【Flutter】Future 异步编程 ( 简介 | then 方法 | 异常捕获 | async、await 关键字 | whenComplete 方法 | timeout 方法 )（一）

2023-11-17 14:23:40

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 ;

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