Patrick Perini Patrick Perini - 2 months ago 10
iOS Question

Centering subview's X in autolayout throws "not prepared for the constraint"

I have a custom UIView subclass which is being initialized via a nib.

In

-awakeFromNib
, I'm creating a subview and attempting to center it in its superview.

[self setInteralView: [[UIView alloc] init]];
[[self internalView] addConstraint: [NSLayoutConstraint constraintWithItem: [self internalView]
attribute: NSLayoutAttributeCenterX
relatedBy: NSLayoutRelationEqual
toItem: self
attribute: NSLayoutAttributeCenterX
multiplier: 1
constant: 0]];


This breaks, and causes the following output:

2013-08-11 17:58:29.628 MyApp[32414:a0b] The view hierarchy is not prepared for the constraint: <NSLayoutConstraint:0xc1dcc80 UIView:0xc132a40.centerX == MyView:0xc1315a0.centerX>
When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. Break on -[UIView _viewHierarchyUnpreparedForConstraint:] to debug.
2013-08-11 17:58:29.630 MyApp[32414:a0b] View hierarchy unprepared for constraint.
Constraint: <NSLayoutConstraint:0xc1dcc80 UIView:0xc132a40.centerX == MyView:0xc1315a0.centerX>
Container hierarchy:
<UIView: 0xc132a40; frame = (0 0; 0 0); clipsToBounds = YES; layer = <CALayer: 0xc132bc0>>
View not found in container hierarchy: <MyView: 0xc1315a0; frame = (-128 -118; 576 804); layer = <CALayer: 0xc131710>>
That view's superview: <UIView: 0xc131a70; frame = (0 0; 320 568); autoresize = W+H; layer = <CALayer: 0xc131b50>>

Answer

As the error states, you must add the constraint to a view such that the views involved in the constraint are the view you're adding it to or a subview of that view. In the case of your code above, you should be adding that constraint to self, rather than [self internalView]. The reason it doesn't work as written is one of the views involved in the constraint (self) is not in the view hierarchy if you consider only [self internalView] and below).

For more details, see the discussion section of the documentation on the addConstraint: method.

Here is your line of code, fixed as I suggest:

UIView* internalView = [[UIView alloc] initWithFrame:CGRectZero];
internalView.translatesAutoresizingMaskIntoConstraints = NO;
[self setInternalView:internalView];

[self addConstraint: [NSLayoutConstraint constraintWithItem: [self internalMapView]
                                         attribute: NSLayoutAttributeCenterX
                                         relatedBy: NSLayoutRelationEqual
                                         toItem: self
                                         attribute: NSLayoutAttributeCenterX
                                         multiplier: 1
                                         constant: 0]];