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Programming iOS 4 by Matt Neuburg

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Visibility and Opacity

A view can be made invisible by setting its hidden property to YES, and visible again by setting it to NO. This takes it (and its subviews, of course) out of the visible interface without the overhead of actually removing it from the view hierarchy. A hidden view does not (normally) receive touch events, so to the user it really is as if the view weren’t there. But it is there, so it can still participate in layout and can be manipulated in other ways.

A view can be assigned a background color through its backgroundColor property, as we’ve been doing in the examples so far in this chapter — indeed, having a background color is the only thing that made our example views visible and distinguishable. A color is a UIColor; this is not a difficult class to use, and I’m not going to go into details. A view whose background color is nil has a transparent background. It is perfectly reasonable for a view to have a transparent background and to do no additional drawing of its own, just so that it can act as a convenient superview to other views, making them behave together.

A view can be made partially or completely transparent through its alpha property: 1.0 means opaque, 0.0 means transparent, and a value may be anywhere between them, inclusive. This affects subviews, as I’ve already mentioned; if a superview has an alpha of 0.5, none of its subviews can have an apparent opacity of more than 0.5, because whatever alpha value they have will be drawn relative to 0.5 ...

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