作用是操作dom
场景：图片加载完以后获取图片的宽高
// window上添加事件监听后，组件销毁前需要移除
class Test extends React.Component {
constructor(props){
super(props);
this.state = {
top: 0
}
this.handleWindowScroll = this.handleWindowScroll.bind(this);
}
handleWindowScroll() {
this.setState({
top: document.body.scrollTop
})
}
componentDidMount() {
window.addEventListener('scroll', this.handleWindowScroll);
}
componentWillUnmount() {
window.removeEventListener('scroll', this.handleWindowScroll);
}
render() {
return <div>{this.state.top}</div>
}
}
.