How to add label programmatically and position it with auto layout programmatically - ios

Question

I'm trying add a label to a UIScrollView. I'm adding a top constraint and a leading constraint.
The label is not appearing.
Here is my code.
self.label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 40, 40)];
self.label.translatesAutoresizingMaskIntoConstraints = NO;
self.label.backgroundColor = [UIColor clearColor];
self.label.textAlignment = NSTextAlignmentCenter;
self.label.textColor=[UIColor whiteColor];
self.label.text = #"Hello";
[self.imageCarouselScrollView addSubview:self.label];
[self bringSubviewToFront:self.label];
self.titleLabelTopConstraint = [NSLayoutConstraint
constraintWithItem:self
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self.imageCarouselScrollView
attribute:NSLayoutAttributeBottom
multiplier:1.0
constant:20];
NSLayoutConstraint *titleLeadingConstraint = [NSLayoutConstraint
constraintWithItem:self
attribute:NSLayoutAttributeLeading
relatedBy:NSLayoutRelationEqual
toItem:self.imageCarouselScrollView
attribute:NSLayoutAttributeTrailing
multiplier:1.0
constant:20];
[self addConstraints:#[self.titleLabelTopConstraint,titleLeadingConstraint]];

Answer 1

You have to include a constraint to the right (for horizontal scrolling) or bottom (for vertical scrolling) of the scroll view so that it can deduce the content size.

Answer 2

I try comment your code:
self.label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 40, 40)]; //if 'translatesAutoresizingMaskIntoConstraints=NO' frame not work work anymore. Use CGRectZero instead
self.label.translatesAutoresizingMaskIntoConstraints = NO;
self.label.backgroundColor = [UIColor clearColor];
self.label.textAlignment = NSTextAlignmentCenter;
self.label.textColor=[UIColor whiteColor];
self.label.text = #"Hello";
[self.imageCarouselScrollView addSubview:self.label]; //Your self.label added on 'self.imageCarouselScrollView' but your constraints added on self
[self bringSubviewToFront:self.label]; // ??? Why?
self.titleLabelTopConstraint = [NSLayoutConstraint
constraintWithItem:self
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self.imageCarouselScrollView
attribute:NSLayoutAttributeBottom
multiplier:1.0
constant:20]; //it constrain means: "self view top edge equal to self.imageCarouselScrollView bottom + offset 20 "
NSLayoutConstraint *titleLeadingConstraint = [NSLayoutConstraint
constraintWithItem:self
attribute:NSLayoutAttributeLeading
relatedBy:NSLayoutRelationEqual
toItem:self.imageCarouselScrollView
attribute:NSLayoutAttributeTrailing
multiplier:1.0
constant:20];//it constrain means: "self view Leading edge equal to self.imageCarouselScrollView Trailing + offset 20 "
[self addConstraints:#[self.titleLabelTopConstraint,titleLeadingConstraint]];
And now i have few questions:
Why constraints for self.label?? Because after translatesAutoresizingMaskIntoConstraints=no it frame equals 0; You didn't see label anymore.
You have 2 constraints what about other edges????
No question, council, try use 'constraintsWithVisualFormat:options:metrics:views:'