Wednesday, May 25, 2011

single-threaded UI update - wpf vs swing

In a WPF app every visual "element" object has a "owning" thread. It can be modified only by the owning thread. This sounds like "every child has a mother". Indeed WPF lets you have 2 owning threads if you have 2 unrelated window instances in one Process[3]. In practice, most developers avoid that unnecessary flexibility, and use a single owning thread -- known as a dispatcher thread.

[3] each having a bunch of visuals.

In WPF, Anytime you update a Visual "element", the work is sent through the Dispatcher to the UI thread. The control itself can only be touched by it's owning thread. If you try to do anything with a control from another thread, you'll get a runtime exception.

myCheckBox.Dispatcher.Invoke(
          System.Windows.Threading.DispatcherPriority.Normal,
          new Action(
            delegate(){myCheckBox.IsChecked = true;}
          )
        );
Very similar to invokeAndWait(new Runnable().... There's also a counterpart for invokeLater().

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