gistfile1.cs

[Fact]
public void FetchImageFromSiteCommandTest()
{
    // Replace the immediate scheduler with an event loop (a thread who just 
    // waits in the background to process stuff as it arrives, one at a time)
    var origSched = RxApp.DeferredScheduler;
    RxApp.DeferredScheduler = new EventLoopScheduler();

    // MyCoolViewModel has an ICommand called FetchImageFromSite
    var fixture = new MyCoolViewModel();
    fixture.FetchImageFromSite("myCoolImage.jpg").Execute();

    // While it's running, make sure we can't execute anything
    Assert.False(fixture.FetchImageFromSite.CanExecute("myCoolImage.jpg"));
    Assert.False(fixture.DownloadedImages.Any(x => x.Name == "myCoolImage.jpg"));

    // Wait until it completes
    fixture.FetchImageFromSite.ItemsInflight
        .Where(count => count == 0)
        .First();

    // Verify that the Image is downloaded
    Assert.True(fixture.DownloadedImages.Any(x => x.Name == "myCoolImage"));

    // Now we *should* be able to execute the command
    Assert.True(fixture.FetchImageFromSite.CanExecute("myCoolImage.jpg"));

    // Replace the old scheduler
    RxApp.DeferredScheduler = origSched;
}

If FetchImageFromSite is an ICommand then line 11 should read:

fixture.FetchImageFromSite.Execute("myCoolImage.jpg");

and line 14 should read:

Assert.False(fixture.FetchImageFromSite.CanExecute("myCoolImage.jpg"));

Unless I'm missing something :)

@GraemeF Oops, this is what happens when you code in a TextArea :)