MVVM：异步事件处理程序的单元测试

Question

我有一个带有异步任务的 viewModel。我现在不知道如何测试它。

public class MyViewModel : BindableBase
{
    public MyViewModel()
    {
        this.PropertyChanged += MyViewModel_PropertyChanged;
    }

    private void MyViewModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        Action action = async () => await DoSomething();
        action();
    }

    public const string BeforeKey = "before";
    public const string AfterKey = "After";

    public string Status { get; private set; } = BeforeKey;

    public async Task DoSomething()
    {
        await Task.Delay(3000);
        Status = AfterKey;
    }

    string bindagleProp;
    public string BindagleProp
    {
        get { return bindagleProp; }
        set { SetProperty(ref bindagleProp, value); }
    }
}

这是我的测试：

[TestMethod]
public async Task TestMyViewModel()
{
    MyViewModel viewModel = new MyViewModel();
    Assert.AreEqual(viewModel.Status, MyViewModel.BeforeKey, "before check");

    viewModel.BindagleProp = "abc";
    Assert.AreEqual(viewModel.Status, MyViewModel.AfterKey, "after check");
}

测试失败，因为它没有等待任务完成。 我不想在单元测试中使用 Task.Delay，因为它不安全。 DoSomething 方法可以有未知的持续时间。

感谢您的帮助。

---

编辑：

事实上，这个问题并不特定于 MVVM，而是任何异步事件处理程序。 例如：

// class with some logic, can be UI or whatever.
public class MyClassA
{
    Size size;

    public Size Size
    {
        get { return size; }
        set
        {
            size = value;
            SizeChanged?.Invoke(this, EventArgs.Empty);
        }
    }

    public event EventHandler SizeChanged;
}

// this class uses the MyClassA class.
public class MyCunsomerClass
{
    readonly MyClassA myClassA = new MyClassA();

    public MyCunsomerClass()
    {
        myClassA.SizeChanged += MyClassA_SizeChanged;
    }

    public string Status { get; private set; } = "BEFORE";

    private async void MyClassA_SizeChanged(object sender, EventArgs e)
    {
        await LongRunningTaskAsync();
        Status = "AFTER";
    }

    public async Task LongRunningTaskAsync()
    {
        await Task.Delay(3000);
        ///await XYZ....;
    }

    public void SetSize()
    {
        myClassA.Size = new Size(20, 30);
    }
}

现在，我想测试一下：

    [TestMethod]
    public void TestMyClass()
    {
        var cunsomerClass = new MyCunsomerClass();
        cunsomerClass.SetSize();
        Assert.AreEqual(cunsomerClass.Status, "AFTER");
    }

测试失败。

Answer 1

好的，首先，我会将worker移到另一个类并为它创建一个接口。这样当我运行测试时，我可以注入另一个工人！

public class MyViewModel : BindableBase
{
    private IWorker _worker;

    private readonly DataHolder _data = new DataHolder(){Test = DataHolder.BeforeKey};
    public string Status { get { return _data.Status; } }

    public MyViewModel(IWorker worker = null)
    {
        _worker = worker;
        if (_worker == null)
        {
            _worker = new Worker();
        }

        this.PropertyChanged += MyViewModel_PropertyChanged;
    }

    private void MyViewModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {

        Action action = async () => await _worker.DoSomething(_data);
        action();
    }


    string bindagleProp;
    public string BindagleProp
    {
        get { return bindagleProp; }
        set { SetProperty(ref bindagleProp, value); }
    }
}

public class DataHolder
{
    public const string BeforeKey = "before";
    public const string AfterKey = "After";

    public string Status;
}

public interface IWorker
{
    Task DoSomething(DataHolder data);
}

public class Worker : IWorker
{
    public async Task DoSomething(DataHolder data)
    {
        await Task.Delay(3000);
        data.Status = DataHolder.AfterKey;
    }
}

现在注入代码如下所示：

[TestMethod]
public async Task TestMyViewModel()
{
    TestWorker w = new TestWorker();

    MyViewModel viewModel = new MyViewModel(w);
    Assert.AreEqual(viewModel.Status, DataHolder.BeforeKey, "before check");

    viewModel.BindagleProp = "abc";
    Assert.AreEqual(viewModel.Status, DataHolder.AfterKey, "after check");
}

public class TestWorker : IWorker
{
    public Task DoSomething(DataHolder data)
    {
        data.Status = DataHolder.BeforeKey;
        return null; //you maybe should return something else here...
    }
}

Answer 2

我问了 Stehphen Cleary [著名的异步教授]，他回答我：

如果“异步事件处理程序”是指async void 事件处理程序， 那么不，那些是不可测试的。但是，它们通常在 UI 中很有用 应用。所以我通常最终做的是让我所有的异步 void 方法正好是一行。它们看起来都是这样的：

async void SomeEventHandler(object sender, EventArgsOrWhatever args)
{
     await SomeEventHandlerAsync(sender, args);
}

async Task SomeEventHandlerAsync(object sender, EventArgsOrWhatever args)
{
      ... // Actual handling logic
}

那么async Task 版本是可单元测试、可组合等的。 async void 处理程序不是，但这是可以接受的，因为它不再 有任何真正的逻辑。

感谢斯蒂芬！你的想法很棒！