Dart Concurrency

异步

Concurrency in Dart | Dart

Asynchronous programming: futures, async, await | Dart

Dart supports concurrent programming with async-await, isolates, and classes such as Future and Stream.

A promise to eventually provide an int value is typed as Future<int>. A promise to provide a series of int values has the type Stream<int>.

The synchronous File method readAsStringSync() reads a file synchronously, blocking until the file is either fully read or an error occurs.

The asynchronous equivalent, readAsString(), immediately returns an object of type Future<String>. At some point in the future, the Future<String> completes with either a string value or an error.

Flutter 教程 Async-1 事件循环 Event Loop 机制_哔哩哔哩_bilibili

JavaScript 只有一个线程；Dart 里面，每个线程都被封装在一个 Isolate 里面。

Future 就相当于 JavaScript 里面的 Promise。

Dart asynchronous programming: Isolates and event loops | by Kathy Walrath | Dart | Medium

Event Loop 可以防止主线程被阻塞，保持主线程的响应。Future 调用会把闭包放进 Event Queue 里面；Dart 还保留了更优先的 Microtask Queue，一般不要使用。

由于 Dart 的单线程模型，如果你真的有很大的计算量要运行，就算你派发到 Event Queue 里面，最终还是要占用主线程来计算，可能导致阻塞，此时你就需要创建 Isolate，用多线程来解决问题。

示例

demo1.dart

import 'dart:async';

void main() {
  print("main 1");

  // 这些会被放到 eventQueue
  Future(() => print("event 1"));
  Future.delayed(Duration.zero, () => print("event 2"));
  Future.delayed(const Duration(seconds: 1), () => print("event 3"));

  // then是在future完成时立刻执行，不会再添加microtask
  Future.delayed(const Duration(seconds: 1)).then((value) {
    scheduleMicrotask(() => print("then microtask"));
    print("then 1");
  }).then((value) => print("then 2"));

  print("main 2");

  // 这些会被放到 microtaskQueue
  scheduleMicrotask(() => print("microtask 1"));
  Future.microtask(() => print("microtask 2"));
  Future.value(1).then((value) => print("microtask 3")); // 已经完成的future使用then，会被添加到microtask

  print("main 3");

  // 这些会立即执行
  Future.sync(() => print("sync 1"));
  Future.value(getName());

  print("main 4");
}

String getName() {
  print("sync 2");
  return "mikko";
}

The async and await keywords provide a declarative way to define asynchronous functions and use their results.

demo2.dart

import 'dart:async';

void main() {
  future1();
  future2();
}

void future1() {
  // future 有三种状态：未完成、已完成、出错
  getOne()
      .then((value) {
        print(value);
        return ++value;
      })
      .then((value) => print(value))
      .catchError((err) => print(err))
      .whenComplete(() => print("complete"));
}

Future<int> getOne() {
  // throw Exception("error");
  return Future.value(1);
  // return Future.error(Exception("error"));
}

void future2() async {
  try {
    int number = await getTwo();
    print(number);
    print(++number);
  } catch (err) {
    print(err);
  }
}

Future<int> getTwo() async {
  throw Exception("error");
  return 2;
}