Center custom UIView vertically and horizontally using Auto Layout

Question

I'm trying to build a rather simple animated custom UI using the Auto Layout API newly available iOS 6. The custom view I'm building has a circle that I want to be both vertically and horizontally centered.

Unfortunately I can't figure out why my constraints appear to work fine for UIButton, and UILabel elements but yield weird results when I use a custom view with and custom CALayer (in this case a circle, that will eventually be animated).

To be clear I don't want my view to expand to fill the whole screen, but rather to have dynamic "padding" so that the view is vertically centered both on the iPhone 4 and 5. I should also note that I'm very new to Cocoa and UIKit.

RootViewController.m:

...
- (void)viewDidLoad {
    [super viewDidLoad];

    // Create Circle View
    CGRect circle_view_rect = CGRectMake(0, 0, 100, 100);
    UIView *circle_view = [[UIView alloc] initWithFrame:circle_view_rect];

    // Create Circle Layer
    CircleLayer *circle_layer = [[CircleLayer alloc] init];
    circle_layer.needsDisplayOnBoundsChange = YES;
    circle_layer.frame = circle_view.bounds;
    [circle_view.layer addSublayer:circle_layer];

    // Enable Auto Layout
    [circle_view setTranslatesAutoresizingMaskIntoConstraints:NO];

    [self.view addSubview:circle_view];

    // Center Vertically
    NSLayoutConstraint *centerYConstraint =
    [NSLayoutConstraint constraintWithItem:circle_view
                attribute:NSLayoutAttributeCenterY
                relatedBy:NSLayoutRelationEqual
                   toItem:self.view
                attribute:NSLayoutAttributeCenterY
               multiplier:1.0
                 constant:0.0];
    [self.view addConstraint:centerYConstraint];

    // Center Horizontally
    NSLayoutConstraint *centerXConstraint =
    [NSLayoutConstraint constraintWithItem:circle_view
                attribute:NSLayoutAttributeCenterX
                relatedBy:NSLayoutRelationEqual
                   toItem:self.view
                attribute:NSLayoutAttributeCenterX
               multiplier:1.0
                 constant:0.0];
    [self.view addConstraint:centerXConstraint];
}
...

CircleLayer.m:

...
- (void)drawInContext:(CGContextRef)context {
    CGContextAddArc(context, 50, 50, 50, 0.0, 2*M_PI, 0);
    CGContextSetFillColorWithColor(context, [UIColor yellowColor].CGColor);
    CGContextFillPath(context);
}
...

Basically the constraints I've implemented are:

• center vertically inside parent view
• center horizontally inside parent view

And this is the result I get:

Any help would be greatly appreciated, I've been pondering this one for a few days now.

Thanks

Answer 1

Try adding a height and width constraint to your circle_view. I couldn't even get just a pain square view to appear at all without adding those (using your code, minus the layer stuff).

NSLayoutConstraint *heightConstraint =
    [NSLayoutConstraint constraintWithItem:circle_view
                                 attribute:NSLayoutAttributeHeight
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:nil
                                 attribute:NSLayoutAttributeNotAnAttribute
                                multiplier:1.0
                                  constant:100.0];
    [circle_view addConstraint:heightConstraint];

    NSLayoutConstraint *widthConstraint =
    [NSLayoutConstraint constraintWithItem:circle_view
                                 attribute:NSLayoutAttributeWidth
                                 relatedBy:NSLayoutRelationEqual
                                    toItem:nil
                                 attribute:NSLayoutAttributeNotAnAttribute
                                multiplier:1.0
                                  constant:100.0];
    [circle_view addConstraint:widthConstraint];

Answer 2

Just to add to rdelmar's answer:

The core issue is that as soon as you go the NSLayoutConstraint route, and specify setTranslatesAutoresizingMaskIntoConstraints:NO , the frame you made with CGRectMake is rendered irrelevant for AutoLayout purposes. That's why it didn't use the info from the frame's height and width.