深入JSX
本质上来讲，JSX是为React.createElement方法提供的语法糖
<MyButton color="blue" shadowSize={}>
Click Me
</MyButton>
编译为
React.createElement(
MyButton,
{color: 'blue', shadowSize: },
'Click Me'
)
点表示法用于JSX类型
const MyComponents = {
DatePicker: function DatePicker(props) {
return <div>Imagine a {props.color} datepicker here.</div>;
}
}
function BlueDatePicker() {
return <MyComponents.DatePicker color="blue" />;
}
JSX的属性
使用JavaScript表达式作为属性
<MyComponent foo={ + + + } />
字符串常量
<MyComponent message="hello world" />
<MyComponent message={'hello world'} />
属性默认为True
<MyTextBox autocomplete />
<MyTextBox autocomplete={true} />
展开属性，建议不要使用可能传递不必要的属性给组件
function App1() {
return <Greeting firstName="Ben" lastName="Hector" />;
}
function App2() {
const props = {firstName: 'Ben', lastName: 'Hector'};
return <Greeting {...props} />;
}
JSX中的子代
字符串子面量
<MyComponent>Hello world!</MyComponent>
JavaScript表达式
function Item(props) {
return <li>{props.message}</li>;
}
function TodoList() {
const todos = ['finish doc', 'submit pr', 'nag dan to review'];
return (
<ul>
{todos.map((message) => <Item key={message} message={message} />)}
</ul>
);
}
布尔，Undefined，Null会被忽略
<div>
{showHeader && <Header />}
<Content />
</div>
Refs & DOM
创建Refs
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.myRef = React.createRef();
}
render() {
return <div ref={this.myRef} />;
}
}
访问Refs
const node = this.myRef.current;
为DOM元素添加Ref
class CustomTextInput extends React.Component {
constructor(props) {
super(props);
// 创建 ref 存储 textInput DOM 元素
this.textInput = React.createRef();
this.focusTextInput = this.focusTextInput.bind(this);
}
focusTextInput() {
// 直接使用原生 API 使 text 输入框获得焦点
// 注意：通过 "current" 取得 DOM 节点
this.textInput.current.focus();
}
render() {
// 告诉 React 我们想把 <input> ref 关联到构造器里创建的 `textInput` 上
return (
<div>
<input
type="text"
ref={this.textInput} />
<input
type="button"
value="Focus the text input"
onClick={this.focusTextInput}
/>
</div>
);
}
}
ref的更新会发生在componentDidMount和componentDidUpdate
为类添加Ref
class AutoFocusTextInput extends React.Component {
constructor(props) {
super(props);
this.textInput = React.createRef();
}
componentDidMount() {
this.textInput.current.focusTextInput();
}
render() {
return (
<CustomTextInput ref={this.textInput} />
);
}
}
回调Refs
class CustomTextInput extends React.Component {
constructor(props) {
super(props);
this.textInput = null;
this.setTextInputRef = element => {
this.textInput = element;
};
this.focusTextInput = () => {
// 直接使用原生 API 使 text 输入框获得焦点
if (this.textInput) this.textInput.focus();
};
}
componentDidMount() {
// 渲染后文本框自动获得焦点
this.focusTextInput();
}
render() {
// 使用 `ref` 的回调将 text 输入框的 DOM 节点存储到 React
// 实例上（比如 this.textInput）
return (
<div>
<input
type="text"
ref={this.setTextInputRef}
/>
<input
type="button"
value="Focus the text input"
onClick={this.focusTextInput}
/>
</div>
);
}
}
非受控组件
使用ref从DOM 获取表单值
class NameForm extends React.Component {
constructor(props) {
super(props);
this.handleSubmit = this.handleSubmit.bind(this);
}
handleSubmit(event) {
alert('A name was submitted: ' + this.input.value);
event.preventDefault();
}
render() {
return (
<form onSubmit={this.handleSubmit}>
<label>
Name:
<input type="text" ref={(input) => this.input = input} />
</label>
<input type="submit" value="Submit" />
</form>
);
}
}
默认值
render() {
return (
<form onSubmit={this.handleSubmit}>
<label>
Name:
<input
defaultValue="Bob"
type="text"
ref={(input) => this.input = input} />
</label>
<input type="submit" value="Submit" />
</form>
);
}
文件输入标签
class FileInput extends React.Component {
constructor(props) {
super(props);
this.handleSubmit = this.handleSubmit.bind(this);
}
handleSubmit(event) {
event.preventDefault();
alert(
`Selected file - ${this.fileInput.files[].name}`
);
}
render() {
return (
<form onSubmit={this.handleSubmit}>
<label>
Upload file:
<input
type="file"
ref={input => {
this.fileInput = input;
}}
/>
</label>
<br />
<button type="submit">Submit</button>
</form>
);
}
}
ReactDOM.render(
<FileInput />,
document.getElementById('root')
);