What is the impact of view.alpha = 0 vs view.hidden = YES?

Question

Transparency is evil, on iOS devices even more so than on heavier machines. I therefore figured to use view removeFromSuperView first, if not applicable view.hidden=YES and as a last resort view.alpha=0. But I actually don't know what's going on behind the scenes. Is there a difference, especially between the latter two?

I have a UIView animateWithDuration:animations:completion: scenario, where if you put hidden=YES in the completion block, it will hide without letting the animation block finish. Therefore I have to resort to alpha=0.

What are the penalties of the one over the other? Cheers, EP.

you shouldn't get a problem with it hiding before the animation has finished. But if you do, place the code within a check, e.g. `if (finished) { view.hidden = YES; }` — jowie, Apr 17 '13 at 16:20

score 76 · Accepted Answer · edited Jun 20 '20 at 09:12

I am not sure that a view with alpha 0.0 is still drawn. Check the documentation library:

Hiding Views

To hide a view visually, you can either set its hidden property to YES or change its alpha property to 0.0. A hidden view does not receive touch events from the system. However, hidden views do participate in autoresizing and other layout operations associated with the view hierarchy. Thus, hiding a view is often a convenient alternative to removing views from your view hierarchy, especially if you plan to show the views again at some point soon.

I also have found this answer here http://www.iphonedevsdk.com/forum/iphone-sdk-development/65525-whats-difference-between-alpha-0-hidden-yes.html

That says:

I believe that Cocoa Touch treats and alpha less than 0.02 as also being hidden, since below that alpha level it's invisible, and Apple's engineers decided that invisible controls should not be clickable.

Using an alpha value requires that the graphics hardware blend each pixel from the object with everything underneath. It's compute-intensive. The hidden flag, on the other hand, is a switch. If you turn it on, the OS knows it doesn't have to draw the object at all.

You have a little typo, less that 0.02 is considered as invisible. Thanks — ebtokyo, Dec 07 '12 at 05:45
You can omit the first `0` if you so desire, both are valid ways of typing out a number with only decimals. — Emil, Jan 04 '13 at 16:38
Well one main difference i have observed is in the accessibility Context, when the alpha is set to 0 the element is still accessible ( even if it is hidden). But when a views property is set true for isHidden, then the particular element is not accessible via accessibility. — Nikilesh Balaji, Aug 08 '22 at 21:47

score 5 · Answer 2 · answered Apr 12 '12 at 14:58

5

One advantage of setting alpha instead of the hidden property is that the alpha property plays more nicely with animations. I had an animation to show/hide my navigation bar and when I used the alpha property the navigation bar faded away while the hidden property was more abrupt.

answered Apr 12 '12 at 14:58

TPoschel

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Well, that's simply because `alpha` is animatable and `hidden` is not (try interpolating a boolean ;-) ). So, yes, you need to use alpha if you want to fade a view in/out. – Daniel Rinser Mar 04 '13 at 19:44
Another "advantage" with using `alpha` is that the subviews of the hidden view will still be shown, whereas they will be hidden if you use the `hidden`property. – turingtested Jan 05 '17 at 11:25

score 5 · Answer 3 · answered Feb 28 '11 at 20:10

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Something with an alpha of zero, still is drawn, however a view that is hidden is not redrawn to the screen. Since this only happens when the view changes anyways, the difference should be insignificant. If you are experiencing performance issues, I would highly recommend profiling with the time profiler and core animation instruments.

answered Feb 28 '11 at 20:10

Skyler Saleh

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That makes sense, thanks. You know whether the device loses time over compositing views if one of them has zero alpha? – epologee Mar 01 '11 at 21:20
@epologee Apple says to assume that it does. – Skyler Saleh Mar 01 '11 at 21:39
Alright, that makes a clear case. You wouldn't happen to have a link to where it says, for referencing? – epologee Mar 01 '11 at 22:07
Can confirm from my own experiences that hidden = YES is better on scrolling performance. – lorean Aug 28 '12 at 15:46

score 0 · Answer 4 · answered Dec 11 '20 at 10:00

Yes, Both will Act Same

Only difference would be if you want to hide some thing with animation like transparent view.

You can set it's alpha to 0.5 than in UIView.animate with spring set it's aplha to 0.

Rest both of them works the same.

What is the impact of view.alpha = 0 vs view.hidden = YES?

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