auto layout modify multiplier of constraint programmatically

Question

How can I modify the multiplier of a constraint programmatically? I have set the following:

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:_button attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0.0]];

and I need to modify it to this:

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:_button attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeWidth multiplier:1.0 constant:0.0]];

Solution 1

As others said, you have to delete the old one and add a new one. If you do not want to store it as a property, just set identifier and search to get it later

-(NSLayoutConstraint *)constraintWithIndientifer:(NSString *)identifer InView:(UIView *)view{
    NSLayoutConstraint * constraintToFind = nil;
    for (NSLayoutConstraint * constraint in view.constraints ) {
        if([constraint.identifier isEqualToString:identifer]){
            constraintToFind = constraint;
            break;
        }
    }
    return constraintToFind;
}

Then

NSLayoutConstraint * constraint = [NSLayoutConstraint constraintWithItem:self.yellowView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0.0];
constraint.identifier = @"1234";
[self.view addConstraint:constraint];

Then you can get this

NSLayoutConstraint * constraint = [self constraintWithIndientifer:@"1234" InView:self.view];

Solution 2

You won't be able to change the multiplier because it is readonly. You would have to store a reference to the constraint. Then when you want to change the multiplier you would remove the constraint and add a new one.

You could include a method like this in your view controllers implementation:

- (void)changeMultiplier:(NSLayoutConstraint **)constraint to:(CGFloat)newMultiplier{
    NSLayoutConstraint *newConstraint = [NSLayoutConstraint constraintWithItem:(*constraint).firstItem attribute:(*constraint).firstAttribute relatedBy:(*constraint).relation toItem:(*constraint).secondItem attribute:(*constraint).secondAttribute multiplier:newMultiplier constant:(*constraint).constant];
    [self.view removeConstraint:(*constraint)];
    [self.view addConstraint:newConstraint];
    *constraint = newConstraint;
}

Related Questions:

Can i change multiplier property for NSLayoutConstraint?

Solution 3

For modifying multiplier I use switching between constraints method.
You must Create constraints with different priorities else it will break the layout.

Objective C

NSLayoutConstraint *constraint1, *constraint2;

// switch between constraints

constraint1.active = NO; 
constraint2.active = YES;
[self.view layoutIfNeeded];

Swift 2.0

self.constraint1.active = true
self.constraint2.active = false
self.view.layoutIfNeeded()

Solution 4

There's an easier way to modify the multiplier of a constraint programmatically using Layout Anchors:

imageView.heightAnchor.constraint(equalTo: imageView.widthAnchor, multiplier: 1.0/2.0).isActive = true

Here's how it works:

View more solutions

Comments

[Jason Bestor]

Thanks, I can't believe that question didn't come up in my searches. Maybe I just glossed over it.

[Jason Bestor]

Thanks for the suggestions. I ended up using the constant instead. to keep it simple... self.constraint = [NSLayoutConstraint constraintWithItem:_button attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:(self.view.frame.size.width / 2)]; [self.view addConstraint:_newsWidthConstraint]; and then later I edited it to make it full width... self.constraint.constant = self.view.frame.size.width;

[TheJeff]

How do I pass in the constraint? This would make this answer clearer for implementation

[Guru Dev]

This worked fine for me easily. I think the easiest way.