Centre the button in view use constraints programmatically in iOS 8 - ios

Question

I have a subview and added 1 button. I want to add this button in the center of that view. I use Autolayout so i need to set the constraints for this button programmatically.
I tried this code but the play button is not in the center.
[self.moviePlayer.view addSubview:self.playbtn];
self.playbtn.translatesAutoresizingMaskIntoConstraints = NO;
[self.moviePlayer.view addConstraint:[NSLayoutConstraint constraintWithItem:self.playbtn attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual
toItem:self.moviePlayer.view attribute:NSLayoutAttributeCenterX multiplier:0.667 constant:0]];
Please help me to correct it. Thanks in advance.

Answer 1

You can use the NSLayoutAttributeCenterX and NSLayoutAttributeCenterY attributes to center your play button like this:
- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
[self initViews];
[self initConstraints];
}
-(void)initViews
{
self.picture = [[UIImageView alloc] init];
self.picture.image = [UIImage imageNamed:#"bg"];
self.playButton = [[UIButton alloc] init];
[self.playButton setImage:[UIImage imageNamed:#"playButton"] forState:UIControlStateNormal];
[self.view addSubview:self.picture];
[self.view addSubview:self.playButton];
}
-(void)initConstraints
{
self.picture.translatesAutoresizingMaskIntoConstraints = NO;
self.playButton.translatesAutoresizingMaskIntoConstraints = NO;
id views = #{
#"picture": self.picture,
#"playButton": self.playButton
};
// picture constraints
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"H:|[picture]|" options:0 metrics:nil views:views]];
[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:#"V:|[picture]|" options:0 metrics:nil views:views]];
// play button constraints
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.playButton attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0.0]];
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.playButton attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0.0]];
}
You get something like this:

Answer 2

You can use this code to center any item (es. self.btn) in its superview:
[self.btn.superview addConstraint:[NSLayoutConstraint
constraintWithItem:self.btn.superview
attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationEqual
toItem:self.btn
attribute:NSLayoutAttributeCenterX
multiplier:1.0
constant:0.0]];
[self.btn.superview addConstraint:[NSLayoutConstraint
constraintWithItem:self.btn.superview
attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationEqual
toItem:self.btn
attribute:NSLayoutAttributeCenterY
multiplier:1.0
constant:0.0]];
I also wrote an article about aligning any UIView objects without using Storyboards on my blog.