UICollectionView Cell + UiLabel with AutoLayout

问题

I'm trying to pin an UILabel to it's parent cell. I added four constraints (top, leading, trailing, bottom) which works fine on iOS 8.0 but not on iOS 7.X. Please see image below:

[Click here for full size]

What am I doing wrong? Please advise!

EDIT #1

It seems it's only broken since Xcode 6 GM. My approach was working fine in Xcode 6 beta 7.

Moreover, if I decrease the inner view's width, it throws the following warning:

2014-09-10 19:58:28.109 Test[57827:60b] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSLayoutConstraint:0x799573a0 H:|-(8)-[UIView:0x798a86e0]   (Names: '|':UIView:0x798ae5d0 )>",
    "<NSLayoutConstraint:0x799573d0 H:[UIView:0x798a86e0]-(43)-|   (Names: '|':UIView:0x798ae5d0 )>",
    "<NSAutoresizingMaskLayoutConstraint:0x798a8b00 h=--& v=--& H:[UIView:0x798ae5d0(50)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x799573d0 H:[UIView:0x798a86e0]-(43)-|   (Names: '|':UIView:0x798ae5d0 )>

Break on objc_exception_throw to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

回答1:

Overriding the custom cell's layoutSubviews is a possible workaround:

override func layoutSubviews() {
    contentView.frame = bounds
    super.layoutSubviews()
}

回答2:

UIView:0x798ae5d0 is the contentView of the CollectionViewCell. Somehow at a certain moment it uses the UICollectionViewCells defaultSize, which is (50.0, 50.0).

<NSAutoresizingMaskLayoutConstraint:0x798a8b00 h=--& v=--& H:[UIView:0x798ae5d0(50)]>

As your horizontal margins 8 + 43 = 51 are bigger than the contentView (50) it is impossible to satisfy the layout.

<NSLayoutConstraint:0x799573a0 H:|-(8)-[UIView:0x798a86e0]   (Names: '|':UIView:0x798ae5d0 )>
<NSLayoutConstraint:0x799573d0 H:[UIView:0x798a86e0]-(43)-|  (Names: '|':UIView:0x798ae5d0 )>

One can make the layout more flexible, so that it also works on a (50.0, 50.0) size. In your case by changing equal 43 to <= 43 or by reducing a priority of 1000 to 750 or 999.

回答3:

The problem is that your custom view (UILabel) has constraints, which conflict with cell's (or better cell's contentView's) constraints. The cell's NSAutoresizingMaskLayoutConstraint are created automatically from what you set in UICollectionView properties in xib (or storyboard) as Cell Size. I have solved my similar problem (*) by explicitly setting

- (void)awakeFromNib {

    [super awakeFromNib];

    self.contentView.translatesAutoresizingMaskIntoConstraints = NO;
}

in my custom UICollectionViewCell subclass. This gets rid of cell size constraint set in Storyboard.

(*) Disclaimer: My collectionView has self sizing cells based on their content view, which is defined by autolayout. I had warnings about conflicting constraints of my content autolayout, and explicit size in Storyboard. This helped me to get rid of those warnings.

回答4:

Instead of giving four constraints (top, leading, trailing, bottom). Try top, leading, width and height. It should work.

回答5:

I hope you have added constraints to the collection with respect to it's parent (which would be the view of the UIViewController) so that it's width and height is equal to the parent view.

回答6:

Just one more possibility that I found in my project.

Usually I copy/paste similar objects, but doing that, sometimes, holds the same ID.
So I fixed my constraint problem by creating an object from scratch.

来源：https://stackoverflow.com/questions/25771026/uicollectionview-cell-uilabel-with-autolayout