Dart - Get the last or the first value of a stream - dart

Question

I have a stream and I need to use the last value of this stream, and if there is no value emitted by this stream I need to wait for the fist value. I only want to use this value once. What is the correct way to do it?

Answer 1

Sounds like you want the most recent event emitted by a stream (which is presumably a broadcast stream, because otherwise there is no events until you listen), or, if there has been no events before, you want the next event instead.
For a plain Dart Stream, that's impossible. It doesn't remember previous events. You need to have listened to that stream previously in order to know what the most recent event was (but if you do that, it doesn't have to be a broadcast stream anyway).
You can build your own memorizing stream wrapper fairly easily (but as always with asynchronous programming, you need to be careful about race conditions)
// Copyright 2021 Google LLC.
// SPDX-License-Identifier: Apache-2.0
import "dart:async";
/// Listens to [source] to returned stream.
///
/// Each listener on the returned stream receives the most recent
/// event sent on [source] followed by all further events of [source]
/// until they stop listening.
/// If there has been no events on [source] yet, only the further events
/// are forwarded.
Stream<T> mostRecentStream<T>(Stream<T> source) {
var isDone = false;
var hasEvent = false;
T? mostRecentEvent;
List<MultiStreamController>? pendingListeners;
var listeners = <MultiStreamController>[];
void forEachListener(void Function(MultiStreamController) action) {
var active = 0;
var originalLength = listeners.length;
for (var i = 0; i < listeners.length; i++) {
var controller = listeners[i];
if (controller.hasListener) {
listeners[active++] = controller;
if (i < originalLength) action(controller);
}
}
listeners.length = active;
}
source.listen((event) {
mostRecentEvent = event;
hasEvent = true;
forEachListener((controller) {
controller.addSync(event);
});
}, onError: (e, s) {
forEachListener((controller) {
controller.addErrorSync(e, s);
});
}, onDone: () {
isDone = true;
for (var controller in listeners) {
controller.close();
}
listeners.clear();
});
return Stream<T>.multi((controller) {
if (hasEvent) controller.add(mostRecentEvent as T);
if (isDone) {
controller.close();
} else {
listeners.add(controller);
}
});
}
With that, you can simply do var recentStream = mostRecentStream(yourStream) and then later do recentStream.first to get either the most recent event or, if there is none, the next event (if there is one, you get an error if the stream is completely empty).