问题引出
对于Angular和React操作DOM的速度，和原生js操作DOM的速度进行了一个比较：
代码如下：
<!DOCTYPE html>DOM Render
<html ng-app="test">
<head>
<title>Performance Comparison for Knockout, Angular and React</title>
<link href="//cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.1/css/bootstrap.css" rel="stylesheet" />
<style type="text/css">
* { box-sizing:border-box; }
body { padding:30px 0; }
h2 { margin:0; margin-bottom:25px; }
h3 { margin:0; padding:0; margin-bottom:12px; }
.test-data { margin-bottom:3px; }
.test-data span { padding:3px 10px; background:#EEE; width:100%; float:left; cursor:pointer; }
.test-data span:hover { background:#DDD; }
.test-data span.selected { background:#3F7AD9; color:white; }
.time { font-weight:bold; height:26px; line-height:26px; vertical-align:middle; display:inline-block; cursor:pointer; text-decoration:underline; }
</style>
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/angular.js/1.3.3/angular.min.js"></script>
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/react/0.12.1/react.js"></script>
<script type="text/javascript">
console.timeEnd("build");
document.addEventListener("DOMContentLoaded", function() {
_knockout();
_react();
_raw();
});
_angular();
function _buildData(count) {
count = count || 1000;
var adjectives = ["pretty", "large", "big", "small", "tall", "short", "long", "handsome", "plain", "quaint", "clean", "elegant", "easy", "angry", "crazy", "helpful", "mushy", "odd", "unsightly", "adorable", "important", "inexpensive", "cheap", "expensive", "fancy"];
var colours = ["red", "yellow", "blue", "green", "pink", "brown", "purple", "brown", "white", "black", "orange"];
var nouns = ["table", "chair", "house", "bbq", "desk", "car", "pony", "cookie", "sandwich", "burger", "pizza", "mouse", "keyboard"];
var data = [];
for (var i = 0; i < count; i++)
data.push({id: i+1, label: adjectives[_random(adjectives.length)] + " " + colours[_random(colours.length)] + " " + nouns[_random(nouns.length)] });
return data;
}
function _random(max) {
return Math.round(Math.random()*1000)%max;
}
function _knockout() {
ko.applyBindings({
selected: ko.observable(),
data: ko.observableArray(),
select: function(item) {
this.selected(item.id);
},
run: function() {
var data = _buildData(),
date = new Date();
this.selected(null);
this.data(data);
document.getElementById("run-knockout").innerHTML = (new Date() - date) + " ms";
}
}, document.getElementById("knockout"));
}
function _angular(data) {
angular.module("test", []).controller("controller", function($scope) {
$scope.run = function() {
var data = _buildData(),
date = new Date();
$scope.selected = null;
$scope.$$postDigest(function() {
document.getElementById("run-angular").innerHTML = (new Date() - date) + " ms";
});
$scope.data = data;
};
$scope.select = function(item) {
$scope.selected = item.id;
};
});
}
function _react() {
var Class = React.createClass({
select: function(data) {
this.props.selected = data.id;
this.forceUpdate();
},
render: function() {
var items = [];
for (var i = 0; i < this.props.data.length; i++) {
items.push(React.createElement("div", { className: "row" },
React.createElement("div", { className: "col-md-12 test-data" },
React.createElement("span", { className: this.props.selected === this.props.data[i].id ? "selected" : "", onClick: this.select.bind(null, this.props.data[i]) }, this.props.data[i].label)
)
));
}
return React.createElement("div", null, items);
}
});
var runReact = document.getElementById("run-react");
runReact.addEventListener("click", function() {
var data = _buildData(),
date = new Date();
React.render(new Class({ data: data, selected: null }), document.getElementById("react"));
runReact.innerHTML = (new Date() - date) + " ms";
});
}
function _raw() {
var container = document.getElementById("raw"),
template = document.getElementById("raw-template").innerHTML;
