看代码注释

import 'dart:async';
import 'dart:collection';
import 'dart:ui';
import 'package:flutter/material.dart';

void main(){
  runApp(MyApp());
  /**
   * WidgetsFlutterBinding就是一个胶水类，用于初始化其他with的minxin类
   * minxin就是一个普通类，用了解决多继承问题，配合with使用解决多继承
   */
  WidgetsFlutterBinding.ensureInitialized().addTimingsCallback(_onReportTimings);
  // window.onReportTimings = _onReportTimings;
  /**
   * 不能直接window.onReportTimings,否则会报错
   * package:flutter/src/scheduler/binding.dart': Failed assertion: line 232 pos 12:
      I/flutter (  569): 'window.onReportTimings == _executeTimingsCallbacks': is not true
   */
}

const maxframes = 100; // 100 帧足够了，对于 60 fps 来说
const _frameInterval = const Duration(microseconds: Duration.microsecondsPerSecond ~/ 60);

final lastFrames = ListQueue<FrameTiming>(maxframes);

void _onReportTimings(List<FrameTiming> timings) {
  //把 Queue 当作堆栈用
  for (FrameTiming timing in timings) {
     //时间从小到大，也就表示老贞在前面小索引位置，新贞在后面的大索引位置
      print(timing.timestampInMicroseconds(FramePhase.buildStart));
      // 时间大的(新贞)被安排在队列头部， 时间小的(老贞)被安排在队列尾部
      lastFrames.addFirst(timing);
  }

  // 只保留 maxframes
  while (lastFrames.length > maxframes) {
    //移除队列尾部的老贞
    lastFrames.removeLast();
  }

  print('fps:'+ fps.toString());
}

double get fps {
  var lastFramesSet = <FrameTiming>[];
  for (FrameTiming timing in lastFrames) {
    if (lastFramesSet.isEmpty) {
      lastFramesSet.add(timing);
    } else {
      var lastStart = lastFramesSet.last.timestampInMicroseconds(FramePhase.buildStart);
      // 相邻两帧如果开始结束相差时间过大，比如大于 frameInterval * 2,是不同绘制时间段产生的,放在一起肯定会拉低fps
      // 因为相当于有一段时间fps=0,根本没有产生绘制
      if (lastStart - timing.timestampInMicroseconds(FramePhase.rasterFinish) > ( _frameInterval.inMicroseconds * 2)) {
        // in different set
        print('break...');
        break;
      }
      print('add...');
      lastFramesSet.add(timing);
    }
  }
  var framesCount = lastFramesSet.length;
  var costCount = lastFramesSet.map((t) {
    // 耗时超过 frameInterval 会导致丢帧
    // 下面这个公式算出来的是实际上应该发生的理论总贞数，结合图理解
    return (t.totalSpan.inMicroseconds ~/ _frameInterval.inMicroseconds) + 1;
  }).fold(0, (a, b)=> a + b);
  return framesCount * 60 / costCount;
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  int frameInterval = const Duration(microseconds: Duration.microsecondsPerSecond ~/ 60).inMilliseconds;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter ++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

计算公司可以结合注释和下面的草图理解

image.png

重点

分母是理论上应该绘制的贞数,也叫成本数量，分子则是实际绘制了多少贞，
取比值则代表实际的绘制能力，再*60则表示fps

代码给你看，真的
因为上面的 duration 其实算的不对。
前面提到，flutter 是按 frameInterval 计划绘制的，所以 duration 算出来偏小，进而导致计算结果超过 60。
换个思路，遵循之前的按计划绘制的逻辑，如果一帧绘制时长超过 frameInterval ，那么它必然会占用下一个帧的绘制计划，这个被挤占的帧即为 丢帧 ，显然，一秒内总绘制的帧数加上总丢掉帧数应该约等于 60。
所以， FPS ≈ 60 * framesCount / (framesCount + droppedCount) = 60 * framesCount / costCount
修改之前的代码，套入公式即为：

参考资料

Dart之Mixin详解
Flutter完整开发实战详解(五、 深入探索)
Dart集合Queue
如何代码获取 Flutter APP 的 FPS
闲鱼Flutter电子书