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UIView performance: opaque, backgroundColor, clearsContextBeforeDrawing?

Asked 2011-11-05T03:13:06.853
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I'm displaying opaque PNGs with UIImageViews inside of a superview with a white background color. What's best for performance?

UIImageView Defaults

opaque = NO, backgroundColor = nil, clearsContextBeforeDrawing = YES.

iOS Developer Library: UIView Class Reference

  1. UIView Class Reference: backgroundColor says, "[nil] results in a transparent background color." If I set a UIViews opaque property to YES, must I also set its backgroundColor to [UIColor clearColor], or is that extra line of code & processing unnecessary? I.e., is [UIColor clearColor] considered opaque (not transparent)?

  2. Does the value of clearsContextBeforeDrawing matter for opaque views?

    The comments for clearsContextBeforeDrawing in UIView.h say it's ignored for opaque views.

    But, UIView Class Reference: clearsContextBeforeDrawing says:

    If the view’s opaque property is also set to YES, the backgroundColor property of the view must not be nil or drawing

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If you set UIView's opaque property to YES, you must ensure your drawing completely fills the view with no transparency. So, you need to set a white background colour (a clear one won't work, because that's not opaque).

I don't think you want to change the defaults for your image view, but you should ensure that your PNGs do not have any transparency (that is, have no alpha channel; you can check this in Preview's image inspector; ensure it doesn't say 'Has Alpha'.). I believe UIImageView will do the right thing if you give it an opaque image.

Depending on what's in the background, you might get better performance with opaque = YES and a white background for your image views, (to prevent redrawing parts of the parent view) but I wouldn't start that way.

answered 2011-11-19T16:10:45.100

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