我正在使用Entity Framework开发WPF应用程序(MVVM)。我想在给定的时间间隔内查询SQL服务器上的表,以向用户显示更新的DB值。为此,我在我的ViewModel中设置了一个计时器,如下所示:
RefreshTimer = new System.Timers.Timer(2000.0);
RefreshTimer.Enabled = true;
RefreshTimer.SynchronizingObject = new Synchronizer();
RefreshTimer.Elapsed += RefreshTimer_Elapsed;
其中Synchronizer是ISynchronizeInvoke的实现:
internal class Synchronizer : ISynchronizeInvoke
{
private object _sync = new object();
public bool InvokeRequired { get { return true; } }
public IAsyncResult BeginInvoke(Delegate method, object[] args)
{
var result = new SimpleAsyncResult();
ThreadPool.QueueUserWorkItem(delegate {
result.AsyncWaitHandle = new ManualResetEvent(false);
try
{
result.AsyncState = Invoke(method, args);
}
catch (Exception exception)
{
result.Exception = exception;
}
result.IsCompleted = true;
});
return result;
}
public object EndInvoke(IAsyncResult result)
{
if (!result.IsCompleted)
{
result.AsyncWaitHandle.WaitOne();
}
return result.AsyncState;
}
public object Invoke(Delegate method, object[] args)
{
lock (_sync)
{
return method.DynamicInvoke(args);
}
}
}
internal class SimpleAsyncResult : IAsyncResult
{
object _state;
public bool IsCompleted { get; set; }
public WaitHandle AsyncWaitHandle { get; internal set; }
public object AsyncState
{
get
{
if (Exception != null)
{
throw Exception;
}
return _state;
}
internal set
{
_state = value;
}
}
public bool CompletedSynchronously { get { return IsCompleted; } }
internal Exception Exception { get; set; }
}
如果我理解正确,SynchronizingObject应该在创建SynchronizingObject的线程上封送Elapsed事件。在我的情况下,这是主线程。现在考虑以下代码来处理事件:
private void RefreshTimer_Elapsed(object sender, ElapsedEventArgs e)
{
System.Diagnostics.Debug.WriteLine($"RefreshTimer ManagedThreadID: {Thread.CurrentThread.ManagedThreadId}");
RefreshTimer.SynchronizingObject.Invoke((Action)(() => { TestMethod(); }), null);
}
public void TestMethod()
{
System.Diagnostics.Debug.WriteLine($"TestMethod ManagedThreadID: {Thread.CurrentThread.ManagedThreadId}");
}
TestMethod是我更新/重新加载我的Entity对象的地方,因此View会自动显示最新数据。当我尝试这样做时,我得到一个InvalidOperationException,指出调用线程无法访问此对象,因为另一个线程拥有它
输出结果为:
RefreshTimer ManagedThreadID: 5
TestMethod ManagedThreadID: 5
RefreshTimer ManagedThreadID: 4
TestMethod ManagedThreadID: 4
RefreshTimer ManagedThreadID: 8
TestMethod ManagedThreadID: 8
我怀疑这是我得到上述异常的原因。我希望输出为:
RefreshTimer ManagedThreadID: 5
TestMethod ManagedThreadID: 1
RefreshTimer ManagedThreadID: 4
TestMethod ManagedThreadID: 1
RefreshTimer ManagedThreadID: 8
TestMethod ManagedThreadID: 1
有谁能告诉我我做错了什么以及如何让SynizingObject.Invoke方法在主线程上执行?
答案 0 :(得分:3)
您可以使用调度程序安排在调度程序线程上执行委托:
private void RefreshTimer_Elapsed(object sender, ElapsedEventArgs e)
{
System.Diagnostics.Debug.WriteLine($"RefreshTimer ManagedThreadID: {Thread.CurrentThread.ManagedThreadId}");
Application.Current.Dispatcher.Invoke(new Action(() => { TestMethod(); }))
}