如何在NUnit中测试此视图模型的异步行为？

Question

My view model has a public method void StartBroadcastingAndDiscovery() . This triggers two private async methods Task StartBroadcasting() and Task StartDiscovering() . Both of these private methods are (endless) while loops like this:

while(!ct.IsCancellationRequested)
{
  var discovered = await this.networkService.Discover();
  this.DiscoveredPeers.Add(discovered);
  await Task.Delay(1000);
}

The idea is that the async method used for discovery is polling the network service regularly. The network service is injected:

this.networkService = ServiceContainer.Resolve<INetworkService> ();

The user of the model can get a list of discovered peers. The list can change over time as peers appear or disappear from the network. At some point, the user calls void StopBroadcastingAndDiscovering().

In order to test my view model I have created a mockup network service that will "discover" three different peers. First request returns first peer, then the second, then the third and then the first again and so on.

My problem is now: how do I actually run the test? What can I test here? I would like to see that the list of discovered peers contains three items after 3 seconds but I don't want to hardcode the 3 secs delay.

Answer 1

The simplest solution would be to refactor Task.Delay to a separate, protected method inside your class like this:

protected Task Wait()
{
    return Task.Delay(1000);
}

Then inside your unit tests, you inherit from your system under test and override the Wait method:

protected override Task Wait()
{
   return Task.FromResult(true);
}

This way, your unit tests will call the overridden version of Wait that returns immediately. This skips the one second wait and makes your unit tests a lot faster and easier to control.

Extracting to a protected method is called creating a seam. You can also create seams by using interfaces and dependency injection but that looks like overkill to me for this situation.