UICollectionView auto layout constraint error - ios

Question

I have a simple UICollectionVewController with one prototype cell. In the cell all I have is a UILabel.
I have size classes and auto layout selected. I have set a constraint for
top (Top Space to cell): = 100
Height >= 17
Trailing space to Cell = 0
Leading Space to Cell = 0
In this configuration it works fine. However I want a left and right margin on the cell, so I changed the Leading and Trailing space to 10.
Although this works, I get the following error:
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:0x1740907c0 H:[UILabel:0x14de1a380'This is a test to see wha...']-(10)-| (Names: '|':UIView:0x14de1a270 )>",
"<NSLayoutConstraint:0x174090810 H:|-(10)-[UILabel:0x14de1a380'This is a test to see wha...'] (Names: '|':UIView:0x14de1a270 )>",
"<NSAutoresizingMaskLayoutConstraint:0x174090d10 h=--& v=--& H:[UIView:0x14de1a270(0)]>"
)
Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x1740907c0 H:[UILabel:0x14de1a380'This is a test to see wha...']-(10)-| (Names: '|':UIView:0x14de1a270 )>
Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.
If I set the leading space to 10 and remove the training space and set a width constraint, I do not get the error.
However, the width of the cell is variable so I can't set the width.
I can work around it, by creating an outlet to the width constraint and doing this:
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
MyCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:reuseIdentifier forIndexPath:indexPath];
CGRect colViewSize = collectionView.bounds;
int width = colViewSize.size.width;
width -= 28; // 10px either size and 8px in middle
width = width/2;
width -=14; // margin of 7 (x 2)
cell.labelconstraintWidth.constant = width;
return cell;
however this seems such a hack.
Does anyone have any clue what is happening here.

Answer 1

Simply put, your view resizes, and when it does, the constraints of 10 leading and 10 trailing space can't be satisfied because your view's width is less than 20.
Solution? Either put a percentual label margins / width to the superview or make the the margins zero, if you don't want to think about it.
However, if you have a encapsulated logic that calculates the cell width, you can easily add margin constraints from code.
EDIT:
You might have an autolayout constraint that's 50%, but that's not the issue here. You got an AutoResizingMaskLayoutConstraint that's affecting you view's width.
In order to create a percentual margin in storyboard, do:
1) Select your view in the view hierarchy
2) Hold CTRL and drag to your superview(or the root view that's taking the full bounds of the superview)
3) Select Horizontal Spacing
4) Select your view and double click on the Horizontal Spacing constraint
5) In the size inspector select:
First item: YourView.Leading
Relation: Equal
Second item: YourSuperview.Trailing
Constant: 0
Priority: 1000
Multiplier: (your margin percentage)
As #Bamsworld said, you should double check for AutoResizing masks. They might be conflicting with your AutoLayout constraints.

Answer 2

The error tells you one or more constraints is not needed and lists one of the constraints as NSAutoresizingMaskLayoutConstraint. It would appear that there is a conflict with auto-layout and autoresizingmasks. I suggest to use one or the other.
Try removing the autoresize-constraint by calling setTranslatesAutoresizingMaskIntoConstraints on the view and pass in NO. Also set the autoresizingMask property of the view to UIViewAutoresizingNone (just to be sure).