使用autolayout为包含子视图的视图设置动画

Question

This is the setup:

• A UIView created on Interface Builder, linked to an IBOutlet variable ( _vAbout )
• A constraint for this view that we want to animate, linked to an IBOutlet variable ( _ctrBottomAboutView )

I am using this code to animate:

_ctrBottomAboutView.constant = -100;
[UIView animateWithDuration:0.5 animations:^{
    [_vAbout layoutIfNeeded];
}

My problem is: whenever the view has any subviews in it, the animation doesn't work . However, if the view has no children, the animation works correctly.

Do you have any idea of a solution? I have tried everything: adding and removing constraints instead of modificating the constant value, adding constraints to the subviews on Interface Builder...

Answer 1

After some experiments starting from the ground with an empty project, this is what I've found:

Given A the view we want to animate and B its superview

• It's very important to keep in mind that the view that receives the layoutIfNeeded message is the view that owns the constraint .
• In the case of NSLayoutAttributeWidth and NSLayoutAttributeHeight the owner of the constraint is actually A, but in all the other cases, the view that owns the constraint is B

HOWEVER

• If A does not have any subviews, we can call [A layoutIfNeeded] at any time on our code and it will work
• If A has one or more subviews but we start the animation on viewDidLoad we can call [A layoutIfNeeded] and it will work