// on impl
void Scheduler::ProcessScheduledActions() {
  ...
      case SchedulerStateMachine::ACTION_SEND_BEGIN_MAIN_FRAME:
        client_->ScheduledActionSendBeginMainFrame();
  ...
}

// on impl
void ThreadProxy::ScheduledActionSendBeginMainFrame() {
  ...
  Proxy::MainThreadTaskRunner()->PostTask(
      FROM_HERE,
      base::Bind(&ThreadProxy::BeginMainFrame,
                 main_thread_weak_ptr_,
                 base::Passed(&begin_main_frame_state)));
  ...
}

// on main
void ThreadProxy::BeginMainFrame(
    scoped_ptr<BeginMainFrameAndCommitState> begin_main_frame_state) {
  ...
* When the bFAC message runs on the main thread we do the following things:
 * Apply any impl-side scrolls to the main thread
 * Call the requestAnimationFrame callback
 * Perform any pending layout (namely, HTML layout)
 * Paint any layers that need to be painted (software rasterization)
  // Paint layers!!!
  bool updated = layer_tree_host_->UpdateLayers(queue.get());
  ...
  {
    ...
    CompletionEvent completion;
    Proxy::ImplThreadTaskRunner()->PostTask(
        FROM_HERE,
        base::Bind(&ThreadProxy::StartCommitOnImplThread,
                   impl_thread_weak_ptr_,
                   &completion,
                   queue.release(),
                   offscreen_context_provider));
    completion.Wait();
    // WAIT!!!
    ...
  }

  layer_tree_host_->CommitComplete();
  layer_tree_host_->DidBeginMainFrame();
}

// on impl
void ThreadProxy::StartCommitOnImplThread(
    CompletionEvent* completion,
    ResourceUpdateQueue* raw_queue,
    scoped_refptr<cc::ContextProvider> offscreen_context_provider) {
  ..
  if (layer_tree_host_->contents_texture_manager()) {
    if (layer_tree_host_->contents_texture_manager()->
            LinkedEvictedBackingsExist()) {
      // Clear any uploads we were making to textures linked to evicted
      // resources
      queue->ClearUploadsToEvictedResources();
      // Some textures in the layer tree are invalid. Kick off another commit
      // to fill them again.
      SetNeedsCommitOnImplThread(); // Queue COMMIT!!! but if not impl-side painting....
    }

    layer_tree_host_->contents_texture_manager()->
        PushTexturePrioritiesToBackings();
  }

  commit_completion_event_on_impl_thread_ = completion;
  current_resource_update_controller_on_impl_thread_ =
      ResourceUpdateController::Create(
          this,
          Proxy::ImplThreadTaskRunner(),
          queue.Pass(),
          layer_tree_host_impl_->resource_provider());
  current_resource_update_controller_on_impl_thread_->PerformMoreUpdates(
      scheduler_on_impl_thread_->AnticipatedDrawTime());
}

// on impl. it looks like impl-side painting abstraction. NEED TO UNDERSTAND!!!!
void ResourceUpdateController::PerformMoreUpdates(
    base::TimeTicks time_limit) {
  time_limit_ = time_limit;

  // Update already in progress.
  if (task_posted_)
    return;

  // Call UpdateMoreTexturesNow() directly unless it's the first update
  // attempt. This ensures that we empty the update queue in a finite
  // amount of time.
  if (!first_update_attempt_)
    UpdateMoreTexturesNow();

  // Post a 0-delay task when no updates were left. When it runs,
  // ReadyToFinalizeTextureUpdates() will be called.
  if (!UpdateMoreTexturesIfEnoughTimeRemaining()) {
    task_posted_ = true;
    task_runner_->PostTask(
        FROM_HERE,
        base::Bind(&ResourceUpdateController::OnTimerFired,
                   weak_factory_.GetWeakPtr()));
  }

  first_update_attempt_ = false;
}

void ResourceUpdateController::OnTimerFired() {
  task_posted_ = false;
  if (!UpdateMoreTexturesIfEnoughTimeRemaining())
    client_->ReadyToFinalizeTextureUpdates();
}

void ThreadProxy::ReadyToFinalizeTextureUpdates() {
  DCHECK(IsImplThread());
  scheduler_on_impl_thread_->FinishCommit();
}

// a while later on impl
void ThreadProxy::ScheduledActionCommit() {
  TRACE_EVENT0("cc", "ThreadProxy::ScheduledActionCommit");
  DCHECK(IsImplThread());
  ...
  layer_tree_host_impl_->BeginCommit();
  layer_tree_host_->BeginCommitOnImplThread(layer_tree_host_impl_.get());
  layer_tree_host_->FinishCommitOnImplThread(layer_tree_host_impl_.get());
  layer_tree_host_impl_->CommitComplete();
  ...
    // wake up main thread
    commit_completion_event_on_impl_thread_->Signal();
  ...
  // SetVisible kicks off the next scheduler action, so this must be last.
  scheduler_on_impl_thread_->SetVisible(layer_tree_host_impl_->visible());
}

void LayerTreeHost::FinishCommitOnImplThread(LayerTreeHostImpl* host_impl) {
  DCHECK(proxy_->IsImplThread());
  ...
  if (needs_full_tree_sync_)
    sync_tree->SetRootLayer(TreeSynchronizer::SynchronizeTrees(
        root_layer(), sync_tree->DetachLayerTree(), sync_tree));
  {
    TRACE_EVENT0("cc", "LayerTreeHost::PushProperties");
    TreeSynchronizer::PushProperties(root_layer(), sync_tree->root_layer());
  }
  ...
}

