WPF进度栏正常工作，但即使使用异步也被阻止在UI线程中

Question

I am trying to implement an indeterminate progress bar into my program. I'm new to threading, but as far as I know one of the best options here is to add an async method, and await the "heavy" function to perform its results. So I wrote this:

public void Window_Loaded(object sender, RoutedEventArgs e)
{
    firstLoad();                         
}

private async void firstLoad()
{
    LW.Title = "Loading...";
    LW.Show();

    filterTextBox.Text = defaultSearch;
    await Task.Run(() => InitializeFilter());          
}

private void InitializeFilter()
{

//Asynchronous??? 
Dispatcher.BeginInvoke(new Action(() => {

//... some lines of code that takes some time to run. 

dataGrid.ItemContainerGenerator.StatusChanged += new EventHandler(closeLoadingWindow);

}));

private void closeLoadingWindow(object sender, EventArgs e)
{
    if (LW != null)
    {
        LW.closable = true;
        LW.Close();
    }
}

firstLoad runs when the window is loaded, showing an indeterminate LW loadingWindow, and running the InitializeFilter() method (the heavy one). Finally, when the grid is populated and loaded, an event fires, allowing the LW window to be closed and closing it (if I didn't make it unclosable, a funny user could just close it clicking or using F4, which is not nice).

The system is working properly and everything works as expected regarding time frames, but the loading bar is frozen, not showing progress. The same LW bar works in the MainWindow with a similar set up What am I missing? Thanks in advance!

Answer 1

as far as I know one of the best options here is to add an async method, and await the "heavy" function to perform its results

The best option is to use Task.Run to move the heavy processing to the thread pool, and use await to retrieve its results.

The code as it currently stands uses Task.Run to move to the thread pool and then immediately turns around and uses Dispatcher to move back to the UI thread before doing the heavy processing. Thus, it's blocking the UI thread.

what this particular DataGrid displays is a CollectionView, which is not thread-safe.

Right, you can't update data-bound objects from a thread pool thread.

The best solution is to separate the heavy processing from the UI updates, something like this:

public async void Window_Loaded(object sender, RoutedEventArgs e)
{
  await firstLoadAsync();
}

private List<FilterType> InitializeFilter()
{
  //... some lines of code that takes some time to run. 
}

private async Task firstLoadAsync()
{
  LW.Title = "Loading...";
  LW.Show();

  filterTextBox.Text = defaultSearch;
  var filterData = await Task.Run(() => InitializeFilter()); // Get the plain data on a background thread
  myCollectionView = new CollectionView(filterData); // Update the UI
  if (LW != null)
  {
    LW.closable = true;
    LW.Close();
  }
}

Answer 2

do not use your dispatcher. Microsoft had the foresight to use it's magic (SynchronizationContext) to be able to update the UI thread in a method that is being executed in an async context. This is demonstrated in their async/await example found here

while under previous/other circumstances, you would have to either marshal back to the main (UI) thread to update the UI thread, or wait until completed and retrieve the results from objects who share state. Since you are using async/await then you should be fine to not use the dispatcher, and update the UI directly.