Why is NSLayoutAttributeCenterX centering around 0 instead of container view center? - ios

Question

I am having a problem with NSLayoutAttributeCenterX that i can't figure out.
I created a NIB for a custom view and within this custom view I inserted a UILabel that I want to center in the x axis. And I am then inserting this custom view in my root ViewController's view.
Now, I overrided the "layoutSubviews" method of the custom view to put my constraints there to center the Label within the custom view.
I called the UILable inside this view "appTitle".
Below is the content of the "layoutSubviews" method of my custom view. As you can see there, I used the following constraint method to center the UILabel at the center of the custom view, in the x axis.
NSLayoutConstraint *titleConstraint_H = [NSLayoutConstraint constraintWithItem:self.appTitle attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self attribute:NSLayoutAttributeCenterX multiplier:1 constant:0];
But, the end result is that the UILabel is centered around x= 0 , instead of x = 160, which is the center of the custom view that contains it. I do not understand why this is happening.
In my simulation I am using an ipHone 5s, in PORTRAIT MODE , and I am setting the width of the custom view to be equal to the width of the phone's frame , which is 320. Thus x = 160 is the x coordinate of the center point.
Below is the code inside the "layoutSubviews"
LLDB output that shows the center of the appTitle label after the constraint has been enacted (po (CGPoint) [self.appTitle center])
LLDB output that shows the center of the custom view that contains the label "appTitle" (po (CGPoint)[self center])
and the final output
I wonder if someone has run into this problem too and has figured it out
- (void)layoutSubviews {
//######### TITLE ####################################
UIFont *font = [UIFont fontWithName:#"Avenir-Black" size:20.0];
UIColor *color = [UIColor blackColor];
NSDictionary *attrsDictionary = [NSDictionary dictionaryWithObjects:#[font,color] forKeys:#[NSFontAttributeName,NSForegroundColorAttributeName]];
NSAttributedString *titleString = [[NSAttributedString alloc] initWithString:#"WHINE-O-METER" attributes: attrsDictionary];
self.appTitle.attributedText = titleString;
// Adjust UILabel frame to size of text
[self.appTitle sizeToFit];
// Center title on horizontal length
NSLayoutConstraint *titleConstraint_H = [NSLayoutConstraint constraintWithItem:self.appTitle attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self attribute:NSLayoutAttributeCenterX multiplier:1 constant:0];
NSDictionary *viewsDictionary = #{ #"title":self.appTitle};
NSArray *titleConstraint_V = [NSLayoutConstraint constraintsWithVisualFormat:#"V:|-20-[title]-20-|" options:0 metrics:nil views:viewsDictionary];
[self addConstraints:titleConstraint_V];
[self addConstraint:titleConstraint_H];
}
thanks!

Answer 1

That's a lot of code to do something that can be done much more easily in your xib file. My recommendation is to remove all that code and set this up in your xib using auto layout. If you are using auto layout, don't call sizeToFit, just set leading and trailing constraints on the label that contains your text. You should not need to implement layoutSubviews to achieve this.

Answer 2

You should not be adding or modifying constraints in -layoutSubviews. From the docs:
You should override this method only if the autoresizing and constraint-based behaviors of the subviews do not offer the behavior you want.
Rather, you should set up constraints in an override of -updateConstraints. And don't forget to call through to super when you override methods like this. (That may actually contribute to your problem, too. Your override of -layoutSubviews does not call through to super.)