如何在 React 中使用钩子绑定函数？

Question

Basically we bind event handler functions in constructor or make them as arrow functions in React class components like below

class Test extends Component{
  constructor(props){
    super(props);
    this.state = { count:0 };
    this.setCount = this.setCount.bind(this);
  }

  setCount() {
    this.setState({count: this.state.count + 1});
  }

  render() {
    return <button onClick={this.setCount}>Increase</button>
  }
}

But after hooks are introduced in React v16.7.0 the class components became functional components with state.

So how can I bind the function with hooks in functional component?

Answer 1

There's no need to bind functions/callbacks in functional components since there's no this in functions. In classes, it was important to bind this because we want to ensure that the this in the callbacks referred to the component's instance itself. However, doing .bind in the constructor has another useful property of creating the functions once during the entire lifecycle of the component and a new callback wasn't created in every call of render() . To do only initialize the callback once using React hooks, you would use useCallback .

Classes

class Foo extends Component {
  constructor(props) {
    super(props);
    this.handleClick = this.handleClick.bind(this);
  }

  handleClick() {
    console.log('Click happened');
  }

  render() {
    return <Button onClick={this.handleClick}>Click Me</Button>;
  }
}

Hooks

function Foo() {
  const memoizedHandleClick = useCallback(
    () => {
      console.log('Click happened');
    },
    [], // Tells React to memoize regardless of arguments.
  );
  return <Button onClick={memoizedHandleClick}>Click Me</Button>;
}

Answer 2

People come to SO and copy-paste code. Leaving this answer here, so the React community doesn't go around incorrectly memoizing everything and potentially doing more work than necessary.

Function components

function Foo() {
  const handleClick = function(){
    // use function statements to avoid creating new instances on every render
    // when you use `bind` or arrow functions
    console.log('memoizing can lead to more work!')
  };
  return <Button onClick={handleClick}>Click Me</Button>;
}

Tip: look at what code is transpiled when using useCallback and see if it's necessary, before dropping it in. if you're not sure you need it, you probably don't. and to be sure it's doing you good, profile it.

Answer 3

You might as well write the component Foo above like this and save yourself some typing. Note the syntax around handleClick ... it defines the closure handleClick as a field on Foo, rather than as a method. This removes the need for you to use bind to overwrite the OBject's 'handleClick' reference in the constructor. (Also, you don't need to define a constructor if you're just calling 'super'!)

class Foo extends Component {
  handleClick = () => {
    console.log('Click happened');
  }

  render() {
    return <Button onClick={this.handleClick}>Click Me</Button>;
  }
}

Similarly, for your original example, just declare state and setCount directly and to simplify your code:

class Test extends Component{
  state = {count: 0}

  setCount = () => {
    this.setState({count: this.state.count + 1});
  }

  render() {
    return <button onClick={this.setCount}>Increase</button>
  }
}