小菜尝试过 Future 和 Stream 实现 Dart 异步处理,但仅限于基本的使用,网上有很多相关的资料,小菜仅从初识者的角度学习了解 Dart 的实现的异步处理;
Dart 是单线程的!这点很重要,是不可变更的;但单线程的 Dart 是如何实现多线程的,这是很值得研究的;
小菜了解的异步操作来处理耗时任务的方式主要是两种:一种是类似于 Java 的开启多线程,并在线程间通信;另一种是类似于 Dart 的单线程和事件循环 Event Loop;其中 Event Loop 事件循环就是把一系列的(点击事件/滑动事件/网络请求/ IO 事件等)事件存放在 Event Queue 队列中,循环执行从 Event Loop 中获取事件进行执行,直到清空队列事件;
Dart 可以通过 Future 和 Stream 实现异步操作,小菜先尽量全面的学习 Future;
Future
Future Methods
Future 的应用主要涉及三种状态,分别是未完成状态(UnCompleted / Pending),完成成功(Completed with Data),完成失败(Completed with Error);
当 Future 任务执行完成之后,通常需要一个回调,这个回调会立即执行,不会添加到事件队列中,其中通过 then() 的成功回调来来监听 Future 执行完成时获取到的结果;通过 catchError() 异常回调来监听 Future 执行失败或者出现异常时的错误信息;通过 whenComplete() 最终回调来获取最终回调,不管成功还是失败;
小菜做了一个不太严谨的对比,这三种状态类似于 try-catch-finally,其中 try 对应 then() 成功回调,catch 对应 catchError() 异常回调,而 finall 对应 whenComplete() 最终回调;
Future 还有两个常用的回调,分别是 timeout 超时回调和 asStream() 转换成 Stream 流回调;Future 的使用可以通过 Future Constructors 构造方法或 async-await 方式;
Future.delayed(Duration(seconds: 3), () {
print('Future.delayed(), CurrentTime = ${DateTime.now().millisecondsSinceEpoch}');
return 'Future.delayed() result! CurrentTime = ${DateTime.now().millisecondsSinceEpoch}';
}).timeout(Duration(seconds: 1), onTimeout: () => 'Future.delayed() TimeOut! CurrentTime = ${DateTime.now().millisecondsSinceEpoch}')
.then((val) => print(val))
.whenComplete(() => print('Future.delayed() whenComplete! CurrentTime = ${DateTime.now().millisecondsSinceEpoch}'));
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Future Constructors
1. Future(FutureOr<T> computation())
用于返回状态结果的基本构造方法;其中 computation 返回的可以是普通类型也可以是 Future;
factory Future(FutureOr<T> computation()) {
_Future<T> result = new _Future<T>();
Timer.run(() {
try {
result._complete(computation());
} catch (e, s) {
_completeWithErrorCallback(result, e, s);
}
});
return result;
}
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分析源码可知,Future 主要是通过 Timer.run() 来执行,在回调方法中执行 computation();
Future(() => print('Future(FutureOr<T> computation()) 01'));
Future(() {
print('Future(FutureOr<T> computation()) 02');
return 'Future(FutureOr<T> computation()) 02 result!';
}).then((val) => print(val));
Future(() {
print('Future(FutureOr<T> computation()) 03');
return 'Future(FutureOr<T> computation()) 03 result!';
}).then((val) => print(val)).whenComplete(() => print('Future(FutureOr<T> computation()) 03 whenComplete!'));
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2. Future.value([FutureOr<T> value])
创建一个固定类型的 Future;
Future.value('Future.value() 01').then((val) => print(val));
Future.value('Future.value() 02').then((val) => print(val));
Future.value('Future.value() 03')
.then((val) => print(val))
.whenComplete(() => print('Future.value() 03 whenComplete!'));
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3. Future.delayed(Duration duration, [FutureOr<T> computation()])
delayed 是延迟执行,任务会到设置的延迟时间之后再执行;
factory Future.delayed(Duration duration, [FutureOr<T> computation()]) {
_Future<T> result = new _Future<T>();
new Timer(duration, () {
if (computation == null) {
result._complete(null);
} else {
try {
result._complete(computation());
} catch (e, s) {
_completeWithErrorCallback(result, e, s);
}
}
});
return result;
}
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分析源码可知,与默认的构造方法相比,Timer 只是多了一个 duration,会在 duration 之后调用 computation();
print('CurrentTime01 = ${DateTime.now().millisecondsSinceEpoch}');
Future.delayed(Duration(seconds: 3), () => print('Future.delayed() 01, CurrentTime = ${DateTime.now().millisecondsSinceEpoch}'));
print('CurrentTime02 = ${DateTime.now().millisecondsSinceEpoch}');
