Executing a function periodically after the function completes its task

问题

I am building a windows store app using C# and xaml. I need to refresh the data after certain interval of time (bring the new data from the server). I used ThreadPoolTimer to execute my refresh function periodically as follows:

   TimeSpan period = TimeSpan.FromMinutes(15); 
   ThreadPoolTimer PeriodicTimer =  ThreadPoolTimer.CreatePeriodicTimer(async(source)=> {  
   n++; 
   Debug.WriteLine("hello" + n);
   await dp.RefreshAsync(); //Function to refresh the data
   await Dispatcher.RunAsync(CoreDispatcherPriority.High,
                () =>
                {
                    bv.Text = "timer thread" + n;

                });

        }, period);

This is working properly. The only problem is that what if refresh function doesnot complete before its next instance is submitted to the thread pool. Is there some way to specify the gap between its execution.

Step 1 : Refresh function executes (takes any amount of time)

Step 2 : Refresh function completes its execution

Step 3 : Gap for 15mins then go to Step 1

Refresh function executes. 15mins after its execution ends, it executes again.

回答1:

The AutoResetEvent will solve this problem. Declare a class-level AutoResetEvent instance.

AutoResetEvent _refreshWaiter = new AutoResetEvent(true);

Then inside your code: 1. wait on it till it is signaled, and 2. pass its reference as an argument to RefreshAsync method.

TimeSpan period = TimeSpan.FromMinutes(15); 
   ThreadPoolTimer PeriodicTimer =  ThreadPoolTimer.CreatePeriodicTimer(async(source)=> {  
   // 1. wait till signaled. execution will block here till _refreshWaiter.Set() is called.
   _refreshWaiter.WaitOne();
   n++; 
   Debug.WriteLine("hello" + n);
   // 2. pass _refreshWaiter reference as an argument
   await dp.RefreshAsync(_refreshWaiter); //Function to refresh the data
   await Dispatcher.RunAsync(CoreDispatcherPriority.High,
                () =>
                {
                    bv.Text = "timer thread" + n;

                });

        }, period);

Finally, at the end of dp.RefreshAsync method, call _refreshWaiter.Set(); so that if 15 seconds have passed then the next RefreshAsync may be called. Note that if RefreshAsync method takes less than 15 minutes, the execution proceeds as normal.

回答2:

I think an easier way to do this is with async:

private async Task PeriodicallyRefreshDataAsync(TimeSpan period)
{
  while (true)
  {
    n++; 
    Debug.WriteLine("hello" + n);
    await dp.RefreshAsync(); //Function to refresh the data
    bv.Text = "timer thread" + n;
    await Task.Delay(period);
  }
}

TimeSpan period = TimeSpan.FromMinutes(15); 
Task refreshTask = PeriodicallyRefreshDataAsync(period);

This solution also provides a Task which can be used to detect errors.

来源：https://stackoverflow.com/questions/14950565/executing-a-function-periodically-after-the-function-completes-its-task