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I have been looking for descriptions of events "Preview******" like every element has events KeyDown and PreviewKeyDown. What is difference (not that one is attached event and one is not, the real conventional difference and programming way difference)
In any class derived from Control, you can override both methods.. OnKeyDown and OnPreviewKeyDown, now I am writing my custom control, which method shall I use? And whats difference between both of them.
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In a WPF application, events are often implemented as a tunneling/bubbling pair. So, you'll have a preview MouseDown and then a MouseDown event. Given below is a simple example of a Routed event in which a button and three text blocks are created with some properties and events.
The difference between the two, as the naming convention implies, is that a tunneling event will start at the highest node in the tree (probably the Window) and going down to the lowest child. A bubbling event will start at the child and then go upwards again.
A bubbling event begins with the element which triggers the event. Then it travels up the visual tree to the topmost element of the visual tree. So in WPF, the topmost element either could be a window or a usercontrol. Basically, an event bubbles up till it reached the topmost element.
From Programming WPF - Chris Sells and Ian Griffith
With the exception of direct events, WPF defines most routed events in pairs - one tunnelling and the other bubbling. The tunnelling event name always begins with 'Preview' and is raised first. This gives parents the chance to see the event before it reaches the child. This is followed by the bubbling counterpart. In most cases, you will handle only the bubbling one. The Preview would be usually used to
- block the event (
e.Handled = true)- cause the parent to do something in advance to normal event handling.
e.g. if UI Tree = Button contains Grid contains Canvas contains Ellipse
Clicking on the ellipse would result in (MouseDownButton is eaten up by Button and Click is raised instead.)
PreviewMouseDownButton PreviewMouseDownGrid PreviewMouseDownCanvas PreviewMouseDownEllipse MouseDownEllipse MouseDownCanvas MouseDownGrid If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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