How can I change constraints priority in run time
Swift version:
myContraint.priority = UILayoutPriority(999.0)
I want to make a small addition to Cyrille's answer.
If you are creating constraint in code, make sure to set its priority before making it active. For instance:
surveyViewHeightConstraint = [NSLayoutConstraint constraintWithItem:self
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:self.superview
attribute:NSLayoutAttributeHeight
multiplier:1
constant:0];
surveyViewHeightConstraint.active = YES;
surveyViewHeightConstraint.priority = 999;
This would result in run-time exception.
Mutating a priority from required to not on an installed constraint (or vice-versa) is not supported.
Correct order is:
surveyViewHeightConstraint.priority = 999;
surveyViewHeightConstraint.active = YES;
For Swift version 4+
constraintName.priority = UILayoutPriority(rawValue: 999)
As stated in NSLayoutConstraint
class reference:
Priorities may not change from nonrequired to required, or from required to nonrequired. An exception will be thrown if a priority of
NSLayoutPriorityRequired
in OS X orUILayoutPriorityRequired
in iOS is changed to a lower priority, or if a lower priority is changed to a required priority after the constraints is added to a view. Changing from one optional priority to another optional priority is allowed even after the constraint is installed on a view.
Use priority 999 instead of 1000. It won't be absolutely required technically speaking, but it'll be a higher priority than anything else.