Future<List<T>> wait<T>(Iterable<Future<T>> futures, { bool eagerError: false, void cleanUp(T successValue) })

Wait for all the given futures to complete and collect their values.

Returns a future which will complete once all the futures in a list are complete. If any of the futures in the list completes with an error, the resulting future also completes with an error. Otherwise the value of the returned future will be a list of all the values that were produced.

If eagerError is true, the future completes with an error immediately on the first error from one of the futures. Otherwise all futures must complete before the returned future is completed (still with the first error to occur, the remaining errors are silently dropped).

If cleanUp is provided, in the case of an error, any non-null result of a successful future is passed to cleanUp, which can then release any resources that the successful operation allocated.

The call to cleanUp should not throw. If it does, the error will be an uncaught asynchronous error.

Source 

static Future<List<T>> wait<T>(Iterable<Future<T>> futures,
                         {bool eagerError: false,
                          void cleanUp(T successValue)}) {
  final _Future<List<T>> result = new _Future<List<T>>();
  List<T> values;  // Collects the values. Set to null on error.
  int remaining = 0;  // How many futures are we waiting for.
  var error;   // The first error from a future.
  StackTrace stackTrace;  // The stackTrace that came with the error.

  // Handle an error from any of the futures.
  void handleError(theError, theStackTrace) {
    remaining--;
    if (values != null) {
      if (cleanUp != null) {
        for (var value in values) {
          if (value != null) {
            // Ensure errors from cleanUp are uncaught.
            new Future.sync(() { cleanUp(value); });
          }
        }
      }
      values = null;
      if (remaining == 0 || eagerError) {
        result._completeError(theError, theStackTrace);
      } else {
        error = theError;
        stackTrace = theStackTrace;
      }
    } else if (remaining == 0 && !eagerError) {
      result._completeError(error, stackTrace);
    }
  }

  try {
    // As each future completes, put its value into the corresponding
    // position in the list of values.
    for (Future future in futures) {
      int pos = remaining;
      future.then((T value) {
        remaining--;
        if (values != null) {
          values[pos] = value;
          if (remaining == 0) {
            result._completeWithValue(values);
          }
        } else {
          if (cleanUp != null && value != null) {
            // Ensure errors from cleanUp are uncaught.
            new Future.sync(() { cleanUp(value); });
          }
          if (remaining == 0 && !eagerError) {
            result._completeError(error, stackTrace);
          }
        }
      }, onError: handleError);
      // Increment the 'remaining' after the call to 'then'.
      // If that call throws, we don't expect any future callback from
      // the future, and we also don't increment remaining.
      remaining++;
    }
    if (remaining == 0) {
      return new Future.value(const []);
    }
    values = new List<T>(remaining);
  } catch (e, st) {
    // The error must have been thrown while iterating over the futures
    // list, or while installing a callback handler on the future.
    if (remaining == 0 || eagerError) {
      // Throw a new Future.error.
      // Don't just call `result._completeError` since that would propagate
      // the error too eagerly, not giving the callers time to install
      // error handlers.
      // Also, don't use `_asyncCompleteError` since that one doesn't give
      // zones the chance to intercept the error.
      return new Future.error(e, st);
    } else {
      // Don't allocate a list for values, thus indicating that there was an
      // error.
      // Set error to the caught exception.
      error = e;
      stackTrace = st;
    }
  }
  return result;
}