React提供了和以往不一样的方式来看待视图,它以组件开发为基础。组件是React的核心概念,React 允许将代码封装成组件(component),然后像插入普通 HTML 标签一样,在网页中插入这个组件。React.createClass 方法就用于生成一个组件类。对React应用而言,你需要分割你的页面,使其成为一个个的组件。也就是说,你的应用是由这些组件组合而成的。你可以通过分割组件的方式去开发复杂的页面或某个功能区块,并且组件是可以被复用的。这个过程大概类似于用乐高积木去瓶装不同的物体。我们称这种编程方式称为**组件驱动开发**。
React是可以同时渲染HTML标签与组件的,但是要注意的是,一般Tags元素都是小写开头,而Component都是以大写字母开头,以下面为例:
var myDivElement = <div className="foo" />;
React.render(myDivElement, document.getElementById('example'));
而如果需要渲染一个Component:
var MyComponent = React.createClass({/*...*/});
var myElement = <MyComponent someProperty={true} />;
React.render(myElement, document.getElementById('example'));
组件的生命周期分成三个状态:
Mounting:已插入真实 DOM,即Initial Render
Updating:正在被重新渲染,即Props与State改变
Unmounting:已移出真实 DOM,即Component Unmount
React 为每个状态都提供了两种处理函数,will 函数在进入状态之前调用,did 函数在进入状态之后调用,三种状态共计五种处理函数。
componentWillMount()
componentDidMount()
componentWillUpdate(object nextProps, object nextState)
componentDidUpdate(object prevProps, object prevState)
componentWillUnmount()
此外,React 还提供两种特殊状态的处理函数。
这里可以看出,Props比State的改变会有多出一个shouldComponentUpdate的回调方法。
总结而言,一个完整的React Component的写法应该如下:
/** * @jsx React.DOM */
var React = require('react'),
MyReactComponent = React.createClass({
// The object returned by this method sets the initial value of this.state
getInitialState: function(){
return {};
},
// The object returned by this method sets the initial value of this.props
// If a complex object is returned, it is shared among all component instances
getDefaultProps: function(){
return {};
},
// Returns the jsx markup for a component
// Inspects this.state and this.props create the markup
// Should never update this.state or this.props
render: function(){
return (<div></div>); }, // An array of objects each of which can augment the lifecycle methods mixins: [], // Functions that can be invoked on the component without creating instances statics: { aStaticFunction: function(){} }, // -- Lifecycle Methods -- // Invoked once before first render componentWillMount: function(){ // Calling setState here does not cause a re-render }, // Invoked once after the first render componentDidMount: function(){ // You now have access to this.getDOMNode() }, // Invoked whenever there is a prop change // Called BEFORE render componentWillReceiveProps: function(nextProps){ // Not called for the initial render // Previous props can be accessed by this.props // Calling setState here does not trigger an an additional re-render }, // Determines if the render method should run in the subsequent step // Called BEFORE a render // Not called for the initial render shouldComponentUpdate: function(nextProps, nextState){ // If you want the render method to execute in the next step // return true, else return false return true; }, // Called IMMEDIATELY BEFORE a render componentWillUpdate: function(nextProps, nextState){ // You cannot use this.setState() in this method }, // Called IMMEDIATELY AFTER a render componentDidUpdate: function(prevProps, prevState){ }, // Called IMMEDIATELY before a component is unmounted componentWillUnmount: function(){ } }); module.exports = MyReactComponent;
设置默认的Props.
this.props 对象的属性与组件的属性一一对应,但是有一个例外,就是 this.props.children 属性。它表示组件的所有子节点。
var NotesList = React.createClass({
render: function() {
return (
<ol> { this.props.children.map(function (child) { return <li>{child}</li> }) } </ol> ); } }); React.render( <NotesList> <span>hello</span> <span>world</span> </NotesList>, document.body );
其效果图如下所示:
[enter description here][5]
React.createClass({
propTypes: {
// You can declare that a prop is a specific JS primitive. By default, these
// are all optional.
optionalArray: React.PropTypes.array,
optionalBool: React.PropTypes.bool,
optionalFunc: React.PropTypes.func,
optionalNumber: React.PropTypes.number,
optionalObject: React.PropTypes.object,
optionalString: React.PropTypes.string,
// Anything that can be rendered: numbers, strings, elements or an array
// containing these types.
optionalNode: React.PropTypes.node,
// A React element.
optionalElement: React.PropTypes.element,
// You can also declare that a prop is an instance of a class. This uses
// JS's instanceof operator.
optionalMessage: React.PropTypes.instanceOf(Message),
// You can ensure that your prop is limited to specific values by treating
// it as an enum.
optionalEnum: React.PropTypes.oneOf(['News', 'Photos']),
// An object that could be one of many types
optionalUnion: React.PropTypes.oneOfType([
React.PropTypes.string,
React.PropTypes.number,
React.PropTypes.instanceOf(Message)
]),
// An array of a certain type
optionalArrayOf: React.PropTypes.arrayOf(React.PropTypes.number),
// An object with property values of a certain type
optionalObjectOf: React.PropTypes.objectOf(React.PropTypes.number),
// An object taking on a particular shape
optionalObjectWithShape: React.PropTypes.shape({
color: React.PropTypes.string,
fontSize: React.PropTypes.number
}),
// You can chain any of the above with `isRequired` to make sure a warning
// is shown if the prop isn't provided.
