React Component Design - Provider

Provider主要是為了解決Props drilling的問題，Props drilling指的是props要傳好幾層往下到想要存取這個props的 component，例如

class Toggle extends React.Component {
  state = { on: false };
  toggle = () => {
    /*...*/
  };
  render() {
    return this.props.children({ ...this.state, toggle: this.toggle });
  }
}
 
const Layer1 = ({ toggle, ...props }) => <Layer2 toggle={toggle} />;
const Layer2 = ({ toggle, ...props }) => <Layer3 toggle={toggle} />;
const Layer3 = ({ toggle, ...props }) => <button onClick={toggle} />;
 
class App extends React.Component {
  handleToggle = () => {};
 
  render() {
    return (
      <Toggle onToggle={this.handleToggle}>
        <Layer1 />
      </Toggle>
    );
  }
}

而React的Provider/Context API 能解決這個問題，讓props中間不用傳遞這麼多層，只要使用Consumer就可以存取Provider裡面的value，來看看實際例子

import React, { Fragment } from 'react';
import { Switch } from '../switch';
 
const ToggleContext = React.createContext();
 
class Toggle extends React.Component {
  static Consumer = ToggleContext.Consumer;
 
  state = { on: false };
 
  toggle = () =>
    this.setState(
      ({ on }) => ({ on: !on }),
      () => this.props.onToggle(this.state.on)
    );
 
  render() {
    return (
      <ToggleContext.Provider
        value={{ on: this.state.on, toggle: this.toggle }}
        {...this.props}
      />
    );
  }
}
 
const Layer1 = () => <Layer2 />;
const Layer2 = () => (
  <Toggle.Consumer>
    {({ on }) => (
      <Fragment>
        {on ? 'The button is on' : 'The button is off'}
        <Layer3 />
      </Fragment>
    )}
  </Toggle.Consumer>
);
const Layer3 = () => <Layer4 />;
const Layer4 = () => (
  <Toggle.Consumer>
    {({ on, toggle }) => <Switch on={on} onClick={toggle} />}
  </Toggle.Consumer>
);
 
function Usage({ onToggle = (...args) => console.log('onToggle', ...args) }) {
  return (
    <Toggle onToggle={onToggle}>
      <Layer1 />
    </Toggle>
  );
}

驗證 Consumer

上面的例子是最簡易版的Provider，但這邊的Consumer要在Provider底下使用才會正常，如果我們希望Consumer不是在Provider底下使用時就噴error，可以改寫成下面的方式

import React, { Fragment } from 'react';
import { Switch } from '../switch';
 
const ToggleContext = React.createContext();
 
function ToggleConsumer(props) {
  return (
    <ToggleContext.Consumer {...props}>
      {(context) => {
        if (!context) {
          throw new Error(
            `Toggle.Consumer cannot be rendered outside the Toggle component`
          );
        }
        return props.children(context);
      }}
    </ToggleContext.Consumer>
  );
}
 
class Toggle extends React.Component {
  static Consumer = ToggleConsumer;
 
  // same as before
}

避免不必要的 re-render

在Provider的value中除了會因state 改變而 re-render 以外，toggle也會在每次 re-render 時產生一個新的實例，如果我們要避免不必要的 re-render，可以用下面的技巧

class Toggle extends React.Component {
  static Consumer = ToggleConsumer;
 
  toggle = () =>
    this.setState(
      ({ on }) => ({ on: !on }),
      () => this.props.onToggle(this.state.on)
    );
 
  state = { on: false, toggle: this.toggle };
 
  render() {
    return <ToggleContext.Provider value={this.state} {...this.props} />;
  }
}

支援 Render Props

在某些情況我們可能會希望Provider支援Render Props的作法，讓上層決定要做客製化的 ui 顯示

class Toggle extends React.Component {
  // same as before
 
  render() {
    const { children, ...rest } = this.props
    const ui =
      typeof children === 'function' ? children(this.state) : children
 
    return (
      <ToggleContext.Provider value={this.state} {...rest}>
        {ui}
      </ToggleContext.Provider>
    )
  }
}
 
function Usage({
  onToggle = (...args) => console.log('onToggle', ...args),
}) {
  return (
    <Toggle onToggle={onToggle}>
		{({ on }) => (
			on ? <Layer1 /> : <OtherLayer>
		)}
    </Toggle>
  )
}

支援 Compound Component

如果我們希望Toggle保留一定的介面供外部使用，並且用Compound Component的形式將一些特定的 component export 出去給外部使用，可以參考下面例子

class Toggle extends React.Component {
  static Consumer = ToggleConsumer;
 
  static On = ({ children }) => (
    <ToggleConsumer>{({ on }) => (on ? children : null)}</ToggleConsumer>
  );
 
  static Off = ({ children }) => (
    <ToggleConsumer>{({ on }) => (on ? null : children)}</ToggleConsumer>
  );
 
  static Button = (props) => (
    <ToggleConsumer>
      {({ on, toggle }) => <Switch on={on} onClick={toggle} {...props} />}
    </ToggleConsumer>
  );
 
  // same as before
}
 
const Layer1 = () => <Layer2 />;
const Layer2 = () => (
  <Fragment>
    <Toggle.On>The button is on</Toggle.On>
    <Toggle.Off>The button is off</Toggle.Off>
    <Layer3 />
  </Fragment>
);
const Layer3 = () => <Layer4 />;
const Layer4 = () => <Toggle.Button />;
 
function Usage({ onToggle = (...args) => console.log('onToggle', ...args) }) {
  return (
    <Toggle onToggle={onToggle}>
      <Layer1 />
    </Toggle>
  );
}

驗證 Consumer

避免不必要的 re-render

支援 Render Props

支援 Compound Component

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