dart - 增加构建 future 的延迟

Question

与Dart一起玩，是否有可能在构建Future时造成延迟？

Future<String>.value("Hello").then((newsDigest) {
        print(newsDigest);
      }) // .delayed(Duration(seconds: 5))

是的，这是可能的:

factory Future.delayed(Duration duration, [FutureOr<T> computation()]) {
  _Future<T> result = new _Future<T>();
  new Timer(duration, () {
    try {
      result._complete(computation?.call());
    } catch (e, s) {
      _completeWithErrorCallback(result, e, s);
    }
  });
  return result;
}

Answer 1

您已经发现Future.delayed构造函数创建了一个在延迟后运行的Future:

从docs:

Future<T>.delayed(
    Duration duration,
    [ FutureOr<T> computation()
]) 

The computation will be executed after the given duration has passed, and the future is completed with the result of the computation.

If computation returns a future, the future returned by this constructor will complete with the value or error of that future.

为了简单起见，以一个立即完成并带有值的 future 为例，此摘要创建了一个延迟的 future ，该延迟在3秒后完成:

import 'dart:async';

main() {
  var future = Future<String>.value("Hello");

  var delayedFuture = Future.delayed(Duration(seconds: 3), () => future);

  delayedFuture.then((value) {
    print("Done: $value");
  });
}