框架操作DOM和原生js操作DOM比较

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>

<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>

DOM Render

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