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);
}