Center SubViews in ParentView in Objective-C - ios

Question

I have the following code:
UIView *parentView = [[UIView alloc] init];
parentView.frame = CGRectMake(0, 0, 100, 100);
parentView.center = self.view.center;
parentView.backgroundColor = [UIColor greenColor];
UIView *subView = [[UIView alloc] init];
subView.backgroundColor = [UIColor redColor];
subView.frame = CGRectMake(0, 0, 50, 50);
subView.center = parentView.center;
[parentView addSubview:subView];
[self.view addSubview:parentView];
Which produces the following result:
Why is the red view not centered in the green view since they have the same center?

Answer 1

The Apple UIView documentation states for the center property:
The center is specified within the coordinate system of its superview and is measured in points. Setting this property changes the values of the frame properties accordingly.
That means the parentView center will be relative to its superview (the white background view by the looks of your screenshot).
To get the desired result you need to do something like this:
subview.center = CGPointMake(CGRectGetMidX(parentView.bounds),
CGRectGetMidY(parentView.bounds));
But you should really be using autolayout for this type of thing.

Answer 2

You are setting the green view to be the center of it's superview, but you are setting the red view to be in the center of it's superview's superview...
The way to fix that with the smallest amount of code change would be to change the red view's superview.
[self.view addSubview:parentView];
[self.view addSubview:subView];
EDIT: To explain a little further what is going on. Say when you set the center of your green view it gets set to (500,500). That's inside it's superview, so it gets set to the middle of the screen. Then you set your red view's center to the same value as the green view's center, (500,500). But it's parent that it is relative to is the red view, so it gets placed (500,500) from the origin point of the green view. It gets placed near the bottom right of the screen.

Answer 3

You are Doing Like this
UIView *parentView = [[UIView alloc] init];
parentView.frame = CGRectMake(0, 0, 100, 100);
parentView.center = self.view.center;
parentView.backgroundColor = [UIColor greenColor];
UIView *subView = [[UIView alloc] init];
subView.backgroundColor = [UIColor redColor];
CGFloat SubviewX = (parentView.frame.size.width - 50)/2;
CGFloat SubviewY = (parentView.frame.size.height - 50)/2;
subView.frame = CGRectMake(SubviewX, SubviewY, 50, 50);
[parentView addSubview:subView];
[self.view addSubview:parentView];

Answer 4

The best way to center a view, and maintain it that way even if the view changes its view frame is using constraints:
UIView *parentView = [[UIView alloc] init];
parentView.translatesAutoresizingMaskIntoConstraints = NO;
// You might want to add constraints for the view's size
parentView.frame = CGRectMake(0, 0, 100, 100);
parentView.backgroundColor = [UIColor greenColor];
[self.view addSubview:parentView];
NSContraint *xContraint = [NSLayoutConstraint constraintWithItem:parentView
attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeCenterX
multiplier:1.0
constant:0.0];
[self.view addConstraint:xContraint];
NSContraint *yContraint = [NSLayoutConstraint constraintWithItem:parentView
attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeCenterY
multiplier:1.0
constant:0.0];
[self.view addConstraint:yContraint];
UIView *subView = [[UIView alloc] init];
subView.translatesAutoresizingMaskIntoConstraints = NO;
// You might want to add constraints for the view's size
subView.frame = CGRectMake(0, 0, 50, 50);
subView.backgroundColor = [UIColor redColor];
[parentView addSubview:subView];
NSContraint *xSubContraint = [NSLayoutConstraint constraintWithItem:subView
attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationEqual
toItem:parentView
attribute:NSLayoutAttributeCenterX
multiplier:1.0
constant:0.0];
[parentView addConstraint:xSubContraint];
NSContraint *ySubContraint = [NSLayoutConstraint constraintWithItem:subView
attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationEqual
toItem:parentView
attribute:NSLayoutAttributeCenterY
multiplier:1.0
constant:0.0];
[parentView addConstraint:ySubContraint];