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
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