How can I change constraints priority in run time

Question

I have a view which has dynamic height and I am trying to change this view height priority in run time.

Here is my part of code;

if (index == 0) {      surveyViewHeightConstraint.constant = 0;     surveyViewHeightConstraint.priority = 1000;  } else if (index == 1) {      surveyViewHeightConstraint.constant = 163;     surveyViewHeightConstraint.priority = 500;  } 

I am changing index with a button action. When I run this code, I am getting this error:

*** Assertion failure in -[NSLayoutConstraint setPriority:], /SourceCache/Foundation/Foundation-1141.1/Layout.subproj/NSLayoutConstraint.m:174 

What is my mistake in here?

Answer 1

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 or UILayoutPriorityRequired 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.

Answer 2

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)