RenderWidget's scheduleComposite should return a bool. (true iff a prompt composite will come).

content/renderer/render_widget.cc

 // Copyright (c) 2012 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 "content/renderer/render_widget.h"
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
@@ skipping 807 matching lines @@
   DoDeferredUpdateAndSendInputAck();
 }
 
 void RenderWidget::DoDeferredUpdateAndSendInputAck() {
   DoDeferredUpdate();
 
   if (pending_input_event_ack_.get())
     Send(pending_input_event_ack_.release());
 }
 
+bool RenderWidget::CanDoDeferredUpdate() const {
+  if (!webwidget_) {
+    TRACE_EVENT0("renderer", "EarlyOut_NoWebWidget");
+    return false;
+  }
+  if (!host_window_set_) {
+    TRACE_EVENT0("renderer", "EarlyOut_NoHostWindow");
+    return false;
+  }
+  if (update_reply_pending_) {
+    TRACE_EVENT0("renderer", "EarlyOut_UpdateReplyPending");
+    return false;
+  }
+  if (is_accelerated_compositing_active_ &&
+      num_swapbuffers_complete_pending_ >= kMaxSwapBuffersPending) {
+    TRACE_EVENT0("renderer", "EarlyOut_MaxSwapBuffersPending");
+    return false;
+  }
+  return true;
+}
+
+bool RenderWidget::ShouldSuppressDeferredUpdates() const {
+  return is_hidden_ || size_.IsEmpty();
+}
+
 void RenderWidget::DoDeferredUpdate() {
   TRACE_EVENT0("renderer", "RenderWidget::DoDeferredUpdate");
 
-  if (!webwidget_)
+  if (!CanDoDeferredUpdate())
     return;
 
-  if (!host_window_set_) {
-    TRACE_EVENT0("renderer", "EarlyOut_NoHostWindow");
-    return;
-  }
-  if (update_reply_pending_) {
-    TRACE_EVENT0("renderer", "EarlyOut_UpdateReplyPending");
-    return;
-  }
-  if (is_accelerated_compositing_active_ &&
-      num_swapbuffers_complete_pending_ >= kMaxSwapBuffersPending) {
-    TRACE_EVENT0("renderer", "EarlyOut_MaxSwapBuffersPending");
-    return;
-  }
-
   // Suppress updating when we are hidden.
-  if (is_hidden_ || size_.IsEmpty()) {
+  if (ShouldSuppressDeferredUpdates()) {
     paint_aggregator_.ClearPendingUpdate();
     needs_repainting_on_restore_ = true;
     TRACE_EVENT0("renderer", "EarlyOut_NotVisible");
     return;
   }
 
   if (is_accelerated_compositing_active_)
     using_asynchronous_swapbuffers_ = SupportsAsynchronousSwapBuffers();
 
   // Tracking of frame rate jitter
@@ skipping 317 matching lines @@
   params.view_size = size_;
   params.plugin_window_moves.swap(plugin_window_moves_);
   params.flags = next_paint_flags_;
   params.scroll_offset = GetScrollOffset();
   params.needs_ack = false;
 
   Send(new ViewHostMsg_UpdateRect(routing_id_, params));
   next_paint_flags_ = 0;
 }
 
-void RenderWidget::scheduleComposite() {
+#if defined(WEBWIDGETCLIENT_SCHEDULECOMPOSITE_RETURNS_BOOL)
+#define SC_RETURN_TYPE bool
+#else
+#define SC_RETURN_TYPE void
+#endif
+
+SC_RETURN_TYPE RenderWidget::scheduleComposite() {
   if (WebWidgetHandlesCompositorScheduling()) {
     webwidget_->composite(false);
+#if defined(WEBWIDGETCLIENT_SCHEDULECOMPOSITE_RETURNS_BOOL)
+    return true;
+#endif
   } else {
     // TODO(nduca): replace with something a little less hacky.  The reason this
     // hack is still used is because the Invalidate-DoDeferredUpdate loop
     // contains a lot of host-renderer synchronization logic that is still
     // important for the accelerated compositing case. The option of simply
     // duplicating all that code is less desirable than "faking out" the
     // invalidation path using a magical damage rect.
     didInvalidateRect(WebRect(0, 0, 1, 1));
+#if defined(WEBWIDGETCLIENT_SCHEDULECOMPOSITE_RETURNS_BOOL)
+    return CanDoDeferredUpdate() && !ShouldSuppressDeferredUpdates();

[jamesr, 2012/05/08 23:42:40]

what happens if ShouldSuppress...() is false, but when the DoDeferredUpdate
actually runs ShouldSuppress..() has become true (and stays that way)? how do we
make sure that WebKit knows about this state transition?

+#endif
   }
 }
 
 void RenderWidget::scheduleAnimation() {
   if (animation_update_pending_)
     return;
 
   TRACE_EVENT0("gpu", "RenderWidget::scheduleAnimation");
   animation_update_pending_ = true;
   if (!animation_timer_.IsRunning()) {
@@ skipping 511 matching lines @@
   }
 }
 
 bool RenderWidget::WillHandleMouseEvent(const WebKit::WebMouseEvent& event) {
   return false;
 }
 
 bool RenderWidget::WebWidgetHandlesCompositorScheduling() const {
   return false;
 }