Why is NSLayoutAttributeCenterX an 'Invalid pairing' with NSLayoutAttributeWidth - ios6

Question

I am trying to have the auto layout manager adjust the center point of a view, based on the width of the super view. I don't understand why that is an 'Invalid Pairing' of the attributes (as told by the crash and NSInvalidArgumentException)
UIView *ac;
NSLayoutConstraint *cXloc = [NSLayoutConstraint constraintWithItem:ac
attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationEqual
toItem:ac.superview
attribute:NSLayoutAttributeWidth
multiplier:.1
constant:x*ac.superview.frame.size.width*.2];
[ac.superview addConstraint:cXloc];
Could someone explain why this is an 'Invalid paring' and how I should approach this?
Thanks

Answer 1

It is the limitation of the current implementation of Auto Layout. However, you can easily work around it since all the constrains are linear and NSLayoutAttributes are correlated. For example, say, the constraint you want is:
subview.centerX = m * superview.width + c;
You can express it as a relationship between tow centerXs:
// Since width == 2 * centerX
subview.centerX = m * 2 * superview.centerX + c;

Answer 2

If you relate ac's AttributeCenterX to its superview's AttributeCenterX, AttributeLeading, or AttributeTrailing, you should be able to express your desired constraint using the multiplier and constraint. Keep in mind that the constant is evaluated only when the constraint is created, and your example's constant wouldn't update as ac.superview's width changes.
If you can express in words how you'd like ac positioned relative to its superview, we can suggest a constraint.
Edit
Here's an example with 5 NSButtons. They themselves and the space between them expand so that the spaces are 30% as wide as the buttons, all the buttons have the same width, and all the spaces have the same width. Creating 4 invisible NSViews just for spacing is pretty cumbersome, especially considering you've got it working outside of autolayout. But in case you're curious:
// Assuming these NSViews and NSButtons exist,
//NSView* superview ;
//NSButton *buttonOne, *buttonTwo, *buttonThree, *buttonFour, *buttonFive ;
[superView removeConstraints:superView.constraints] ;
// Create empty NSViews to fill the space between the 5 buttons.
NSView* spaceOne = [NSView new] ;
NSView* spaceTwo = [NSView new] ;
NSView* spaceThree = [NSView new] ;
NSView* spaceFour = [NSView new] ;
spaceOne.translatesAutoresizingMaskIntoConstraints = NO ;
spaceTwo.translatesAutoresizingMaskIntoConstraints = NO ;
spaceThree.translatesAutoresizingMaskIntoConstraints = NO ;
spaceFour.translatesAutoresizingMaskIntoConstraints = NO ;
[superView addSubview:spaceOne] ;
[superView addSubview:spaceTwo] ;
[superView addSubview:spaceThree] ;
[superView addSubview:spaceFour] ;
NSDictionary* views = NSDictionaryOfVariableBindings(superView,buttonOne,buttonTwo,buttonThree,buttonFour,buttonFive,spaceOne,spaceTwo,spaceThree,spaceFour) ;
// Vertically align buttonOne to its superview however you like.
[superView addConstraints: [NSLayoutConstraint constraintsWithVisualFormat:#"V:|-[buttonOne]" options:0 metrics:nil views:views ] ] ;
// Make the "space" NSViews' widths equal and >= 10. Make the buttons' widths equal.
[superView addConstraints: [NSLayoutConstraint constraintsWithVisualFormat:#"H:|[buttonOne][spaceOne(>=10)][buttonTwo(==buttonOne)][spaceTwo(==spaceOne)][buttonThree(==buttonOne)][spaceThree(==spaceOne)][buttonFour(==buttonOne)][spaceFour(==spaceOne)][buttonFive(==buttonOne)]|" options: NSLayoutFormatAlignAllCenterY metrics:nil views:views ] ] ;
// Make the "space" NSViews' widths 30% of the NSButtons' widths.
[superView addConstraint: [NSLayoutConstraint constraintWithItem: spaceOne
attribute: NSLayoutAttributeWidth
relatedBy: NSLayoutRelationEqual
toItem: buttonOne
attribute: NSLayoutAttributeWidth
multiplier: 0.3
constant: 0 ] ] ;

Answer 3

Based on an0's answer, and assuming you have an NSArray containing your buttons, the following should space the buttons equally within the superview:
NSUInteger currentButton = 1;
for (UIButton *button in self.buttons)
{
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:button attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:2.0 * (CGFloat) currentButton / (CGFloat) (self.buttons.count + 1) constant:0.0]];
currentButton++;
}

Answer 4

If you're looking to have your programmatically generated views to fit its superview's width. You can use the constraint pairing of NSLayoutAttributeLeading and attribute:NSLayoutAttributeCenterX
You have to make the proper calculation in order to get the right multiplier. Calculation involves the total count of the views to be laid out and the index of the current view.
//Caculate constraint multiplier from parentView CenterX.
//This sets the width of the button relative to parentView.
// A value of 2 = Full width.
CGFloat multiplier = 2/(arr.count/counter);
[parentView addConstraint:[NSLayoutConstraint constraintWithItem:btn
attribute:NSLayoutAttributeTrailing
relatedBy:NSLayoutRelationEqual toItem:parentView
attribute:NSLayoutAttributeCenterX multiplier:multiplier constant:0]];
This will distribute the views width to fill its superview.