requestAnimationFrame在Chrome里的实现

requestAnimationFrame是HTML5游戏和动画必不可少的函数，相对于setTimeout或setInterval它有两个优势，一是它注册的回调函数与浏览器的渲染同步，不用担心Timer的时间间隔太长或太短。二是时间间隔相对与Timer要稳定，requestAnimationFrame注册的回调函数最高执行频率是60FPS，虽然在HTML5游戏里通常是达不到的，但是它两次调用之间时间间隔要比Timer稳定一些。前段时间我在CanTK Runtime里自己模拟过requestAnimationFrame，为了深入的理解Chrome里实现requestAnimationFrame方法，花了一点时间去读Blink的代码。

requestAnimationFrame的基本用法如下：

var start = null;
var element = document.getElementById("SomeElementYouWantToAnimate");

function step(timestamp) {
if (!start) start = timestamp;
var progress = timestamp - start;
element.style.left = Math.min(progress/10, 200) + "px";
if (progress < 2000) {
window.requestAnimationFrame(step);
}
}

window.requestAnimationFrame(step);

我比较关注的是回调函数的注册和调用过程：

1.注册回调函数的实现。

(WebKit/Source/core/dom/ScriptedAnimationController.cpp)

ScriptedAnimationController::CallbackId ScriptedAnimationController::registerCallback(FrameRequestCallback* callback)
{
CallbackId id = m_callbackCollection.registerCallback(callback);
scheduleAnimationIfNeeded();
return id;
}

注册之后还需要请求重绘，scheduleAnimationIfNeeded最终会调到ThreadProxy::SendCommitRequestToImplThreadIfNeeded：

(cc/trees/thread_proxy.cc)

bool ThreadProxy::SendCommitRequestToImplThreadIfNeeded(
CommitPipelineStage required_stage) {
DCHECK(IsMainThread());
DCHECK_NE(NO_PIPELINE_STAGE, required_stage);
bool already_posted =
main().max_requested_pipeline_stage != NO_PIPELINE_STAGE;
main().max_requested_pipeline_stage =
std::max(main().max_requested_pipeline_stage, required_stage);
if (already_posted)
return false;
Proxy::ImplThreadTaskRunner()->PostTask(
FROM_HERE,
base::Bind(&ThreadProxy::SetNeedsCommitOnImplThread,
impl_thread_weak_ptr_));
return true;
}

void ThreadProxy::SetNeedsCommitOnImplThread() {
TRACE_EVENT0("cc", "ThreadProxy::SetNeedsCommitOnImplThread");
DCHECK(IsImplThread());
impl().scheduler->SetNeedsBeginMainFrame();
}

void Scheduler::SetNeedsBeginMainFrame() {
state_machine_.SetNeedsBeginMainFrame();
ProcessScheduledActions();
}

2.执行回调函数的实现。

WebKit/Source/core/dom/ScriptedAnimationController.cpp

void ScriptedAnimationController::executeCallbacks(double monotonicTimeNow)
{
// dispatchEvents() runs script which can cause the document to be destroyed.
if (!m_document)
return;

double highResNowMs = 1000.0 * m_document->loader()->timing().monotonicTimeToZeroBasedDocumentTime(monotonicTimeNow);
double legacyHighResNowMs = 1000.0 * m_document->loader()->timing().monotonicTimeToPseudoWallTime(monotonicTimeNow);
m_callbackCollection.executeCallbacks(highResNowMs, legacyHighResNowMs);
}
void FrameRequestCallbackCollection::executeCallbacks(double highResNowMs, double highResNowMsLegacy)
{
// First, generate a list of callbacks to consider.  Callbacks registered from this point
// on are considered only for the "next" frame, not this one.
ASSERT(m_callbacksToInvoke.isEmpty());
m_callbacksToInvoke.swap(m_callbacks);

for (size_t i = 0; i < m_callbacksToInvoke.size(); ++i) {
FrameRequestCallback* callback = m_callbacksToInvoke[i].get();
if (!callback->m_cancelled) {
TRACE_EVENT1("devtools.timeline", "FireAnimationFrame", "data", InspectorAnimationFrameEvent::data(m_context, callback->m_id));
InspectorInstrumentationCookie cookie = InspectorInstrumentation::willFireAnimationFrame(m_context, callback->m_id);
if (callback->m_useLegacyTimeBase)
callback->handleEvent(highResNowMsLegacy);
else
callback->handleEvent(highResNowMs);
InspectorInstrumentation::didFireAnimationFrame(cookie);
TRACE_EVENT_INSTANT1(TRACE_DISABLED_BY_DEFAULT("devtools.timeline"), "UpdateCounters", TRACE_EVENT_SCOPE_THREAD, "data", InspectorUpdateCountersEvent::data());
}
}

m_callbacksToInvoke.clear();
}

这行代码m_callbacksToInvoke.swap(m_callbacks);比较有意思，在循环执行的callbacks会有新的callback注册进来，我在实现这个功能时，没看过这段代码，当时费了点功夫才想明白怎么搞。

上面的代码是由RenderWidgetCompositor::BeginMainFrame调过来的：

(src/content/renderer/gpu/render_widget_compositor.cc)

void RenderWidgetCompositor::BeginMainFrame(const cc::BeginFrameArgs& args) {
double frame_time_sec = (args.frame_time - base::TimeTicks()).InSecondsF();
double deadline_sec = (args.deadline - base::TimeTicks()).InSecondsF();
double interval_sec = args.interval.InSecondsF();
WebBeginFrameArgs web_begin_frame_args =
WebBeginFrameArgs(frame_time_sec, deadline_sec, interval_sec);
compositor_deps_->GetRendererScheduler()->WillBeginFrame(args);
widget_->webwidget()->beginFrame(web_begin_frame_args);
}