requiredFunc: React.PropTypes.func.isRequired,
// A value of any data type
requiredAny: React.PropTypes.any.isRequired,
// You can also specify a custom validator. It should return an Error
// object if the validation fails. Don't `console.warn` or throw, as this
// won't work inside `oneOfType`.
customProp: function(props, propName, componentName) {
if (!/matchme/.test(props[propName])) {
return new Error('Validation failed!');
}
}
},
/* ... */
});
React不提倡数据的双向绑定,而在用户行为下面产生的数据更新,React建议还是通过事件机制来处理。譬如下述例子中,输入框文本内容的改变,还是通过onChange事件,然后出发状态机的变化。
var LikeButton = React.createClass({
getInitialState: function() {
return {liked: false};
},
handleClick: function(event) {
this.setState({liked: !this.state.liked});
},
render: function() {
var text = this.state.liked ? 'like' : 'haven\'t liked';
return (
<p onClick={this.handleClick}> You {text} this. Click to toggle. </p> ); } }); React.render( <LikeButton />, document.getElementById('example') );
参考资料
[Props VS State][6]
组件的主要职责是将原始数据转化为HTML中的富文本格式,而Props与State协作完成这件事,换言之,Props与State的并集即是全部的原始数据。Props与State之间也是有很多交集的,譬如:
不过Props顾名思义,更多的是作为Component的配置项存在。Props往往是由父元素指定并且传递给自己的子元素,不过自身往往不会去改变Props的值。另一方面,State在组件被挂载时才会被赋予一个默认值,而常常在与用户的交互中发生更改。往往一个组件独立地维护它的整个状态机,可以认为State是一个私有属性。他们的对比如下:
描述 | Props | State |
---|---|---|
是否可以从父元素获取初始值 | Yes | Yes |
是否可以被父元素改变 | Yes | No |
是否可以设置默认值 | Yes | Yes |
是否可以在组件内改变 | No | Yes |
是否可以设置为子元素的初始值 | Yes | Yes |
是否可以在子元素中改变 | Yes | No |
上文中提及过,一种利用组件内包裹的方式动态定义组件的方式,可以利用Props的children属性来获取所有包裹住的Dom对象。
React主打的是组件驱动型编程,往往可以将一个大的组件拆分为几个小的组件,这里以头像控件为例:
var Avatar = React.createClass({
render: function() {
return (
<div> <ProfilePic username={this.props.username} /> <ProfileLink username={this.props.username} /> </div> ); } }); var ProfilePic = React.createClass({ render: function() { return ( <img src={'https://graph.facebook.com/' + this.props.username + '/picture'} /> ); } }); var ProfileLink = React.createClass({ render: function() { return ( <a href={'https://www.facebook.com/' + this.props.username}> {this.props.username} </a> ); } }); React.render( <Avatar username="pwh" />, document.getElementById('example') );
有时候在某个组件内调用另一个组件,并不会进行渲染,譬如:
class Home extends React.Component {
render() {
return (
<div> <map/> </div> ); } } var map = React.createClass({ render: function() { return ( <div id="map-canvas"> <span>hello</span> </div> ); } });
这里的map并不会被识别,应该把map变为Map,可以参考[这里](https://facebook.github.io/react/docs/jsx-in-depth.html#html-tags-vs.-react-components)。
虽然组件的原则就是模块化,彼此之间相互独立,但是有时候不同的组件之间可能会共用一些功能,共享一部分代码。所以 React 提供了 `mixins` 这种方式来处理这种问题。Mixin 就是用来定义一些方法,使用这个 mixin 的组件能够自由的使用这些方法(就像在组件中定义的一样),所以 mixin 相当于组件的一个扩展,在 mixin 中也能定义“生命周期”方法。
比如一个定时器的 mixin:
var SetIntervalMixin = {
componentWillMount: function() {
this.intervals = [];
},
setInterval: function() {
this.intervals.push(setInterval.apply(null, arguments));
},
componentWillUnmount: function() {
this.intervals.map(clearInterval);
}
};
var TickTock = React.createClass({
mixins: [SetIntervalMixin], // Use the mixin
getInitialState: function() {
return {seconds: 0};
},
componentDidMount: function() {
this.setInterval(this.tick, 1000); // Call a method on the mixin
},
tick: function() {
this.setState({seconds: this.state.seconds + 1});
},
render: function() {
return (
<p> React has been running for {this.state.seconds} seconds. </p> ); } }); React.render( <TickTock />, document.getElementById('example') );
React 的 `mixins` 的强大之处在于,如果一个组件使用了多个 mixins,其中几个 `mixins` 定义了相同的“生命周期方法”,这些方法会在组件相应的方法执行完之后按 mixins 指定的数组顺序执行。