反应组件安装正时

componentDidMount 独立于同级组件的生命周期方法?从下面的示例来看,似乎只有在装载了所有同级组件之后才调用它。

假设我们有一个顶层组件,它呈现两个子组件,首先是简单的 render() 另一个相对较慢 渲染() .

https://codesandbox.io/s/j43klml9py?expanddevtools=1

TL;DR:

class SlowComponent extends Component {
  componentDidMount() {
    // perf mark
  }

  render() {
    // Simulate slow render
    // Takes 50ms
    return <h3>Slow component</h3>;
  }
}

class FastComponent extends Component {
  componentDidMount() {
    // perf mark
  }

  render() {
    return <h3>Fast component</h3>;
  }
}

class App extends Component {
  constructor(props) {
    super(props);

    // perf mark start
  }

  componentDidMount() {
    // perf mark
    // measure all marks and print
  }

  render() {
    return (
      <div>
        <FastComponent />
        <SlowComponent />
      </div>
    );
  }
}

ReactDOM.render(<App />, document.getElementById('root'));

组件安装 时间安排如下:

1. FastComponent 10毫秒
2. SlowComponent 50毫秒
3. App

组件安装

1. 快速组件
2. 慢组件 51毫秒
3. 52毫秒

当前的sample和repro代码使用mount回调,但同样适用于 componentDidUpdate .

完整来源:

import ReactDOM from "react-dom";
import React, { Component } from "react";

class SlowComponent extends Component {
  componentDidMount() {
    performance.mark("slow-mounted");
  }

  render() {
    // Simulate slow render
    for (var i = 0; i < 10000; i++) {
      for (var j = 0; j < 100; j++) {
        const b = JSON.parse(
          JSON.stringify({
            test: "test" + i,
            test1: i * i * i
          })
        );
      }
    }
    return <h3>Slow component</h3>;
  }
}

class FastComponent extends Component {
  componentDidMount() {
    performance.mark("fast-mounted");
  }

  render() {
    return <h3>Fast component</h3>;
  }
}

class App extends Component {
  constructor(props) {
    super(props);

    performance.mark("init");
  }

  componentDidMount() {
    performance.mark("app-mounted");

    performance.measure("slow", "init", "slow-mounted");
    performance.measure("fast", "init", "fast-mounted");
    performance.measure("app", "init", "app-mounted");

    console.clear();

    console.log(
      "slow",
      Math.round(performance.getEntriesByName("slow")[0].duration)
    );
    console.log(
      "fast",
      Math.round(performance.getEntriesByName("fast")[0].duration)
    );
    console.log(
      "app",
      Math.round(performance.getEntriesByName("app")[0].duration)
    );

    performance.clearMarks();
    performance.clearMeasures();
  }

  render() {
    return (
      <div>
        <h1>Demo</h1>
        <FastComponent />
        <SlowComponent />
      </div>
    );
  }
}

ReactDOM.render(<App />, document.getElementById("root"));