Testing Dart class that runs code on Isolate - unit-testing

Question

I have a Dart class that performs computations on Isolate. Here is my code:
class Mapper {
SendPort _isolateSendPort;
Isolate _isolate;
Mapper() {
_asyncInit();
}
void _asyncInit() async {
final receivePort = ReceivePort();
_isolate = await Isolate.spawn(
_mappingFunction,
receivePort.sendPort,
);
_isolateSendPort = await receivePort.first;
}
static void _mappingFunction(SendPort callerSendPort) {
final newIsolateReceivePort = ReceivePort();
callerSendPort.send(newIsolateReceivePort.sendPort);
newIsolateReceivePort.listen((dynamic message) {
final crossIsolatesMessage =
message as CrossIsolatesMessage<Input>;
// some computations...
crossIsolatesMessage.sender.send(output);
});
}
Future<Output> map(Input input) async {
final port = ReceivePort();
_isolateSendPort.send(CrossIsolatesMessage<Input>(
sender: port.sendPort,
message: input,
));
return port.map((event) => event as Output).first;
}
void dispose() {
_isolate?.kill(priority: Isolate.immediate);
_isolate = null;
}
}
class CrossIsolatesMessage<T> {
final SendPort sender;
final T message;
CrossIsolatesMessage({
#required this.sender,
this.message,
});
}
This code works well when I run Flutter app. But unit test for public method Future<Output> map(Input input) throws an error NoSuchMethodError which meens _isolateSendPort is null.
Here is the unit test code:
test('Mapper map', () {
final sut = Mapper();
final inputDummy = Input('123');
final resultFuture = sut.map(inputDummy);
final expectedResult = Output('321');
expectLater(resultFuture, completion(expectedResult));
});
Here is an error:
NoSuchMethodError: The method 'send' was called on null.
Receiver: null
Tried calling: send(Instance of 'CrossIsolatesMessage<Input>')
dart:core Object.noSuchMethod
Why this error occurs in tests? And what is the right way to write tests for this class?

Answer 1

Problem solved.
Create of _isolate and _isolateSendPort is an async operation. Thats why _isolateSendPort was null on tests. Call method _asyncInit() from Mapper constructor is wrong way to create an isolate.
Here is working solution with lazy isolate initialization:
class Mapper {
SendPort _isolateSendPort;
Isolate _isolate;
void _initIsolate() async {
final receivePort = ReceivePort();
_isolate = await Isolate.spawn(
_mappingFunction,
receivePort.sendPort,
);
_isolateSendPort = await receivePort.first;
}
...
Future<Output> map(Input input) async {
final port = ReceivePort();
if (_isolateSendPort == null) {
await _initIsolate();
}
_isolateSendPort.send(CrossIsolatesMessage<Input>(
sender: port.sendPort,
message: input,
));
return port.map((event) => event as Output).first;
}
...
}