrawAllLayers(frame);