Dart是一门单线程的语言,我们在开发进行耗时操作（比如网络请求、数据库存储、文件读取）时会阻塞我们的程序，Dart的并发机制可以在等待一个操作完成的同时进行别的操作，在Dart中执行异步操作，可以用Future类和async、await关键字；这里主要介绍Future以及async和await的使用，具体原理参考下一篇文章；
Future的使用
- then
通过then获取Future的执行结果
void test1() { print("start"); thenCase().then((value) { print("then - value = $value"); }); print("end"); } Future<String> thenCase() { return Future(() { return "测试then"; }); }
运行结果
flutter: start flutter: end flutter: then - value = 测试then
- onError & catchError
catchError既能接收then里面的错误又能接收onError里面的错误
class FltError implements Exception { int errCode = 0; String? msg; } void test2() { print("start"); catchErrorCase().then((value) { print("then - value = $value"); }).onError((error, stackTrace) { if (error is FltError) { FltError err = error as FltError; print("FltError - obj - ${err.msg}"); }else { print("other - error - $error"); } throw error!; }).catchError((error) { print("catchError - ${error.runtimeType}"); }); print("end"); } Future<String> catchErrorCase() { return Future(() { FltError error = FltError(); error.msg = "测试返回error"; error.errCode = 200; throw error; }); }
运行结果
flutter: start flutter: end flutter: FltError - obj - 测试返回error flutter: catchError - FltError
- whenComplete
无论正常还是异常，Future均会执行该回调
void test3() { print("start"); catchErrorCase().then((value) { print("then - value = $value"); }).onError((error, stackTrace) { if (error is FltError) { FltError err = error as FltError; print("FltError - obj - ${err.msg}"); }else { print("other - error - $error"); } throw error!; }).catchError((error) { print("catchError - ${error.runtimeType}"); }).whenComplete(() { print("whenComplete"); }); print("end"); }
运行结果
flutter: start flutter: end flutter: FltError - obj - 测试返回error flutter: catchError - FltError flutter: whenComplete
- delay
异步延时执行一个任务
void test4() { print("start - ${DateTime.now()}"); delayed().then((value) { print("value = $value"); }); print("end - ${DateTime.now()}"); } Future<String> delayed() { return Future<String>.delayed(Duration(seconds: 3), () { return "异步延时执行任务 + ${DateTime.now()}"; }); }
运行结果
flutter: start - 2023-03-20 15:29:09.997512 flutter: end - 2023-03-20 15:29:10.006552 flutter: value = 异步延时执行任务 + 2023-03-20 15:29:13.007373
- value
直接创建一个带值的Future，如下代码：
void valueTest() { Future.value("1").then((value) { print("v1 - $value"); }); Future(() { return "2"; }).then((value) { print("v2 - $value"); }); }
运行结果
flutter: v1 - 1 flutter: v2 - 2 • wait
等待数组中的每一个都执行完成，再执行then
void test5() { Future v1 = delayed(1); Future v2 = delayed(2); Future.wait([v1, v2]).then((value) { print("then - \nvalue0 = ${value[0]} \nvalue1 = ${value[1]}\ntime = ${DateTime.now()}"); }); } Future delayed(int value) { return Future.delayed(Duration(seconds: value), () { return "异步延时执行任务 - value = $value - time = ${DateTime.now()}"; }); }
运行结果（注意换行符）
flutter: then - value0 = 异步延时执行任务 - value = 1 - time = 2023-03-20 15:44:25.578118 value1 = 异步延时执行任务 - value = 2 - time = 2023-03-20 15:44:26.579086 time = 2023-03-20 15:44:26.579965
- any
有任意一个Future完成就结束
void test6() { Future v1 = delayed(1); Future v2 = delayed(2); Future.any([v1, v2]).then((value) { print("then - value = $value - time = ${DateTime.now()}"); }).whenComplete(() { print("whenComplete"); }); } Future delayed(int value) { return Future.delayed(Duration(seconds: value), () { return "异步延时执行任务 - value = $value - time = ${DateTime.now()}"; }); }
运行结果
flutter: then - value = 异步延时执行任务 - value = 1 - time = 2023-03-20 15:47:39.451636 - time = 2023-03-20 15:47:39.453878 flutter: whenComplete
- timeout
注意参数传入的是3，duration是2
void test7() { print("${DateTime.now()} - start"); delayed(3).then((value) { print("${DateTime.now()} - then - $value"); }).timeout(Duration(seconds: 2)).then((value) { print("${DateTime.now()} - timeout = $value"); }).catchError((error) { print("${DateTime.now()} - timeout - error - ${error.runtimeType}"); }).whenComplete(() { print("${DateTime.now()} - whenComplete"); }); print("${DateTime.now()} - end"); } Future delayed(int value) { return Future.delayed(Duration(seconds: value), () { return "异步延时执行任务"; }); }
运行结果
flutter: 2023-03-20 15:56:14.055962 - start flutter: 2023-03-20 15:56:14.065781 - end flutter: 2023-03-20 15:56:16.071233 - timeout - error - TimeoutException flutter: 2023-03-20 15:56:16.071578 - whenComplete flutter: 2023-03-20 15:56:17.064688 - then - 异步延时执行任务
将参数传入的是1，duration是2，运行结果
flutter: 2023-03-20 15:58:02.800739 - start
flutter: 2023-03-2015:58:02.811208 - end
flutter: 2023-03-2015:58:03.811116 - then - 异步延时执行任务
flutter: 2023-03-2015:58:03.811733 - timeout = null
flutter: 2023-03-2015:58:03.811959 - whenComplete
async、await
异步处理：在返回的 Future 对象上注册一个then，等 Future 的执行体结束以后，再进行异步处理同步等待；
同步等待：Future 执行体结束，此时需要在调用处使用await，在调用处的函数体使用 async；
void test8() { print("start"); asyncTest(); print("end"); } void asyncTest() async { var result = await Future.value(1); print("result = $result"); }
运行结果
flutter: start flutter: end flutter: result = 1
注意：await关键字必须在async函数内部使用
