Flutter:setState() 在封闭函数返回之前不会执行

Question

所以我在 flutter 中尝试这段代码:

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  int _i = 1;
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: MaterialButton(
            child: Text('You Pressed Me $_i'),
            onPressed: () {
              setState(() {
                _i++;
                print('inside i = $_i');
              });
              sleep(Duration(seconds: 10));
              _i++;
              print('outside i = $_i');
            }
          ),
        ),
      ),
    );
  }
}

预期行为(运行并按下按钮一次后):按钮显示文字“You Pressed Me 2”，

然后变量 _i 会增加到 3 而不会影响视觉效果。

实际行为: setState() 被执行，然后 _i 再次递增，并且没有发生视觉变化，即屏幕上的文本没有t 更新，当 onPressed() 返回时，setState() 会导致小部件重建并更新屏幕，这是 10 秒后出现在屏幕上的内容: “你压了我 3”.

这句话来自 docs关于setState():

The provided callback is immediately called synchronously. It must not return a future (the callback cannot be async), since then it would be unclear when the state was actually being set.

我知道调用是同步的(因此它是阻塞的，根据 this 的回答)所以它应该首先返回(并且这已经发生了)然后更新屏幕(或安排在将来的某个时间)和然后将控制权返回到下一行(后两件事不会发生)。

我什至在没有 sleep 的情况下尝试过，但结果相同。

那么我错过了什么或误解了什么？

Answer 1

事件循环

• 有一种叫做事件循环的东西

• 事件循环按顺序处理事件

您有两个事件

事件 A => 用户点击 =>

    0.onPressed: () {
        1.setState(() {
            3. i++
            4. Mark as widget dirty
            5. Add to the global dirty widgets list
        });
        6.i++
    });

事件 B => Vsync 信号 => 操作系统提供

    7. check dirty widgets list
    8. repaint

更多

• 这是来自 Flutter in Focus 系列的 YouTube 视频

  • Isolates and Event Loops

    • 来自 flutter 团队成员 Andrew Brogdon (@RedBrogdon)

• 或者在这里阅读

  • Dart asynchronous programming: Isolates and event loops

引用:

1. setState method - State class - widgets library - Dart API

2. markNeedsBuild method - Element class - widgets library - Dart API

3. scheduleBuildFor method - BuildOwner class - widgets library - Dart API

4. drawFrame method - WidgetsBinding class - widgets library - Dart API

5. handleDrawFrame method - SchedulerBinding class - scheduler library - Dart API

6. buildScope method - BuildOwner class - widgets library - Dart API

7. dart engine loop - Google Search

8. Dart Programming - Loops - Tutorialspoint

9. optimization - What is the optimal render loop in Dart 2? - Stack Overflow

10. Understanding Flutter Render Engine - Stack Overflow

11. Technical overview - Flutter

12. Flutter - Dart API docs

13. flutter/spinning_square.dart at master · flutter/flutter

14 。 dart engine - Google Search

1. scheduler library - Dart API

2. flutter/binding.dart at master · flutter/flutter

3. scheduler library - Dart API

4. frame scheduling flutter - Google Search

5. scheduleFrame method - SchedulerBinding class - scheduler library - Dart API

6. scheduler library - Dart API

7. packages/flutter/lib/scheduler.dart - external/github.com/flutter/flutter - Git at Google

8. flutter/spinning_square.dart at master · flutter/flutter

9. dart engine - Google Search

10. threading | Dart Package

11. isolate flutter - Google Search