How to update constraint in code using objective c

Question

In my xib file, I have a constraint for height for the label in my xib file. And in my objective c, I tried to change it using

self.heightConstraint.constant = newHeight;

But nothing is changed when I run it in simulator. I have used debugger and make sure that line is executed. And in Spark tool, I see the label height is the old height.

How can I adjust the height dynamically?

Updated:

I have changed my code to add a nil check.

if (self.heightConstraint != nil) {
self.heightConstraint.constant = newHeight;
}

My code still get executed, but nothing get changed.

possible duplicate of [Creating layout constraints programmatically](http://stackoverflow.com/questions/12826878/creating-layout-constraints-programmatically) — rebello95, Jan 26 '15 at 19:19
Have you tried adding `[self setNeedsLayout]; [self layoutIfNeeded];`? — Ian MacDonald, Jan 26 '15 at 19:29
I have added[self setNeedsLayout]; [self layoutIfNeeded]; after I update the heightConstraint. But what does not help either. — n179911, Jan 26 '15 at 20:09
Your updated code does not help: if self.heightConstraint == nil, your update code does nothing, exactly the same as without your update code. Add an else {assert(NO);} and see, if it stops there! — Reinhard Männer, Jan 26 '15 at 20:34

score 0 · Accepted Answer · answered Jan 26 '15 at 19:25

There's nothing wrong with your code itself:

self.heightConstraint.constant = newHeight;

But the reason this changes nothing in your interface, even though it is being executed, is that self.heightConstraint is not a reference to a constraint in your interface. (It is probably nil.)

score 0 · Answer 2 · answered Jan 26 '15 at 19:30

I agree with Matt. The likely reason your code isn't working is that you have a broken outlet link. In general, when code that tries to do something with an outlet or action doesn't work, the most likely cause is a broken outlet or action connection.

Rule of thumb: When debugging stereo or computer hardware, check the cables.

Corollary for for iOS or Mac OS apps in Xcode: When debugging UI code, check your IBOutlet and IBAction connections.

score 0 · Answer 3 · answered Jan 26 '15 at 19:42

0

At what part of the view controller lifecycle are you changing the constraints? You may need to call setNeedsLayout on the parent view.

answered Jan 26 '15 at 19:42

psobko

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How to update constraint in code using objective c

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