Using iOS Auto Layout to establish constraints between a child view controller and its parent view

Question

Within my root view controller, I'm adding a child view controller's view as a subview, as follows:

ChildViewController *cvc = [[ChildViewController alloc] init];
[self addChildViewController:cvc];
[self.view addSubview:cvc.view];
[cvc didMoveToParentViewController:self];

and I'd now like to use a NSLayoutConstraint to position the cvc.view within the parent view (self.view), such that cvc.view is positioning 25 pts above the bottom of the parent view. My understanding is that the following should work:

UIView *superview = self.view;
UIView *childview = cvc.view;
NSLayoutConstraint *cn = 
    [NSLayoutConstraint withItem:childview 
      attribute:NSLayoutAttributeBottom 
      relatedBy:NSLayoutRelationEqual 
      toItem:superview attribute:NSLayoutAttributeBottom 
      multiplier: 1.0 
      constant: -25.0];
[superview addConstraint: cn];

But the constraint fails at runtime. I thought initially maybe the autoresizing mask in the child view was causing problems (and following the WWDC 2012 Intro video on auto layout), so I set [childview setTranslatesAutoresizingMaskIntoConstraints:NO], but then the childview simply fails to appear.

What am I doing wrong?

Answer 1

I'm not sure but the following should work, or something very similar:

UIView *superview = self.view;
UIView *childview = cvc.view;
NSDictionary *constrainedViews = NSDictionaryOfVariableBindings(childview);
NSArray *constraints =
[superview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[childview]-25-|"
                                                                  options:0
                                                                  metrics:nil
                                                                    views:constrainedViews];

If not make sure that you are actually setting a size for the childview, something like:

UIView *superview = self.view;
UIView *childview = cvc.view;
NSDictionary *constrainedViews = NSDictionaryOfVariableBindings(childview);
NSArray *constraints =
[superview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[childview]|"
                                                                  options:0
                                                                  metrics:nil
                                                                    views:constrainedViews];

Say, to make it fill the width of the view. Or:

UIView *superview = self.view;
UIView *childview = cvc.view;
NSDictionary *constrainedViews = NSDictionaryOfVariableBindings(childview);
NSArray *constraints =
[superview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[childview(>100,<304)]-|"
                                                                  options:0
                                                                  metrics:nil
                                                                    views:constrainedViews];

Which means something like childview should have a width bigger than 100 but less than 304 with default margins to the superview. Please note I don't know if the above constraint actually makes sense (e.g. it may just always give you 304 width childview as that would leave default margins), but it serves as an example.