Question

This question is similar to another one I asked, but whereas in that case I wanted to know about forcing a binding to update from XAML or a view model, in this case I’d like to know about wrapping this up into a custom WPF FrameworkElement.

The functionality I’m after is a span of text that indicates how long ago something happened.

<TextBlock Text="It happened " />
<my:AgeTextBlock SinceTime="{Binding OccurredAtUtc}" />
<TextBlock Text=" ago" />

This would render as (for example):

It happened 1 min 13 sec ago

I have code that converts from a TimeSpan to the human-readable form shown.

In order to have the UI update every second I’m considering using a static DispatcherTimer (idea from Kent Boogaart’s answer).

So here’s what I have:

public class AgeTextBlock : TextBlock
{
    private static readonly DispatcherTimer _timer;

    static AgeTextBlock()
    {
        _timer = new DispatcherTimer { Interval = TimeSpan.FromSeconds(1) };
    }

    public AgeTextBlock()
    {
        _timer.Tick += ...;
    }

    // How can I unsubscribe from the Tick event to avoid a memory leak?
}

The comment indicates my problem. I can’t see how I’d clean up properly with this approach. There’s no Dispose method to override and remove the event handler.

What is the recommended pattern here?

Answer 1

Why not subscribe to the Tick event in the Loaded event of the control, and unsubscribe in the Unloaded event ?