document.getElementById("run-raw").addEventListener("click", function() {
var data = _buildData(),
date = new Date(),
html = "";
for (var i = 0; i < data.length; i++) {
var render = template;
render = render.replace("{{className}}", "");
render = render.replace("{{label}}", data[i].label);
html += render;
}
container.innerHTML = html;
var spans = container.querySelectorAll(".test-data span");
for (var i = 0; i < spans.length; i++)
spans[i].addEventListener("click", function() {
var selected = container.querySelector(".selected");
if (selected)
selected.className = "";
this.className = "selected";
});
document.getElementById("run-raw").innerHTML = (new Date() - date) + " ms";
});
}
ko.observableArray.fn.reset = function(values) {
var array = this();
this.valueWillMutate();
ko.utils.arrayPushAll(array, values);
this.valueHasMutated();
};
</script>
</head>
<body ng-controller="controller">
<div class="container">
<div class="row">
<div class="col-md-12">
<h2>Performance Comparison for React, Angular and Knockout</h2>
</div>
</div>
<div class="col-md-3">
<div class="row">
<div class="col-md-7">
<h3>React</h3>
</div>
<div class="col-md-5 text-right time" id="run-react">Run</div>
</div>
<div id="react"></div>
</div>
<div class="col-md-3">
<div class="row">
<div class="col-md-7">
<h3>Angular</h3>
</div>
<div class="col-md-5 text-right time" id="run-angular" ng-click="run()">Run</div>
</div>
<div>
<div class="row" ng-repeat="item in data">
<div class="col-md-12 test-data">
<span ng-class="{ selected: item.id === $parent.selected }" ng-click="select(item)">{{item.label}}</span>
</div>
</div>
</div>
</div>
<div id="knockout" class="col-md-3">
<div class="row">
<div class="col-md-7">
<h3>Knockout</h3>
</div>
<div class="col-md-5 text-right time" id="run-knockout" data-bind="click: run">Run</div>
</div>
<div data-bind="foreach: data">
<div class="row">
<div class="col-md-12 test-data">
<span data-bind="click: $root.select.bind($root, $data), text: $data.label, css: { selected: $data.id === $root.selected() }"></span>
</div>
</div>
</div>
</div>
<div class="col-md-3">
<div class="row">
<div class="col-md-7">
<h3>Raw</h3>
</div>
<div class="col-md-5 text-right time" id="run-raw">Run</div>
</div>
<div id="raw"></div>
</div>
</div>
<script type="text/html" id="raw-template">
<div class="row">
<div class="col-md-12 test-data">
<span class="{{className}}">{{label}}</span>
</div>
</div>
</script>
</body>
</html>
运行结果：
原生最快，为什么还要用框架？
问题分析
框架封装了很多操作，为了提升可维护性，会进行分层，只提供出API。很多应用其实类似，为了提升可读性可复用性，往往会分层、封装，单一的简单应用上性能上可能并不会有明显提升。但是，对于复杂的需要很多模块的应用，框架的优势就很明显。在不用自己优化性能的同时，可维护性也较好。框架会尽最大程度保证性能。复杂的情况，比如多次更新一块DOM，或者多块DOM同时更新等等。
React的VirtualDOM
如果没有VirtualDOM的情况下，想重置DOM，需要使用innerHTML.一个大型列表的数据如果都变更了，那么重置所有DOM无可厚非，但是更多的情况是，只有几个数据变更了，这种情况下，重新渲染大片DOM，就有些浪费了。
可以比较下虚拟DOM和innerHTML的差异：
innerHTML: render html string O(template size) + 重新创建所有 DOM 元素 O(DOM size) Virtual DOM: render Virtual DOM + diff O(template size) + 必要的 DOM 更新 O(DOM change)可以看出，虚拟DOM在渲染环节和比较环节都会比InnerHTML慢些。但是，与操作DOM比起来，js运算是快很多的了。所以这一定程度上，速度会快些。所以，不管每次有多少数据变更，性能都可以接受，React也并没有强调说一定比原生快这样。 MVVM与Virtual DOM比较MVVM采用的都是数据绑定：通过Directive/Binding对象来观察数据变化，有数据变化时改变对应的DOM显示。Angular的Dirty Checking:scope digest O(watcher out)+必要的DOM更新 O(DOM change)Angular比较受诟病的地方，就在于任何小的变动，都有和watcher数量相关的性能代价。MVVM在渲染列表时，每一行数据都有自己的作用域，通常是每行都有一个对应的的ViewModel实例。或者有的是利用原型继承的scope对象，都是有代价的。一般ViewModel初始化都比Virtual DOM慢。VirtualDOM的检测是DOM侧的，而ViewModel如果有数据更新，会销毁所有实例，重新创建实例，再进行一次渲染。所以较React会慢些。对于React，就算是全新的数据，只要渲染之后的DOM没变，不需要更新。Angular提供了列表重绘的优化机制，就是让框架有效的复用实例和DOM元素。例如数据库里面的同一个数据，前端两次拉取会取到不同的值，但是他们有一样的uid.可以添加track by uid让ng知道，这俩对象是同一份数据。原来实例的对象和DOM就可以复用，只会更新变动的部分。上面的例子里面如果加上track by $index的话，会快一些。可以看看加了track by $index的运行结果:
没错，ng明显快了很多。应用过程中，总是应该选择合适的框架，如果可能，对其做一些优化。一般情况：初始化渲染：Virtual DOM>Dirty Checking >=依赖收集小范围数据更新：依赖收集>>Virtual DOM+优化>Dirty Checking大量数据更新：Dirty Checking+优化>=依赖收集+优化>Virtual DOM 参考：DOM操作速度