Test that timeout should cancel EventChannel and throw TimeoutException - unit-testing

Question

I am the maintainer of the open source Geolocator plugin for Flutter and working on refactoring the plugin. On popular request I am trying to add a timeout option to the plugin method which retrieved the current location.
The getCurrentLocation method basically listens to a stream of location updates, graps the first element and then cancels the stream. Now I would like to offer the option to supply a timeLimit which when reached should cancel the stream and throw a TimeoutException. Before I start implementing this in my code base I decided to start with a small proof-of-concept to see if I understand everything. Turns out I don't ;)
I have the following code:
import 'dart:async';
import 'package:flutter/services.dart';
import 'package:flutter_test/flutter_test.dart';
void main() {
TestWidgetsFlutterBinding.ensureInitialized();
final eventChannel = EventChannel('demo');
final log = <MethodCall>[];
setUp(() async {
log.clear();
MethodChannel(eventChannel.name)
.setMockMethodCallHandler((methodCall) async {
log.add(methodCall);
switch (methodCall.method) {
case 'listen':
await Future.delayed(Duration(milliseconds: 10));
await ServicesBinding.instance.defaultBinaryMessenger
.handlePlatformMessage(eventChannel.name,
eventChannel.codec.encodeSuccessEnvelope('one'), (_) {});
await ServicesBinding.instance.defaultBinaryMessenger
.handlePlatformMessage(eventChannel.name,
eventChannel.codec.encodeSuccessEnvelope('two'), (_) {});
await ServicesBinding.instance.defaultBinaryMessenger
.handlePlatformMessage(eventChannel.name,
eventChannel.codec.encodeSuccessEnvelope('three'), (_) {});
break;
case 'cancel':
default:
return null;
}
});
});
test('EventChannel returns the first value and cancels the stream', () async {
final one = await eventChannel.receiveBroadcastStream().first;
expect(one, 'one');
expect(log, <Matcher>[
isMethodCall('listen', arguments: null),
isMethodCall('cancel', arguments: null),
]);
});
test('Timeout should cancel the stream', () async {
final future = eventChannel
.receiveBroadcastStream()
.timeout(Duration(milliseconds: 5), onTimeout: (s) {
s.close();
throw TimeoutException('Timeout expired');
}).first;
expect(
future,
throwsA(predicate((e) {
final isTimeout = e is Timeout;
final streamCancelled = log.contains('cancel');
return isTimeout && streamCancelled;
})),
);
});
}
The first test (called "") works as expected, it retrieves the first value "one" and then cancels the stream.
In the second test (called "") I am trying to test that the stream is cancelled and a TimeoutException is thrown when the timeout is reached. This is where I am struggling, it seems I am not understanding how to accomplish this. Probably I am not understanding the inner workings correctly. The future variable in the second test seems to return the last value from the stream and not timeout at all. See the following output:
00:00 +1: Timeout should cancel the stream [E]
Expected: throws satisfies function
Actual: <Instance of '_Future<dynamic>'>
Which: emitted 'two'
https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 6327:7 DartError
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https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 30956:29 handleValue
https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 31466:49 handleValueCallback
https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 31498:17 _propagateToListeners
https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 31348:25 [_complete]
https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 35874:24 _cancelAndValue
https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 18161:17 get first.<fn>
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https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 29569:26 [_add]
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===== asynchronous gap ===========================
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===== asynchronous gap ===========================
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https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 35756:13 _microtaskLoop
https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 35762:13 _startMicrotaskLoop
https://storage.googleapis.com/compilation_artifacts/2.9.0-8.2.beta/dart_sdk.js 31704:9 _scheduleImmediateWithPromise.<fn>
What am I doing wrong? Is it possible to write such a test? Any help is greatly appreciated.
UPDATE: I have removed the link to DartPad since it will not run the code in a test environment (like flutter test), potentially resulting in false negatives.

Answer 1

With some solid help from the Flutter Community (and Simon Lightfoot in particular) I was able to solve the problem.
I had two mistakes in my code.
Since I am dealing with a stream, I should not throw an TimeoutException but rather add an error to the sink;
To simulate the a long running task I was blocking the event channel with a delay of 10 milliseconds. This means that after the timeout is thrown I still should wait for the remaining 5 milliseconds before the cancel method call is processed.
So after some refactoring I ended up with the following working solution:
test('Timeout should cancel the stream', () async {
final future = eventChannel
.receiveBroadcastStream()
.timeout(Duration(milliseconds: 5), onTimeout: (s) {
s.addError(TimeoutException('Timeout expired'));
s.close();
}).first;
expect(
future,
throwsA(isA<TimeoutException>()),
);
await Future.delayed(Duration(milliseconds: 5));
expect(log, <Matcher>[
isMethodCall('listen', arguments: null),
isMethodCall('cancel', arguments: null),
]);
});