Ios – Setting alpha on UIView sets the alpha on its subviews which should not happen

alpha-transparencyiosobjective cuiview

According to the documentation for UIVIew @property(nonatomic) CGFloat alpha

The value of this property is a floating-point number in the range 0.0
to 1.0, where 0.0 represents totally transparent and 1.0 represents
totally opaque.
This value affects only the current view and does not affect any of its embedded subviews.

I have a container view configured as follows:

self.myView.backgroundColor = [UIColor blackColor];
self.myView.alpha = 0.5;
[self addSubview:self.myView];

And then add subviews to 'myView'

[myView addSubView anotherView];
anotherView.alpha = 1;
NSLog(@"anotherView alpha = %f",anotherView.alpha); // prints 1.0000 as expected

But 'anotherView' does have alpha on screen (it is not opaque as expected)

How can this be and what can be done?

Best Answer

I think this is a bug in the documentation. You should file it at bugreport.apple.com.

Everything I can see after a bit of quick research suggests what you are seeing is how it always has behaved, and my own testing shows it too.

The alpha of a view is applied to all subviews.

Perhaps all you need is [[UIColor blackColor] colorWithAlphaComponent:0.5] but if not you will need to make the view a sibling instead of a child.

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