// on main
void ThreadProxy::BeginMainFrame(
    scoped_ptr<BeginMainFrameAndCommitState> begin_main_frame_state) {
  ...
  scoped_ptr<ResourceUpdateQueue> queue =
      make_scoped_ptr(new ResourceUpdateQueue);

  bool updated = layer_tree_host_->UpdateLayers(queue.get());
  ...
}

bool LayerTreeHost::UpdateLayers(Layer* root_layer,
                                 ResourceUpdateQueue* queue) {
  ...
  PaintLayerContents(
      update_list, queue, &did_paint_content, &need_more_updates);
  ...
  return did_paint_content;
}

void LayerTreeHost::PaintLayerContents(
    const RenderSurfaceLayerList& render_surface_layer_list,
    ResourceUpdateQueue* queue,
    bool* did_paint_content,
    bool* need_more_updates) {
  // PrioritizeTextures by occlusion, viewport and etc..
  PrioritizeTextures(render_surface_layer_list,
                     occlusion_tracker.overdraw_metrics());
  ...
  for (LayerIteratorType it) {
   ...
    if (it.represents_target_render_surface()) {
      // PAINT Mask
      PaintMasksForRenderSurface(
          *it, queue, did_paint_content, need_more_updates);
    } else if (it.represents_itself() && !it->draw_properties().skip_drawing) {
      ...
      // HERE!!!
      *did_paint_content |= it->Update(queue, &occlusion_tracker);
      *need_more_updates |= it->NeedMoreUpdates();
    }

    occlusion_tracker.LeaveLayer(it);
  }
  ...
}

bool ContentLayer::Update(ResourceUpdateQueue* queue,
                          const OcclusionTracker* occlusion) {
  {
    base::AutoReset<bool> ignore_set_needs_commit(&ignore_set_needs_commit_,
                                                  true);

    CreateUpdaterIfNeeded();
    UpdateCanUseLCDText();
  }

  bool updated = TiledLayer::Update(queue, occlusion);
  return updated;
}
// or
bool PictureLayer::Update(ResourceUpdateQueue* queue,
                          const OcclusionTracker* occlusion) {
  ...
  pile_->Resize(paint_properties().bounds);
  ...
  updated |= pile_->Update(client_,
                           SafeOpaqueBackgroundColor(),
                           contents_opaque(),
                           pile_invalidation_,
                           visible_layer_rect,
                           rendering_stats_instrumentation());
  ...
  return updated;
}

SchedulerStateMachine::Action SchedulerStateMachine::NextAction() const {
  ...
  if (ShouldDraw()) {
    if (readback_state_ == READBACK_STATE_WAITING_FOR_DRAW_AND_READBACK)
      return ACTION_DRAW_AND_READBACK;
    else if (PendingDrawsShouldBeAborted())
      return ACTION_DRAW_AND_SWAP_ABORT;
    else if (forced_redraw_state_ == FORCED_REDRAW_STATE_WAITING_FOR_DRAW)
      return ACTION_DRAW_AND_SWAP_FORCED;
    else
      return ACTION_DRAW_AND_SWAP_IF_POSSIBLE;
  }
  ...
}

void Scheduler::ProcessScheduledActions() {
  ...
      case SchedulerStateMachine::ACTION_DRAW_AND_SWAP_IF_POSSIBLE:
        DrawAndSwapIfPossible();
        break;
      case SchedulerStateMachine::ACTION_DRAW_AND_SWAP_FORCED:
        DrawAndSwapForced();
        break;
  ...
}

DrawSwapReadbackResult ThreadProxy::ScheduledActionDrawAndSwapIfPossible() {
  TRACE_EVENT0("cc", "ThreadProxy::ScheduledActionDrawAndSwap");
  bool forced_draw = false;
  bool swap_requested = true;
  bool readback_requested = false;
  return DrawSwapReadbackInternal(
      forced_draw, swap_requested, readback_requested);
}
// or
DrawSwapReadbackResult ThreadProxy::ScheduledActionDrawAndSwapForced() {
  TRACE_EVENT0("cc", "ThreadProxy::ScheduledActionDrawAndSwapForced");
  bool forced_draw = true;
  bool swap_requested = true;
  bool readback_requested = false;
  return DrawSwapReadbackInternal(
      forced_draw, swap_requested, readback_requested);
}

DrawSwapReadbackResult ThreadProxy::DrawSwapReadbackInternal(
    bool forced_draw,
    bool swap_requested,
    bool readback_requested) {
  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_)
    return result;

  DCHECK(layer_tree_host_impl_->renderer());
  if (!layer_tree_host_impl_->renderer())
    return result;

  base::TimeTicks start_time = base::TimeTicks::HighResNow();
  base::TimeDelta draw_duration_estimate = DrawDurationEstimate();
  base::AutoReset<bool> mark_inside(&inside_draw_, true);

