App crashing while adding constraints programatically - ios

Question

I am using visual format to define constraints. The goal is to place a UIView at the bottom of the super view say self.view with height fixed as 40, and width automatically. I have done this using Storyboard but I am unable to do it programatically. Here is the code what I have written. The app is crashing if i am not giving a fixed width. It is crashing with the following constraint: "H:[redView]|". If I change this to "H:[redView(100)]", it works. I don't want to use self.bounds and get width from there. It should stick from left side of super view, bottom and right side of the view.
Please help!
NSDictionary *viewsDictionary = #{#"redView":self.redView};
NSArray *constraint_H = [NSLayoutConstraint constraintsWithVisualFormat:#"V:[redView(40)]"
options:0
metrics:nil
views:viewsDictionary];
NSArray *constraint_V = [NSLayoutConstraint constraintsWithVisualFormat:#"H:[redView]|"
options:0
metrics:nil
views:viewsDictionary];
// 3. Define the redView Position
NSArray *constraint_POS_V = [NSLayoutConstraint constraintsWithVisualFormat:#"V:[redView]|"
options:0
metrics:nil
views:viewsDictionary];
NSArray *constraint_POS_H = [NSLayoutConstraint constraintsWithVisualFormat:#"H:|[redView]"
options:0
metrics:nil
views:viewsDictionary];
[self.redView addConstraints:constraint_H];
[self.redView addConstraints:constraint_V];
[self.view addConstraints:constraint_POS_H];
[self.view addConstraints:constraint_POS_V];

Answer 1

You need to add redView to its super view first.
Also, the constraints are to be added to the superview, not redView.
[self.view addSubview:self.redView];
// create your constraints here
[self.view addConstraints:#[constraint_H, constraint_V, constraint_POS_V, constraint_POS_H]];
[self.view updateConstraintsIfNeeded];

Answer 2

In the visual constraints syntax the | refers to the parent view. The way you had that horizontal constraint before, adding it to the redview, it was interpreted as redview had a child, called the same, which obviously did not exist, so it crashed.
The vertical constraint works the way it is because it does not reference the parent view.
You could change your code a bit so that the constraints are more clear:
NSArray *verticalConstraints = [NSLayoutConstraint constraintsWithVisualFormat:#"V:[redView(40)]|" options:0 metrics:nil
views:viewsDictionary];
NSArray *horizontalConstraints = [NSLayoutConstraint constraintsWithVisualFormat:#"H:|[redView]|" options:0 metrics:nil
views:viewsDictionary];
[self.view addConstraints:horizontalConstraints];
[self.view addConstraints:verticalConstraints];
Just remember you should always add the constraints for an UIView to it's superview.

Answer 3

Changing this line
[self.redView addConstraints:constraint_V];
with
[self.view addConstraints:constraint_V];
Solved myself. Thanks to me..:)

Answer 4

If you want to take the width of the view and apply it to red view, you only need to add a constraint like that:
NSArray *constraint_POS_H = [NSLayoutConstraint constraintsWithVisualFormat:#"H:|-[redView]-|"
options:0
metrics:nil
views:viewsDictionary];