Mouse Input and the WPF - Mouse Input and Hit Testing
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WPF always takes the shapes of your elements into account when handling mouse input. Many graphical systems just use the rectangular bounding box of elements to perform hit testing (i.e., testing to see which element the mouse input “hit”). WPF does not employ this shortcut, no matter what shapes your elements may be. For example, if you create a donut-shaped control and click on the hole in the middle, the click will be delivered to whatever was visible behind your control through the hole.
Occasionally it is useful to subvert the standard hit testing behavior. You might wish to create a donut-shaped control with a visible hole, but which doesn’t let clicks pass through it. Alternatively, you might want to create an element that is visible to the user, but transparent to the mouse. WPF lets you do both of these things.
To achieve the first trick—transparent to the eye but opaque to the mouse—you can paint an object with a transparent brush. For example, anEllipsewith itsFillset toTransparent will be invisible to the eye, but not to the mouse. Alternatively, you can use a nontransparent brush, but make the whole element transparent by setting itsOpacityproperty to0. If a donut-shaped control paints such an ellipse over the hole, this enables it to receive any clicks on the hole. As far as the mouse is concerned, an element is a valid mouse target as long as it is painted with some kind of brush. The mouse doesn’t even look at the level of transparency on the brush, so it treats a completely transparent brush in exactly the same way as a completely opaque brush.
If you want a shape with a transparent fill that does not receive mouse input, simply supply noFillat all. For example, you might want the shape to have an outline but no fill. If theFillis null, as opposed to being a completely transparent brush, the shape will not act as an input target.
WPF supports the second trick—creating a visible object that is transparent to the mouse—with theIsHitTestVisibleproperty, which can be applied to any element. Setting this to false ensures that the element will not receive mouse input; instead, input will be delivered to whatever is under the element. For example, suppose you had written code to make some sort of graphical embellishment follow the mouse around, such as a semi-transparent ellipse to act as a halo for the pointer. SettingIsHitTestVisibleto false would ensure that this visual effect had no impact on the interactive behavior.
If you are using 3D (as described in Chapter 17), hit testing can be an expensive process. If you don’t require hit testing for your 3D content, making it invisible to hit testing can offer a useful performance boost.
Next: Mouse State >>
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 This article is excerpted from Programming WPF, Second Edition, written by Chris Sells and Ian Griffiths (O'Reilly, 2007; ISBN: 0596510373). Check it out today at your favorite bookstore. Buy this book now.
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