Stream<T>.fromFutures constructor Null safety

  1. Iterable<Future<T>> futures

Create a single-subscription stream from a group of futures.

The stream reports the results of the futures on the stream in the order in which the futures complete. Each future provides either a data event or an error event, depending on how the future completes.

If some futures have already completed when Stream.fromFutures is called, their results will be emitted in some unspecified order.

When all futures have completed, the stream is closed.

If futures is empty, the stream closes as soon as possible.


factory Stream.fromFutures(Iterable<Future<T>> futures) {
  _StreamController<T> controller =
      new _SyncStreamController<T>(null, null, null, null);
  int count = 0;
  // Declare these as variables holding closures instead of as
  // function declarations.
  // This avoids creating a new closure from the functions for each future.
  void onValue(T value) {
    if (!controller.isClosed) {
      if (--count == 0) controller._closeUnchecked();

  void onError(Object error, StackTrace stack) {
    if (!controller.isClosed) {
      controller._addError(error, stack);
      if (--count == 0) controller._closeUnchecked();

  // The futures are already running, so start listening to them immediately
  // (instead of waiting for the stream to be listened on).
  // If we wait, we might not catch errors in the futures in time.
  for (var future in futures) {
    future.then(onValue, onError: onError);
  // Use schedule microtask since controller is sync.
  if (count == 0) scheduleMicrotask(controller.close);