# 19.Future\&Microtask

### 多个 Future

下面多个 `Future` 的执行顺序会是什么样的呢？

```
void main() {
  Future( () {
    sleep(const Duration(seconds: 1));
    return "${DateTime.now()}: 1";
  })
  .then((value){
    print(value);
  });

  Future( () {
    sleep(const Duration(seconds: 1));
    return "${DateTime.now()}: 2";
  })
      .then((value){
    print(value);
  });

  Future( () {
    sleep(const Duration(seconds: 1));
    return "${DateTime.now()}: 3";
  })
      .then((value){
    print(value);
  });

  Future( () {
    sleep(const Duration(seconds: 1));
    return "${DateTime.now()}: 4";
  })
      .then((value){
    print(value);
  });
}
```

分析： 多个 `Future` 是按照代码顺序，依次添加到任务队列的，会依次被执行。所以会按顺序输出。

```
flutter: 2021-11-27 15:34:53.829132: 1
flutter: 2021-11-27 15:34:54.855321: 2
flutter: 2021-11-27 15:34:55.863015: 3
flutter: 2021-11-27 15:34:56.870869: 4
```

### 依赖连续调用

如果有几个异步任务是有一定的顺序或者结果依赖的，那么可以通过 `then` 的链式调用进行：

```
int number = 1;

void main() {
  Future(() {
    sleep(const Duration(seconds: 1));
    number += 1;
    print("${DateTime.now()}: $number");
    return number;
  }).then((value) {
    sleep(const Duration(seconds: 1));
    value += 1;
    print("${DateTime.now()}: $value");
    return value;
  }).then((value) {
    sleep(const Duration(seconds: 1));
    value += 1;
    print("${DateTime.now()}: $value");
    return value;
  }).then((value) {
    sleep(const Duration(seconds: 1));
    value += 1;
    print("${DateTime.now()}: $value");
    return value;
  });
}
```

输出的结果：

```
flutter: 2021-11-27 15:43:29.512735: 2
flutter: 2021-11-27 15:43:30.537787: 3
flutter: 2021-11-27 15:43:31.544347: 4
flutter: 2021-11-27 15:43:32.549391: 5
```

### 多个无依赖的任务结束后处理

开发中，我们还会遇到一些多个异步任务都执行完再去做一些事情的场景。 `Future` 也有提供这种歌场景的功能：

```
void main() {
  Future.wait([
    Future(() {
      sleep(Duration(seconds: 1));
      print('${DateTime.now()}: 1');
      return "任务1";
    }),
    Future(() {
      sleep(Duration(seconds: 1));
      print('${DateTime.now()}: 2');
      return "任务2";
    }),
    Future(() {
      sleep(Duration(seconds: 1));
      print('${DateTime.now()}: 3');
    }),
    Future(() {
      sleep(Duration(seconds: 1));
      print('${DateTime.now()}: 4');
    }),
    Future(() {
      sleep(Duration(seconds: 1));
      print('${DateTime.now()}: 5');
      return "任务5";
    }),
  ]).then((value) {
    print(value);
  });
}
```

输出：

```
flutter: 2021-11-27 15:50:47.972739: 1
flutter: 2021-11-27 15:50:49.000895: 2
flutter: 2021-11-27 15:50:50.008548: 3
flutter: 2021-11-27 15:50:51.015465: 4
flutter: 2021-11-27 15:50:52.023024: 5
flutter: [任务1, 任务2, null, null, 任务5]
```

这里和使用单个 `Future` 一样，也可以通过 `then` 来处理结果。通过输出内容我们可以发现，这里的 `then` 里的 `value` 其实是一个数组，并且如果数组内部的顺序是和 `Future` 数组的顺序保持一致的。

### scheduleMicrotask

[scheduleMicrotask](https://api.flutter.dev/flutter/dart-async/scheduleMicrotask.html) 也是异步编程常用的一个 `API`，它的优先级比 `Future` 会高，同一队列中，会优先将 `微任务` 处理完成再去处理 `Future`。

#### 题目

这里通过一个题目考验一下：

```
void main() {
  Future((){
    print('1');
  }).then((value){
    print('2');
    scheduleMicrotask((){
      print('3');
    });
  }).then((value){
    print('4');
  });

  scheduleMicrotask((){
    print('5');
  });

  Future((){
    print('6');
  });

  scheduleMicrotask((){
    print('7');
  });

  print('8');
}
```

思考一下这里的输出会是什么样的顺序呢？不要运行，欢迎在评论中告诉我哦。


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