The opaque
property of a UIView is by default always set to "YES". But the UIView class reference states this:
An opaque view is expected to fill its bounds with entirely opaque content—that is, the content should have an alpha value of 1.0. If the view is opaque and either does not fill its bounds or contains wholly or partially transparent content, the results are unpredictable.
Since changing the alpha
of a view is quite common, especially during transitions or animations, then the above statement would imply that you must always manually set opaque
to NO
if you are going to change the alpha
property as well.
But I have never manually adjusted opaque
and haven't had any noticeable symptoms. How necessary is it to make this consideration?
Best Answer
The answer is that iOS is smart enough to recognize that if your view's
alpha
is less than 1, it needs to draw the content behind your view, regardless of your view'sopaque
property.In response to comments: from my limited experimentation, I don't think the view's
opaque
property has any effect. (I think the documentation is wrong.) The view's layer'sopaque
property does have an effect: it controls whether theCGContext
passed todrawRect:
has an alpha channel. If the layer'sopaque
property is YES, the context has no alpha channel (and is treated as if every pixel has alpha of 1.0).Changing the view's
opaque
property has no effect on the layer'sopaque
property. This is different than (for example) the view'salpha
property, which is just a wrapper for the layer'sopacity
property.In theory, having documented that the
opaque
property allows them to optimize drawing, Apple could implement that optimization in the future. In practice, doing so would probably break a lot of apps, so they probably won't make such a change apply to apps linked against older SDKs. (They have the ability to make UIKit behave differently depending on which version the app was linked with.)