在 iOS 中以编程方式更改 NSLayoutConstraint 的关系

Question

I have a UITextView at the bottom of screen.
It has leading, trailing bottom space constraints to the superview .
It has also height constraint with value (constant) 40 .

I want to make this text view of flexible height as I go on typing more text in it. I can make flexible height by changing height constraint value to greater than or equal to as:

It works fine and text view height goes on increasing.

But when I make it, storyboard show this error:

I want to make constraint value greater than or equal to programmatically, not from storyboard.

I know that there is one property of NSLayoutConstraint called

public var relation: NSLayoutRelation { get }

but as we can see it is readonly.

How can I change this relation to one of:

public enum NSLayoutRelation : Int {   
    case LessThanOrEqual
    case Equal
    case GreaterThanOrEqual
}

Or is there any workaround make a view of flexible height in auto-layout based storyboard?

---

I know about making IBOutlet of constraint and changing it's constant value, but in my situation I can not do this, it should be changed dynamically.

Answer 1

Related to: auto layout modify multiplier of constriaint programmatically

With ARC:

- (void)changeConstraintRelationToEqual:(NSLayoutConstraint * __strong *)constraint{
    NSLayoutConstraint *newConstraint = [NSLayoutConstraint constraintWithItem:(*constraint).firstItem attribute:(*constraint).firstAttribute relatedBy:NSLayoutRelationEqual toItem:(*constraint).secondItem attribute:(*constraint).secondAttribute multiplier:(*constraint).multiplier constant:(*constraint).constant];
    [self.view removeConstraint:(*constraint)];
    [self.view addConstraint:newConstraint];
    *constraint = newConstraint;
}

Answer 2

我过去解决了这个问题，方法是在接口构建器中将我需要在运行时设置的约束标记为“占位符 - 在构建时删除”，然后通过代码添加约束以编程方式提供基于动态值的约束。