在 UI 线程上创建并启动任务

当在工作线程上调用的方法需要在 UI 线程上运行代码并等待其完成后再执行其他操作时，可以这样做：

    public int RunOnUi(Func<int> f)
    {
        int res = Application.Current.Dispatcher.Invoke(f);

        return res;
    }

但如果我想用任务来做呢？有没有办法让 RunOnUi 方法创建一个在 UI 上启动的任务并返回它，以便调用者（在工作线程上运行）可以等待它？符合以下签名的东西：public Task<int> StartOnUi(Func<int> f) ?

一种方法如下：

public Task<int> RunOnUi(Func<int> f)
{
    var task = new Task<int>(f);
    task.Start(_scheduler);

    return task;
}

在这里，假设_schduler持有用户界面TaskScheduler。但我不太习惯创建“冷”任务并使用 start 方法来运行它们。这是“推荐”的方式还是有更优雅的方式来做到这一点？

只需使用InvokeAsync https://msdn.microsoft.com/en-us/library/hh199385(v=vs.110).aspx代替Invoke然后返回Task<int>在 - 的里面DispatcherOperation<int> https://msdn.microsoft.com/en-us/library/hh199421(v=vs.110).aspx函数返回。

//Coding conventions say async functions should end with the word Async.
public Task<int> RunOnUiAsync(Func<int> f)
{
    var dispatcherOperation = Application.Current.Dispatcher.InvokeAsync(f);
    return dispatcherOperation.Task;
}

如果您无法访问 .NET 4.5，情况会稍微复杂一些。您将需要使用BeginInvoke https://msdn.microsoft.com/en-us/library/cc190824(v=vs.110).aspx and a TaskCompletionSource https://msdn.microsoft.com/en-us/library/dd449174(v=vs.110).aspx包裹DispaterOperation https://msdn.microsoft.com/en-us/library/System.Windows.Threading.DispatcherOperation(v=vs.100).aspx that BeginInvoke returns

    public Task<int> RunOnUi(Func<int> f)
    {
        var operation = Application.Current.Dispatcher.BeginInvoke(f);
        var tcs = new TaskCompletionSource<int>();
        operation.Aborted += (sender, args) => tcs.TrySetException(new SomeExecptionHere());
        operation.Completed += (sender, args) => tcs.TrySetResult((int)operation.Result);

        //The operation may have already finished and this check accounts for 
        //the race condition where neither of the events will ever be called
        //because the events where raised before you subscribed.
        var status = operation.Status;
        if (status == DispatcherOperationStatus.Completed)
        {
            tcs.TrySetResult((int)operation.Result);
        }
        else if (status == DispatcherOperationStatus.Aborted)
        {
            tcs.TrySetException(new SomeExecptionHere());
        }

        return tcs.Task;
    }