Future.delayed(Duration(seconds: 1), () => print('Future.delayed() 02, CurrentTime = ${DateTime.now().millisecondsSinceEpoch}'));
print('CurrentTime03 = ${DateTime.now().millisecondsSinceEpoch}');
Future.delayed(Duration(seconds: 2), () {
print('Future.delayed() 03, CurrentTime = ${DateTime.now().millisecondsSinceEpoch}');
return 'Future.delayed() 03 result! CurrentTime = ${DateTime.now().millisecondsSinceEpoch}';
}).then((val) => print(val)).whenComplete(() => print('Future.delayed() 03 whenComplete! CurrentTime = ${DateTime.now().millisecondsSinceEpoch}'));
print('CurrentTime04 = ${DateTime.now().millisecondsSinceEpoch}');
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4. Future.sync(FutureOr<T> computation())
sync 是同步方法,任务会被立即执行;
factory Future.sync(FutureOr<T> computation()) {
try {
var result = computation();
if (result is Future<T>) {
return result;
} else {
return new _Future<T>.value(result);
}
} catch (error, stackTrace) {
var future = new _Future<T>();
AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
if (replacement != null) {
future._asyncCompleteError(
_nonNullError(replacement.error), replacement.stackTrace);
} else {
future._asyncCompleteError(error, stackTrace);
}
return future;
}
}
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分析源码可知,与默认的构造方法相比,sync 直接调用 computation(),并未通过 Timer() 的回调;
Future.sync(() => print('Future.sync() 01'));
Future.sync(() {
print('Future.sync() 02');
return 'Future.sync() 02 result!';
}).then((val) => print(val));
Future.sync(() {
print('Future.sync() 03');
return 'Future.sync() 03 result!';
}).then((val) => print(val)).whenComplete(() => print('Future.value() 03 whenComplete!'));
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5. Future.error(Object error, [StackTrace stackTrace])
error 主要创建一个异常状态的 Future;
Future.error(ArgumentError.notNull('Input'));
Future.error('Future error01')
.then((val) => print('Future.error01 result = $val'))
.catchError((val) => print(val));
Future.error('Future error02')
.then((val) => print('Future.error02 result = $val'))
.catchError((val) => print(val))
.whenComplete(() => print('Future.error03 whenComplete!'));
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6. Future.microtask(FutureOr<T> computation())
microtask 比较特殊,会创建一个微队列事件的 Future,Dart 的异步任务队列主要包括 Event Queue 普通事件队列和 Microtask Queue 微事件队列,其中 Microtask Queue 的执行优先级更高,Future 默认是 Event Queue 普通队列事件,小菜会在后面的博客中进一步研究;
Future.microtask(() => print('Future.microtask() 01'));
Future.microtask(() {
print('Future.microtask() 02');
return 'Future.microtask() 02 result!';
}).then((val) => print(val)).whenComplete(() => print('Future.microtask() 02 whenComplete!'));
Future.microtask(() => print('Future.microtask() 03'));
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Future Static Methods
1. any<T>(Iterable<Future<T>> futures)
静态方法 any() 是 Futures 数组中完成第一个 Future 的回调监听,包括成功回调 then() 和异常回调 catchError(),其中第一个 Future 完成之后,并不影响其他的 Future 执行;
Future.any([
Future.delayed(Duration(seconds: 4)).then((val) => 4),
Future.delayed(Duration(seconds: 2)).then((val) => '2'),
Future.delayed(Duration(seconds: 3)).then((val) => null),
]).then((val) => print(val));
Future.any([
Future.error(ArgumentError.notNull('Input')),
Future.error('Future error01').then((val) => print('Future.error01 result = $val')).catchError((val) => print(val)),
Future.error('Future error02').then((val) => print('Future.error02 result = $val')).catchError((val) => print(val))
.whenComplete(() => print('Future.error03 whenComplete!'))