  // Advance our animations.
  base::TimeTicks monotonic_time =
      layer_tree_host_impl_->CurrentFrameTimeTicks();
  base::Time wall_clock_time = layer_tree_host_impl_->CurrentFrameTime();

  // TODO(enne): This should probably happen post-animate.
  if (layer_tree_host_impl_->pending_tree())
    layer_tree_host_impl_->pending_tree()->UpdateDrawProperties();
  layer_tree_host_impl_->Animate(monotonic_time, wall_clock_time);

  // This method is called on a forced draw, regardless of whether we are able
  // to produce a frame, as the calling site on main thread is blocked until its
  // request completes, and we signal completion here. If CanDraw() is false, we
  // will indicate success=false to the caller, but we must still signal
  // completion to avoid deadlock.

  // We guard PrepareToDraw() with CanDraw() because it always returns a valid
  // frame, so can only be used when such a frame is possible. Since
  // DrawLayers() depends on the result of PrepareToDraw(), it is guarded on
  // CanDraw() as well.

  bool drawing_for_readback =
      readback_requested && !!readback_request_on_impl_thread_;
  bool can_do_readback = layer_tree_host_impl_->renderer()->CanReadPixels();

  LayerTreeHostImpl::FrameData frame;
  bool draw_frame = false;

  if (layer_tree_host_impl_->CanDraw() &&
      (!drawing_for_readback || can_do_readback)) {
    // If it is for a readback, make sure we draw the portion being read back.
    gfx::Rect readback_rect;
    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;
  }

  if (draw_frame) {
    layer_tree_host_impl_->DrawLayers(
        &frame,
        scheduler_on_impl_thread_->LastBeginImplFrameTime());
    result.did_draw = true;
  }
  layer_tree_host_impl_->DidDrawAllLayers(frame);

  bool start_ready_animations = draw_frame;
  layer_tree_host_impl_->UpdateAnimationState(start_ready_animations);

  // Check for a pending CompositeAndReadback.
  if (drawing_for_readback) {
    DCHECK(!swap_requested);
    result.did_readback = false;
    if (draw_frame && !layer_tree_host_impl_->IsContextLost()) {
      layer_tree_host_impl_->Readback(readback_request_on_impl_thread_->pixels,
                                      readback_request_on_impl_thread_->rect);
      result.did_readback = true;
    }
    readback_request_on_impl_thread_->success = result.did_readback;
    readback_request_on_impl_thread_->completion.Signal();
    readback_request_on_impl_thread_ = NULL;
  } else if (draw_frame) {
    DCHECK(swap_requested);
    result.did_swap = layer_tree_host_impl_->SwapBuffers(frame);

    // We don't know if we have incomplete tiles if we didn't actually swap.
    if (result.did_swap) {
      DCHECK(!frame.has_no_damage);
      SetSwapUsedIncompleteTileOnImplThread(frame.contains_incomplete_tile);
    }
  }

  // Tell the main thread that the the newly-commited frame was drawn.
  if (next_frame_is_newly_committed_frame_on_impl_thread_) {
    next_frame_is_newly_committed_frame_on_impl_thread_ = false;
    Proxy::MainThreadTaskRunner()->PostTask(
        FROM_HERE,
        base::Bind(&ThreadProxy::DidCommitAndDrawFrame, main_thread_weak_ptr_));
  }

  if (draw_frame) {
    CheckOutputSurfaceStatusOnImplThread();

    base::TimeDelta draw_duration = base::TimeTicks::HighResNow() - start_time;
    draw_duration_history_.InsertSample(draw_duration);
    base::TimeDelta draw_duration_overestimate;
    base::TimeDelta draw_duration_underestimate;
    if (draw_duration > draw_duration_estimate)
      draw_duration_underestimate = draw_duration - draw_duration_estimate;
    else
      draw_duration_overestimate = draw_duration_estimate - draw_duration;
    UMA_HISTOGRAM_CUSTOM_TIMES("Renderer.DrawDuration",
                               draw_duration,
                               base::TimeDelta::FromMilliseconds(1),
                               base::TimeDelta::FromMilliseconds(100),
                               50);
    UMA_HISTOGRAM_CUSTOM_TIMES("Renderer.DrawDurationUnderestimate",
                               draw_duration_underestimate,
                               base::TimeDelta::FromMilliseconds(1),
                               base::TimeDelta::FromMilliseconds(100),
                               50);
    UMA_HISTOGRAM_CUSTOM_TIMES("Renderer.DrawDurationOverestimate",
                               draw_duration_overestimate,
                               base::TimeDelta::FromMilliseconds(1),
                               base::TimeDelta::FromMilliseconds(100),
                               50);
  }

  return result;
}


.
.
.

void TiledLayerImpl::AppendQuads(QuadSink* quad_sink,
                                 AppendQuadsData* append_quads_data) {
  DCHECK(tiler_);
  DCHECK(!tiler_->has_empty_bounds());
  DCHECK(!visible_content_rect().IsEmpty());
  ...
}