Getting an existing NSLayoutConstraint for the width?

Question

I'm trying to animate a control in Cocoa with auto layout.

Now, I can set [[constraint animator] setConstant:newWidth];, which works. But how can I get the right constraint?

With [self constraints] you can get all the constraints, and in this case I can just select constraints[0], but the order of the constraints may vary.

How can I be certain I always have the right constraint? The constraints are set in Interface Builder. I have seen that you can add a IBOutlet to it, but it doesn't seem necessary.

---

My solution

Thanks, it worked great. I wrote a little category.

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NSView+NSLayoutConstraintFilter.h

#import <Cocoa/Cocoa.h>

@interface NSView (NSLayoutConstraintFilter)
- (NSLayoutConstraint *)constraintForAttribute:(NSLayoutAttribute)attribute;
- (NSArray *)constaintsForAttribute:(NSLayoutAttribute)attribute;
@end

---

NSView+NSLayoutConstraintFilter.m

#import "NSView+NSLayoutConstraintFilter.h"

@implementation NSView (NSLayoutConstraintFilter)

- (NSArray *)constaintsForAttribute:(NSLayoutAttribute)attribute {
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"firstAttribute = %d", attribute];
    NSArray *filteredArray = [[self constraints] filteredArrayUsingPredicate:predicate];

    return filteredArray;
}

- (NSLayoutConstraint *)constraintForAttribute:(NSLayoutAttribute)attribute {
    NSArray *constraints = [self constaintsForAttribute:attribute];

    if (constraints.count) {
        return constraints[0];
    }

    return nil;
}

@end

Answer 1

Every contraint has an attribute [constraint firstAttribute] It returns an enum NSLayoutAttribute

typedef NS_ENUM(NSInteger, NSLayoutAttribute) {
    NSLayoutAttributeLeft = 1,
    NSLayoutAttributeRight,
    NSLayoutAttributeTop,
    NSLayoutAttributeBottom,
    NSLayoutAttributeLeading,
    NSLayoutAttributeTrailing,
    NSLayoutAttributeWidth,
    NSLayoutAttributeHeight,
    NSLayoutAttributeCenterX,
    NSLayoutAttributeCenterY,
    NSLayoutAttributeBaseline,

    NSLayoutAttributeNotAnAttribute = 0
};

so you can check NSLayoutAttributeWidth for width.

Sample code:

NSArray constraints = [self constraints];
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"firstAttribute = %d", NSLayoutAttributeWidth];
NSArray *filteredArray = [constraints filteredArrayUsingPredicate:predicate];
if(filteredArray.count == 0){
      return nil;
}
NSLayoutConstraint *constraint =  [constraints objectAtIndex:0];

Answer 2

Here is the swift 3 version tested on Xcode 8.2.1 and macOS 10.12.2.

The code shows how to get a button's width and height constraints, but you could filter whatever you want from NSLayoutAttribute enum.

let cons = signInButton.constraints.filter {
    $0.firstAttribute == NSLayoutAttribute.width || $0.firstAttribute == NSLayoutAttribute.height /// or other from `NSLayoutAttribute`
}
// do something with the constraints array, e.g.
NSLayoutConstraint.deactivate(cons)