]).then((val) => print('Future.any() --> then()'))
.catchError((val) => print('Future.any() --> catchError() --> $val'))
.whenComplete(() => print('Future.any() whenComplete!'));
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2. doWhile(FutureOr<bool> action())
静态方法 doWhile() 可以用来执行循环任务,只有当返回内容是 false 或异常时停止;
var i = 0;
Future.doWhile(() {
++i;
return Future.delayed(Duration(seconds: 2), () {
print('Future.doWhile() $i, CurrentTime = ${DateTime.now().millisecondsSinceEpoch}');
return i < 6;
}).then((val) {
print(val);
return val;
});
}.then((val) => print('Future.doWhile -> then() -> $val'))
.whenComplete(() => print('Future.doWhile -> whenComplete()'));
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3. forEach<T>(Iterable<T> elements, FutureOr action(T element))
静态方法 forEach() 用来遍历执行 elements 中各 item,不止于单一数据类型;
Future.forEach([
1,
Future(() => print('Future.forEach() 01')),
Future(() {
print('Future.forEach() 02');
return 'Future.forEach() 02 result!';
}).then((val) => print(val)),
'Hello Flutter!',
Future(() {
print('Future.forEach() 03');
return 'Future.forEach() 03 result!';
}).then((val) => print(val))
.whenComplete(() => print('Future.forEach() 03 whenComplete!'))
], (val) => print(val))
.then((val) => print('Future.forEach() -> then() -> $val'))
.whenComplete(() => print('Future.forEach() -> whenComplete()'));
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4. wait<T>(Iterable<Future<T>> futures, {bool eagerError: false, void cleanUp(T successValue)})
静态方法 wait() 用来等待多个 Futures 完成之后再执行,并收集结果,其中 Futures 时按延迟或其他顺序执行,只有所有的 Futures 结束之后会按 Futures 数组收集;
Future.wait([
Future.delayed(Duration(seconds: 4)).then((val) {
print('Future -> 4');
return 4;
}).whenComplete(() => print('Future -> 4 whenComplete!')),
Future.delayed(Duration(seconds: 2)).then((val) {
print('Future -> 2.toString()');
return '2';
}).whenComplete(() => print('Future -> 2.toString() whenComplete!')),
Future.delayed(Duration(seconds: 3)).then((val) {
print('Future -> null');
return null;
}).whenComplete(() => print('Future -> null whenComplete!')),
]).then((val) => print(val))
.catchError((val) => print('Future.wait() --> catchError() --> $val'))
.whenComplete(() => print('Future.wait() whenComplete!'));
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a. Future.wait() 只会捕获 Futures 中第一个异常,且当前 Future 未进行捕获,即:Future 中都进行了 catchError 则不会进入 wait().catchError();
Future.wait([
Future.delayed(Duration(seconds: 4)).then((val) {
print('Future -> 4');
return 4;
}).whenComplete(() => print('Future -> 4 whenComplete!')),
Future.error('Future error01')
.then((val) => print('Future.error01 result = $val'))
.catchError((val) => print(val)),
Future.error(ArgumentError.notNull('Input')),
Future.error('Future error02')
.then((val) => print('Future.error02 result = $val'))
.catchError((val) => print(val))
.whenComplete(() => print('Future.error02 whenComplete!'))
]).then((val) => print('Future.wait() --> then()'))
.catchError((val) => print('Future.wait() --> catchError() --> $val'))
.whenComplete(() => print('Future.wait() whenComplete!'));
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b. wait() 但异常捕获时机与 eagerError 参数相关,eagerError = true 时,Futures 中第一个返回异常时 wait() 立即捕获;eagerError = false 时,需等 Futures 全部执行结束之后才会捕获;
c. Futures 中出现异常时,cleanUp 会给每项正常执行的 Future 进行清理操作,传递给 cleanUp 的参数为每个正常执行项的返回内容;
Future.wait([
Future.delayed(Duration(seconds: 4)).then((val) {
print('Future -> 4');
return 4;
}).whenComplete(() => print('Future -> 4 whenComplete!')),
Future.error('Future error01')
.then((val) => print('Future.error01 result = $val'))
.catchError((val) => print(val)),
Future.error(ArgumentError.notNull('Input')),
Future.delayed(Duration(seconds: 2)).then((val) {
print('Future -> 2.toString()');
return '2';
}),
Future.error('Future error02')
.then((val) => print('Future.error02 result = $val'))
.catchError((val) => print(val))
.whenComplete(() => print('Future.error02 whenComplete!')),
], cleanUp: (val) => print('Future.wait() --> cleanUp --> $val'))
.then((val) => print('Future.wait() --> then()'))
.catchError(
(val) => print('Future.wait() --> catchError() --> $val'))
.whenComplete(() => print('Future.wait() whenComplete!'));
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Dart Future 案例尝试
小菜对 Dart 异步的认知还不完全,接下来会继续尝试 await-async 以及 EventLoop 执行顺序等;如有错误和遗漏请多多指导!
来源: 阿策小和尚
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