1. 问题
代码语言：javascript复制[TestClass]
public class UnitTest1
{
[TestMethod]
public void TestMethod1()
{
ContainerLocator.Container.Resolve<TestViewModel>();
}
}
public class TestViewModel
{
public TestViewModel(IEventAggregator eventAggregator)
{
var testEvent = eventAggregator.GetEvent<TestEvent>();
testEvent.Subscribe(() => { }, ThreadOption.UIThread);
}
}
public class TestEvent : PubSubEvent
{
}上面是一段使用了 Prism 的单元测试，它主要的逻辑是在 EventAggregator 中订阅了 TestEvent，当接收到消息后在 UI 线程上执行后续的逻辑。这种代码在正常程序中没有问题，但在单元测试中会报错：
System.InvalidOperationException: To use the UIThread option for subscribing, the EventAggregator must be constructed on the UI thread.
2. 原因
翻翻源码，可以发现这个 Exception 在 PubSubEvent 的 Subscribe 函数中抛出：
switch (threadOption)
{
case ThreadOption.PublisherThread:
subscription = new EventSubscription(actionReference);
break;
case ThreadOption.BackgroundThread:
subscription = new BackgroundEventSubscription(actionReference);
break;
case ThreadOption.UIThread:
if (SynchronizationContext == null) throw new InvalidOperationException(Resources.EventAggregatorNotConstructedOnUIThread);
subscription = new DispatcherEventSubscription(actionReference, SynchronizationContext);
break;
default:
subscription = new EventSubscription(actionReference);
break;当 SynchronizationContext 为 null 时就会判断当前不在 UI 线程，然后抛出 Exception。而 SynchronizationContext 又是在 EventAggregator 中赋值：
private readonly SynchronizationContext syncContext = SynchronizationContext.Current;
public TEventType GetEvent<TEventType>() where TEventType : EventBase, new()
{
lock (events)
{
EventBase existingEvent = null;
if (!events.TryGetValue(typeof(TEventType), out existingEvent))
{
TEventType newEvent = new TEventType();
newEvent.SynchronizationContext = syncContext;
events[typeof(TEventType)] = newEvent;
return newEvent;
}
else
{
return (TEventType)existingEvent;
}
}
}问题就出在 SynchronizationContext.Current 这里。这个属性用于获取当前线程的同步上下文。不是每一个线程都有一个 SynchronizationContext 对象。一个总是有 SynchronizationContext 对象的是UI线程。由于单元测试并不是运行在 UI 线程，所以这个属性在单元测试中一直为 null。
3. 解决方案
现在我们知道问题原因了，解决方案也很简单，只要自定义一个 EventAggregator，源码全部照抄，但是把这句：
代码语言：javascript复制private readonly SynchronizationContext syncContext = SynchronizationContext.Current;替换成这句：
代码语言：javascript复制private readonly SynchronizationContext syncContext = new SynchronizationContext();就不会出现 PubSubEvent 中 SynchronizationContext 等于 null 的情况了。然后再把这个类注册到容器中作为 IEventAggregator：
ContainerLocator.Current.RegisterSingleton<IEventAggregator, MyEventAggregator>();4. 最后
根据单元测试项目的结构，容器的初始化会有不同的方式，如果想尽量模仿 PrismApplication 的话可以参考 PrismApplicationBase 和 PrismInitializationExtensions 写一个初始化类，大概差不多这样（简化了部分代码）：
代码语言：javascript复制[TestClass]
public abstract class TestInitializerBase
{
public void Initialize()
{
ContainerLocator.SetContainerExtension(() => new UnityContainerExtension());
ContainerExtension = ContainerLocator.Current;
ContainerExtension.RegisterSingleton<IDialogService, DialogService>();
ContainerExtension.RegisterSingleton<IModuleInitializer, ModuleInitializer>();
ContainerExtension.RegisterSingleton<IModuleManager, ModuleManager>();
ContainerExtension.RegisterSingleton<RegionAdapterMappings>();
ContainerExtension.RegisterSingleton<IRegionManager, RegionManager>();
ContainerExtension.RegisterSingleton<IRegionNavigationContentLoader, RegionNavigationContentLoader>();
ContainerExtension.RegisterSingleton<IEventAggregator, EventAggregator>();
ContainerExtension.RegisterSingleton<IRegionViewRegistry, RegionViewRegistry>();
ContainerExtension.RegisterSingleton<IRegionBehaviorFactory, RegionBehaviorFactory>();
ContainerExtension.Register<IRegionNavigationJournalEntry, RegionNavigationJournalEntry>();
ContainerExtension.Register<IRegionNavigationJournal, RegionNavigationJournal>();
ContainerExtension.Register<IRegionNavigationService, RegionNavigationService>();
RegisterRequiredTypes(ContainerExtension);
}
public IContainerExtension ContainerExtension { get; private set; }
protected abstract void RegisterRequiredTypes(IContainerRegistry containerRegistry);
}
public class TestInitializer : TestInitializerBase
{
[AssemblyInitialize]
public static void InitializeAseemble(TestContext testContext)
{
var testInitializer = new TestInitializer();
testInitializer.Initialize();
}
protected override void RegisterRequiredTypes(IContainerRegistry containerRegistry)
{
containerRegistry.RegisterSingleton<IEventAggregator, MyEventAggregator>();
}
}这样在 TestInitializer 中可以注册各种方便单元测试的伪对象。
