Setting Constraints programmatically different from setting them in IB?

Question

I am new to Auto Layout in iOS. I really like the concept in principle, but it's driving me nuts trying to get the simplest things done. I suspect I'm still missing some simple underlying principle. I am trying to learn by doing and get the basics right before actually working with it in an app, so I'm creating very simple test projects. Here's one as simple as it gets that doesn't work as expected. First the part that works. In IB, I add a View to fill the entire viewcontroller, and XCode automatically sets the constraints to Top/Bottom/Leading/Trailing and the Space to 0. When done with IB, it works as expected:

enter image description here

rotates to

enter image description here

Great!

Now I am trying to do the same thing in code:

- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
UIView *redView = [[UIView alloc] initWithFrame:self.view.bounds];
redView.backgroundColor = [UIColor redColor];
[self.view addSubview:redView];

redView.translatesAutoresizingMaskIntoConstraints = NO;

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0f constant:0.0f]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeading multiplier:1.0f constant:0.0f]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeTrailing relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0f constant:0.0f]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeading multiplier:1.0f constant:0.0f]];
}

That's all the code there is other than the default code for a Single-view application.

When I run the above code, trying to mirror the same thing as I get with IB programmatically, I get this after rotation:

enter image description here

How come that the same constraints lead to different results? It's probably something really simple and embarrassingly stupid that I am missing. Help!!!

Sasi · Accepted Answer · 2015-12-25 15:47:17Z

I know this is old, but I think the issue with your constraints was that you were confusing what trailing etc. represent. I've amended your code to the following and it now fills the screen as expected:

       UIView *redView = [[UIView alloc] initWithFrame:self.view.bounds];
redView.backgroundColor = [UIColor redColor];
[self.view addSubview:redView];

redView.translatesAutoresizingMaskIntoConstraints = NO;

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0f constant:0.0f]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeading multiplier:1.0f constant:0.0f]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeTrailing relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTrailing multiplier:1.0f constant:0.0f]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0f constant:0.0f]];

You were setting the trailing constraint equal to the bottom (trailing is actually the right hand side not the bottom and you were relating the top constraint to the left hand side (leading)

score 6 · Accepted Answer · 2013-06-21 20:37:41Z

6

Here's how I would do this:

[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[redView]|"
                                                                  options:0
                                                                  metrics:nil
                                                                    views:NSDictionaryOfVariableBindings(redView)]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[redView]|"
                                                                  options:0
                                                                  metrics:nil
                                                                    views:NSDictionaryOfVariableBindings(redView)]];

AutoLayout in IB can be a pain and I found that familiarizing myself with the Visual Format Language was less work than constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:

edited Jun 21, 2013 at 20:37

answered Jun 21, 2013 at 19:47

user1078170

5 Comments

user1459524 Over a year ago

Thanks! That works (after adding the missing | in V:|[redView]... but why the heck doesn't my other code work??? It's equivalent to what IB shows, isn't it???

user1078170 Over a year ago

Yes, I was missing a pipe there. Fixed. And, to be honest, I don't know. I found VFL and looked no further for my AutoLayout needs.

shinyuX Over a year ago

Hi guys. 6 months later i encountered the same problem, but i want to do that with mycontroller.view directly, not a view inside mycontroller.view. And it just does ... nothing !

Raptor Over a year ago

@AmitBattan Yes, you can: developer.apple.com/library/ios/documentation/UserExperience/…

Raptor Over a year ago

@shinyuX i know this's old (hope you've already solved the problem), in order to use these constraints made in codes, you have to set translatesAutoresizingMaskIntoConstraints to NO.

Ben Wheeler · Accepted Answer · 2013-06-22 00:53:30Z

1

Instead of Leading and Trailing, use Top and Bottom. Note that you are mixing these up in original post. Leading and Trailing seem to confuse people!

answered Jun 22, 2013 at 0:53

Ben Wheeler

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