文章目录
一、Future 简介
二、Future.then 使用
三、Future 异常捕获
四、Dart 练习网站
五、async、await 关键字
六、whenComplete 方法
七、timeout 方法
八、相关资源
一、Future 简介
Future 指的是在 将来 的 某个时刻 的 结果 , 可以是一个值 , 也可以是一个报错信息 ;
借助 Future 可以实现异步操作 ;
Future 是在 dart:async 包中的类 , 系统会默认导入该包中的类 , 直接使用即可 , 不需要刻意导入 ;
Future 有两种状态 :
① 执行中 , Pending 状态 ;
② 执行结果 , Complete 状态 ;
二、Future.then 使用
调用 then 方法 , 可以在该方法中 , 获取 Future 中的值 , 其类型是 Future 泛型中的类型 ;
调用 testFuture 方法后 , 调用 then 方法 , 可以获取 testFuture 方法返回的 String 字符串 , 就是 s 参数 , 打印该字符串 ;
Future<String> testFuture() { return Future.value('success'); } main() { testFuture().then((s) { print(s); }); }
Future 的 then 方法原型如下 :
/// Register callbacks to be called when this future completes. /// /// When this future completes with a value, /// the [onValue] callback will be called with that value. /// If this future is already completed, the callback will not be called /// immediately, but will be scheduled in a later microtask. /// /// If [onError] is provided, and this future completes with an error, /// the `onError` callback is called with that error and its stack trace. /// The `onError` callback must accept either one argument or two arguments /// where the latter is a [StackTrace]. /// If `onError` accepts two arguments, /// it is called with both the error and the stack trace, /// otherwise it is called with just the error object. /// The `onError` callback must return a value or future that can be used /// to complete the returned future, so it must be something assignable to /// `FutureOr<R>`. /// /// Returns a new [Future] /// which is completed with the result of the call to `onValue` /// (if this future completes with a value) /// or to `onError` (if this future completes with an error). /// /// If the invoked callback throws, /// the returned future is completed with the thrown error /// and a stack trace for the error. /// In the case of `onError`, /// if the exception thrown is `identical` to the error argument to `onError`, /// the throw is considered a rethrow, /// and the original stack trace is used instead. /// /// If the callback returns a [Future], /// the future returned by `then` will be completed with /// the same result as the future returned by the callback. /// /// If [onError] is not given, and this future completes with an error, /// the error is forwarded directly to the returned future. /// /// In most cases, it is more readable to use [catchError] separately, /// possibly with a `test` parameter, /// instead of handling both value and error in a single [then] call. /// /// Note that futures don't delay reporting of errors until listeners are /// added. If the first `then` or `catchError` call happens /// after this future has completed with an error, /// then the error is reported as unhandled error. /// See the description on [Future]. Future<R> then<R>(FutureOr<R> onValue(T value), {Function? onError});
then 方法的第一个参数 FutureOr<R> onValue(T value) 就是 Future 的 onValue 代表的值 , 类型是 Future 泛型类型 R ;
then 方法的第二个参数 {Function? onError} 是可选的 , 用于捕获异常的方法 ;
三、Future 异常捕获
方式一 : then 方法传入 onError 参数 ;
在执行 返回值是 Future 类型的 testFuture 方法时 , 在 then 方法中 , 第二个参数 onError
Future<String> testFuture() { return Future.value('success'); } main() { testFuture().then((s) { print(s); }, onError: (e) { print('onError:'); print(e); }); }
方式二 : 继续链式调用 , 在 then 方法后 , 继续调用 Future 的 catchError 方法 ;
Future<String> testFuture() { return Future.value('success'); } main() { testFuture().then((s) { print(s); }).catchError((e) { print('catchError:'); print(e); }); }
注意 : 上述两个方法只能二选其一 , 如果都设置了 , 那么只有 方式一 生效 , 方式二 会被覆盖 ;
