Allow cc::Scheduler begin frame source to be changed

cc/scheduler/scheduler.cc

 // Copyright 2011 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 "cc/scheduler/scheduler.h"
 
 #include <algorithm>
 
 #include "base/auto_reset.h"
 #include "base/logging.h"
@@ skipping 31 matching lines @@
     SchedulerClient* client,
     const SchedulerSettings& settings,
     int layer_tree_host_id,
     base::SingleThreadTaskRunner* task_runner,
     BeginFrameSource* begin_frame_source,
     scoped_ptr<CompositorTimingHistory> compositor_timing_history)
     : settings_(settings),
       client_(client),
       layer_tree_host_id_(layer_tree_host_id),
       task_runner_(task_runner),
-      begin_frame_source_(begin_frame_source),
+      begin_frame_source_(nullptr),
       observing_begin_frame_source_(false),
       compositor_timing_history_(std::move(compositor_timing_history)),
       begin_impl_frame_deadline_mode_(
           SchedulerStateMachine::BEGIN_IMPL_FRAME_DEADLINE_MODE_NONE),
       begin_impl_frame_tracker_(BEGINFRAMETRACKER_FROM_HERE),
       state_machine_(settings),
       inside_process_scheduled_actions_(false),
       inside_action_(SchedulerStateMachine::ACTION_NONE),
       weak_factory_(this) {
   TRACE_EVENT1("cc", "Scheduler::Scheduler", "settings", settings_.AsValue());
   DCHECK(client_);
   DCHECK(!state_machine_.BeginFrameNeeded());
 
   begin_retro_frame_closure_ =
       base::Bind(&Scheduler::BeginRetroFrame, weak_factory_.GetWeakPtr());
   begin_impl_frame_deadline_closure_ = base::Bind(
       &Scheduler::OnBeginImplFrameDeadline, weak_factory_.GetWeakPtr());
 
+  SetBeginFrameSource(begin_frame_source);
   ProcessScheduledActions();
 }
 
 Scheduler::~Scheduler() {
   if (observing_begin_frame_source_)
     begin_frame_source_->RemoveObserver(this);
 }
 
 base::TimeTicks Scheduler::Now() const {
   base::TimeTicks now = base::TimeTicks::Now();
@@ skipping 25 matching lines @@
   state_machine_.NotifyReadyToActivate();
   ProcessScheduledActions();
 }
 
 void Scheduler::NotifyReadyToDraw() {
   // Future work might still needed for crbug.com/352894.
   state_machine_.NotifyReadyToDraw();
   ProcessScheduledActions();
 }
 
+void Scheduler::SetBeginFrameSource(BeginFrameSource* source) {
+  DCHECK(source);
+  if (source == begin_frame_source_)
+    return;
+  if (begin_frame_source_ && observing_begin_frame_source_)
+    begin_frame_source_->RemoveObserver(this);
+  begin_frame_source_ = source;
+  if (observing_begin_frame_source_)
+    begin_frame_source_->AddObserver(this);
+}
+
 void Scheduler::SetNeedsBeginMainFrame() {
   state_machine_.SetNeedsBeginMainFrame();
   ProcessScheduledActions();
 }
 
 void Scheduler::SetNeedsOneBeginImplFrame() {
   state_machine_.SetNeedsOneBeginImplFrame();
   ProcessScheduledActions();
 }
 
@@ skipping 611 matching lines @@
 
   state->BeginDictionary("state_machine");
   state_machine_.AsValueInto(state);
   state->EndDictionary();
 
   // Only trace frame sources when explicitly enabled - http://crbug.com/420607
   bool frame_tracing_enabled = false;
   TRACE_EVENT_CATEGORY_GROUP_ENABLED(
       TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.frames"),
       &frame_tracing_enabled);
-  if (frame_tracing_enabled) {
+  if (frame_tracing_enabled && begin_frame_source_) {
     state->BeginDictionary("begin_frame_source_");
     begin_frame_source_->AsValueInto(state);
     state->EndDictionary();
   }
 
   state->BeginDictionary("scheduler_state");
   state->SetBoolean("throttle_frame_production_",
                     settings_.throttle_frame_production);
   state->SetDouble("estimated_parent_draw_time_ms",
                    estimated_parent_draw_time_.InMillisecondsF());
@@ skipping 93 matching lines @@
 }
 
 bool Scheduler::IsBeginMainFrameSentOrStarted() const {
   return (state_machine_.begin_main_frame_state() ==
               SchedulerStateMachine::BEGIN_MAIN_FRAME_STATE_SENT ||
           state_machine_.begin_main_frame_state() ==
               SchedulerStateMachine::BEGIN_MAIN_FRAME_STATE_STARTED);
 }
 
 }  // namespace cc