Detailed explanation of life cycle usage in React
This time I will bring you a detailed explanation of the use of life cycle in React. What are the precautions for using the life cycle in React? Here are practical cases, let’s take a look.
Life cycle
React is a process of rendering virtual DOM into real DOM. This process is called the life cycle of the component. React divides this cycle into three stages, and each stage provides two processing methods: will and did. Will refers to before it happens, and did refers to after it happens.
Mounting: Component rendering process
componentWillMount()
componentDidMount()
Updating: Component update process
componentWillReceiveProps(nextProps)
shouldComponentUpdate(nextProps, nextState)
- ##componentWillUpdate(object nextProps, object nextState)
- componentDidUpdate(object prevProps, object prevState)
- Unmounting: component removal process
- componentWillUnmount()
- This stage There is no corresponding did method
##Execution order
- getDefaultProps
- ##getInitialState
##componentWillMount
render
- ##componentDidMount
- componentWillMountThis method is called before render, also That is to say, the real DOM elements cannot be obtained in this method. This method is triggered before rendering for the first time and before rendering again when state changes.
componentDidMount
This method is called after render, that is, this method can directly obtain the real DOM element.
This method is triggered after the first rendering and after rendering again when the state changes.
var MountingComponent = React.createClass({
componentWillMount: function(){
console.log(this.refs.h1) // undefined
},
componentDidMount: function(){
console.log(this.refs.h1) // h1 对象
},
render: function(){
return <h1 ref="h1">Lifecycle-Mounting</h1>;
}
})
ReactDOM.render(<MountingComponent />, document.getElementById('p1'));Updating
It will be triggered when the props or state of the component are changed
- shouldComponentUpdate
- componentWillUpdate
- render
- componentDidUpdate
- componentWillReceivePropsCalled when the component receives a new prop. This method will not be called when initializing render. The method accepts one parameter
Note: props cannot be changed manually. The normal scenario is that the current component is called as a child component, and then the props of the component are changed in the parent component
shouldComponentUpdate
After the component is mounted, shouldComponentUpdate will be called every time setState is called to determine whether the component needs to be re-rendered. Returns true by default and needs to be re-rendered. In more complex applications, some data changes do not affect the interface display. You can make judgments here to optimize rendering efficiency.
The method accepts two parameters
newState: updated state
The method must return boolen. If true is returned, subsequent componentWillUpdate, render, and componentDidUpdate will be executed. Otherwise it will not be executed.
componentWillUpdate
is called when the component receives new props or state but has not yet rendered. Will not be called during initialization.
The method accepts two parameters
nextState: state to be updated
componentDidUpdate
Called immediately after the component completes the update. Will not be called during initialization.
The method accepts two parameters
nextState: state before update
var UpdatingComponent = React.createClass({
getInitialState: function() {
return {
data:0
};
},
setNewNumber: function() {
//当 state 发生改变的时候，state 对应的组件会重新挂载
//会触发 componentWillUpdate、componentDidUpdate
this.setState({data: this.state.data + 1})
},
//参数 newProps：已更新的 props
componentWillReceiveProps:function(newProps) {
console.log('Component WILL RECEIVE PROPS!', newProps)
},
//参数 newProps：已更新的 props
//参数 newState：已更新的 state
//必须要返回 boolen，true 则执行componentWillUpdate、render、componentDidUpdate。反之则不执行。
shouldComponentUpdate: function(newProps, newState){
console.log('shouldComponentUpdate',newProps, newState);
return (newState.data > 0 && newState.data % 2 == 0);
},
//参数 nextProps：将要更新的 props
//参数 nextState：将要更新的 state
componentWillUpdate: function(nextProps, nextState){
console.log(nextProps, nextState, this.refs.p1)
},
//参数 prevProps：更新前的 props
//参数 nextState：更新前的 state
componentDidUpdate: function(prevProps, prevState){
console.log(prevProps, prevState)
},
render: function(){
return (
<p>
<button onClick={this.setNewNumber}>INCREMENT</button>
<h3>{this.state.data}</h3>
</p>
);
}
})
ReactDOM.render(<UpdatingComponent/>, document.getElementById('p2'));Unmounting
在组件从 DOM 中移除的时候立刻被调用，这个阶段没有对应的 did 方法
componentWillUnmount
方法适用在父子组件的结构中，当某个条件符合的场景下，该子组件会被渲染
重新渲染的执行顺序
getInitialState
componentWillMount
render
componentDidMount
var ChildrenComponent = React.createClass({
componentWillUnmount: function(){
console.log('componentWillUnmount');
},
render: function(){
return <h3>{this.props.myNumber}</h3>
}
})
var UnmountingComponent = React.createClass({
getInitialState: function() {
return {
data:0
};
},
setNewNumber: function() {
this.setState({data: this.state.data + 1})
},
render: function () {
var content;
//当条件不符合时 ChildrenComponent 会被移除，然后会触发方组件的 componentWillUnmount 方法
//当条件重新符合时，会重新渲染组件 ChildrenComponent
if(this.state.data % 2 == 0){
content = <ChildrenComponent myNumber = {this.state.data}></ChildrenComponent>;
} else {
content = <h3>{this.state.data}</h3>;
}
return (
<p>
<button onClick = {this.setNewNumber}>INCREMENT</button>
{content}
</p>
);
}
})
ReactDOM.render(<UnmountingComponent/>, document.getElementById('p3'));相信看了本文案例你已经掌握了方法，更多精彩请关注php中文网其它相关文章！
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