WPF: Waiting for an ItemContainerGenerator to have its containers ready - wpf

Question

I'm writing a WPF control that contains an ItemsControl. The control adds and removes items based on certain user actions. Once an item has been added, the control needs to access a FrameworkElement inside the ItemTemplate instance that was just created.
I'm using ItemContainerGenerator.ContainerFromIndex to do this. I also get a ContentPresenter back, but it is empty: it appears it takes a few milliseconds on a separate thread to instantiate the template objects.
I read that I need to use ItemContainerGenerator.Status to determine whether or not the containers are fully created, so I wrote the following method:
private async Task<TextBox> GetMainInputControl(int index)
{
// _selectedItemsEditor is the ItemsControl inside my main control that contains the items
var evt = new ManualResetEvent(false);
_selectedItemsEditor.ItemContainerGenerator.StatusChanged += (sender, args) =>
{
var status = _selectedItemsEditor.ItemContainerGenerator.Status;
if (status == GeneratorStatus.ContainersGenerated || status == GeneratorStatus.Error)
{
evt.Set();
}
};
ContentPresenter container = null;
await Task.Run(() =>
{
var status = _selectedItemsEditor.ItemContainerGenerator.Status;
if (status == GeneratorStatus.GeneratingContainers
|| status == GeneratorStatus.NotStarted)
{
evt.WaitOne();
}
container =
_selectedItemsEditor.ItemContainerGenerator.ContainerFromIndex(index) as ContentPresenter;
});
return container?.ContentTemplate.FindName("PART_ItemEditorMainInput", container) as TextBox;
}
I know that there are a few things I need to fix here, but mostly, it just doesn't work, because _selectedItemsEditor.ItemContainerGenerator.Status immediately returns GeneratorStatus.ContainersGenerated, so the code doesn't wait - but then the code container?.ContentTemplate.FindName throws an exception indicating that the container is NOT ready.
How can I make this work, or alternatively use a better way of achieving this?

Answer 1

That code looks like you're trying to access ui controls on a background thread. So I'm not at all surprised it doesn't work.
There are two approaches I would consider.
You could defer your code so it waits until the dispatcher ( the ui thread essentially ) has done it's stuff for whatever you just asked it to do.
Application.Current.Dispatcher.InvokeAsync(new Action(() =>
{
// Your code which is to run after the items are rendered
}), DispatcherPriority.ContextIdle);
Or
You could force the layout process so you make the items do their thing. This will potentially lock the ui up whilst it's working. If the user clicks something and his obvious intent is to wait for layout to update or there's not so much going on then this won't be a problem.
You could just call .UpdateLayout() on your control.
https://msdn.microsoft.com/en-us/library/system.windows.uielement.updatelayout(v=vs.110).aspx