测试时Dispatcher.Invoke(Action)挂起，PushFrame方法无效求助

核心问题拆解 #

• 测试环境无WPF应用实例，Application.Current为null，导致Dispatcher使用测试线程的Dispatcher，但测试线程无消息循环，Invoke调用直接阻塞。
• async void的命令执行方法无法被测试代码追踪完成状态，导致手动调用的PushFrame无法正确终止，永久卡住。
• 测试中未正确处理异步任务的完成时机，依赖未就绪的UI线程逻辑引发死锁。

具体解决步骤 #

1. 替换async void命令逻辑为async Task #

async void在非UI场景（如测试）中会导致任务无法被等待，异常无法被捕获。修改PopulatePeople方法：

private async Task PopulatePeople() // 改为async Task
{
    Progress = 0;
    IsBusy = true;
    try
    {
        await Task.Run(async () =>
        {
            ShowToastMessage("About to get a load of records");
            var restPeople = await peopleRestService.PeopleAllAsync();
            ShowToastMessage("Got a load of records");
            
            List<IBaseViewModel> localList = new List<IBaseViewModel>();

            var pvm = Container.Resolve<IPersonViewModel>();
            pvm.Person = pe;
            pvm.Title = pe.Name;
            pvm.RestPeople = new ObservableCollection<RestPerson>(restPeople.Select(p => new RestPerson(p)));

            localList.Add(pvm);
            GenerateExpensiveViewModels(numberOfPeople, maxVmIndex, vmIndex, localList, restPeople);
            ShowToastMessage("Got them all!");
            UI(() => People = new ObservableCollection<IBaseViewModel>(localList));
        });
    }
    finally
    {
        IsBusy = false;
    }
}

2. 实现支持异步的DelegateCommand #

原DelegateCommand不支持异步方法，需扩展适配：

public class AsyncDelegateCommand : IDelegateCommand
{
    public event EventHandler CanExecuteChanged;

    readonly Func<Task> _executeAsync;
    readonly Func<bool> _canExecute;

    public AsyncDelegateCommand(Func<Task> executeAsync, Func<bool> canExecute = null)
    {
        _executeAsync = executeAsync;
        _canExecute = canExecute ?? (() => true);
    }

    public bool CanExecute(object parameter) => _canExecute.Invoke();

    public async void Execute(object parameter) // 保持void实现ICommand接口，内部await异步逻辑
    {
        if (CanExecute(parameter)) await _executeAsync();
    }

    public void Refresh() => CanExecuteChanged?.Invoke(this, EventArgs.Empty);
}

ViewModel中初始化命令：

PopulatePeopleCommand = new AsyncDelegateCommand(PopulatePeople);

3. 测试中Mock Dispatcher调用（最优方案） #

测试场景无需依赖真实WPF Dispatcher，直接MockIDispatcherService跳过UI线程限制：

// 提前封装BaseViewModel的UI方法为依赖注入的服务
// BaseViewModel代码：
// private readonly IDispatcherService _dispatcherService;
// public BaseViewModel(IDispatcherService dispatcherService) => _dispatcherService = dispatcherService;
// public void UI(Action action) => _dispatcherService.Invoke(action);

// 测试中设置Mock：直接在当前线程执行Action
dispatcherServiceMock.Setup(s => s.Invoke(It.IsAny<Action>())).Callback<Action>(action => action());

4. 测试中正确等待异步任务完成 #

通过TaskCompletionSource追踪命令执行状态，避免手动调用PushFrame：

[Fact]
public async Task OpenPersonDetailsAndChangeTheirNameTest() // 改为async Task测试方法
{
    // ... 原有Mock代码保持不变 ...

    var tcs = new TaskCompletionSource<bool>();
    // 拦截UI方法，任务完成时触发tcs
    dispatcherServiceMock.Setup(s => s.Invoke(It.IsAny<Action>()))
        .Callback<Action>(action =>
        {
            action();
            tcs.TrySetResult(true);
        });

    var sut = new MainViewModel(RestMoq.Object, windowServiceMoq.Object, dispatcherServiceMock.Object, designerServiceMock.Object, viewModelRepositoryMock.Object, containerRepositoryMock.Object);

    // 执行命令并等待完成
    if (sut.PopulatePeopleCommand.CanExecute(null))
    {
        sut.PopulatePeopleCommand.Execute(null);
        await tcs.Task; // 等待异步任务完成
    }

    sut.SelectedPerson = sut.People.FirstOrDefault();

    if (sut.OpenPersonDetailsCommand.CanExecute(person.Object))
    {
        sut.OpenPersonDetailsCommand.Execute(person.Object);
    }

    // 验证逻辑
    windowServiceMoq.Verify(s => s.Show(It.IsAny<IBaseViewModel>()), Times.Once());            
    providerMock.Verify(p => p.Resolve(typeof(IWindowService)), Times.AtLeastOnce());
}

关键注意事项 #

• 测试中永远避免依赖真实WPF UI组件（如Dispatcher），Mock相关服务是最可靠的解耦方式。
• 命令执行逻辑优先使用async Task而非async void，便于测试追踪任务状态。
• 不要在测试中手动调用PushFrame，通过异步等待确保所有任务执行完毕。

内容的提问来源于stack exchange，提问作者XAMlMAX