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Browser implements high-performance CSS3 animation on mobile terminal
Browser implements high-performance CSS3 animation on mobile terminal
在现有的前端动画体系中，通常有两种模式：JS动画与CSS3动画。 JS动画是通过JS动态改写样式实现动画能力的一种方案，在PC端兼容低端浏览器中不失为一种推荐方案。 而在移动端，我们选择性能更优浏览器原生实现方案：CSS3动画
高性能移动Web相较PC的场景需要考虑的因素也相对更多更复杂，我们总结为以下几点： 流量、功耗与流畅度。 在PC时代我们更多的是考虑体验上的流畅度，而在Mobile端本身丰富的场景下，需要额外关注对用户基站网络流量使用的情况，设备耗电量的情况。
关于流畅度，主要体现在前端动画中，在现有的前端动画体系中，通常有两种模式：JS动画与CSS3动画。 JS动画是通过JS动态改写样式实现动画能力的一种方案，在PC端兼容低端浏览器中不失为一种推荐方案。 而在移动端，我们选择性能更优浏览器原生实现方案：CSS3动画。
然而，CSS3动画在移动多终端设备场景下，相比PC会面对更多的性能问题，主要体现在动画的卡顿与闪烁。
目前对提升移动端CSS3动画体验的主要方法有几点：
尽可能多的利用硬件能力，如使用3D变形来开启GPU加速
代码如下:
-webkit-trans form: translate3d(0, 0, 0); -moz-transform: translate3d(0, 0, 0); -ms-transform: translate3d(0, 0, 0); transform: translate3d(0, 0, 0);
如动画过程有闪烁（通常发生在动画开始的时候），可以尝试下面的Hack：
代码如下:
-webkit-backface-visibility: hidden; -moz-backface-visibility: hidden; -ms-backface-visibility: hidden; backface-visibility: hidden; -webkit-perspective: 1000; -moz-perspective: 1000; -ms-perspective: 1000; perspective: 1000;
如下面一个元素通过translate3d右移500px的动画流畅度会明显优于使用left属性：
代码如下:
#b
all-1 {
transition: -webkit-transform .5s ease;
-webkit-transform: translate3d(0, 0, 0);
}
#ball-1.slidein {
-webkit-transform: translate3d(500px, 0, 0);
}
#ball-2 {
transition: left .5s ease;
left：0;
}
#ball-2.slidein {
left：500px;
}注：3D变形会消耗更多的内存与功耗，应确实有性能问题时才去使用它，兼在权衡
尽可能少的使用box-shadows与gradients
box-shadows与gradients往往都是页面的性能杀手，尤其是在一个元素同时都使用了它们，所以拥抱扁平化设计吧。
尽可能的让动画元素不在文档流中，以减少重排
代码如下:
position: fixed; position: absolute;
优化 DOM layout 性能
我们从实例开始描述这个主题：
代码如下:
var newWidth = ap.offset Width + 10; ap.style.width = newWidth + 'px'; var newHeight = ap.offsetHeight + 10; ap.style.height = newHeight + 'px'; var newWidth = ap.offsetWidth + 10; var newHeight = ap.offsetHeight + 10; ap.style.width = newWidth + 'px'; ap.style.height = newHeight + 'px';
这是两段能力上完全等同的代码，显式的差异正如我们所见，只有执行顺序的区别。但真是如此吗？下面是加了说明注释的代码版本，很好的阐述了其中的进一步差异：
代码如下:
// 触发两次 layout var newWidth = ap.offsetWidth + 10; // Read ap.style.width = newWidth + 'px'; // Write var newHeight = ap.offsetHeight + 10; // Read ap.style.height = newHeight + 'px'; // Write // 只触发一次 layout var newWidth = ap.offsetWidth + 10; // Read var newHeight = ap.offsetHeight + 10; // Read ap.style.width = newWidth + 'px'; // Write ap.style.height = newHeight + 'px'; // Write
从注释中可找到规律，连续的读取offsetWidth/Height属性与连续的设置width/height属性，相比分别读取设置单个属性可少触发一次layout。
从结论看似乎与执行队列有关，没错，这是浏览器的优化策略。所有可触发layout的操作都会被暂时放入 layout-queue 中，等到必须更新的时候，再计算整个队列中所有操作影响的结果，如此就可只进行一次的layout，从而提升性能。
关键一，可触发layout的操作，哪些操作下会layout的更新（也称为reflow或者relayout）？
我们从浏览器的源码实现入手，以开源Webkit/Blink为例， 对layout的更新，Webkit 主要通过 Document::updateLayout 与Document::updateLayoutIgnorePendingStylesheets 两个方法：
代码如下:
void Document::updateLayout()
{
ASSERT(is
Main
Thread());
FrameView* frameView = view();
if
(frameView && frameView->isInLayout()) {
ASSERT_NOT_R
EACH
ED();
return
;
}
if (Element* oe = ownerElement())
oe->document()->updateLayout();
updateStyleIfNeeded();
StackStats::LayoutCheckPoint layoutCheckPoint;
if (frameView && renderer() && (frameView->layoutPending() || renderer()->needsLayout()))
frameView->layout();
if (m_focusedNode && !m_didPostCheckFocusedNodeTask) {
postTask(CheckFocusedNodeTask::create());
m_didPostCheckFocusedNodeTask = true;
}
}
void Document::updateLayoutIgnorePendingStylesheets()
{
bool oldIgnore = m_ignorePendingStylesheets;
if (!haveStylesheetsLoaded()) {
m_ignorePendingStylesheets = true;
HTMLElement* bodyElement = body();
if (bodyElement && !bodyElement->renderer() && m_pendingSheetLayout == NoLayoutWithPendingSheets) {
m_pendingSheetLayout = Di
dL
ayoutWithPendingSheets;
styleResolverChanged(RecalcStyleImmediately);
}
else
if (m_hasNodesWithPlaceholderStyle)
recalcStyle(Force);
}
updateLayout();
m_ignorePendingStylesheets = oldIgnore;
}从 updateLayoutIgnorePendingStylesheets 方法的内部实现可知，其也是对 updateLayout 方法的扩展，并且在现有的 layout 更新模式中，大部分场景都是调用 updateLayoutIgnorePendingStylesheets 来进行layout的更新。
搜索 Webkit 实现中调用 updateLayoutIgnorePendingStylesheets 方法的代码, 得到以下可导致触发 layout 的操作：
Element: clientHeight, clientLeft, clientTop, clientWidth, focus(), getBoundingClientRect(), getClientRects(), innerText, offsetHeight, offsetLeft, offsetParent, offsetTop, offsetWidth, outerText, scrollByLines(), scrollByPages(), scrollHeight, scrollIntoView(), scrollIntoViewIfNeeded(), scrollLeft, scrollTop, scrollWidth
Frame, HTMLImageElement: height, width
Range: getBoundingClientRect(), getClientRects()
SVGLocatable: computeCTM(), getBBox()
SVGTextContent: getCharNumAtPosition(), getComputedTextLength(), getEndPositionOfChar(), getExtentOfChar(), getNumberOfChars(), getRotationOfChar(), getStartPositionOfChar(), getSubStringLength(), selectSubString()
SVGUse: instanceRoot
window: getComputedStyle(), scrollBy(), scrollTo(), scrollX, scrollY, webkitConvertPointFromNodeToPage(), webkitConvertPointFromPageToNode()
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