I have a fixed size custom OS X view that I load from a nib and want to centre
within a host view using auto layout.
Can this be done using VFL alone?
My best shot at it follows, but it is incorrect:
[self.window.contentView addConstraints:[NSLayoutConstraint
constraintsWithVisualFormat:@"|-(>=0)-[subview(width)]-(>=0)-|"
options:NSLayoutFormatAlignAllCenterY
metrics:@{@"width": @(width)}
views:@{@"subview":
self.subview}]];
[self.window.contentView addConstraints:[NSLayoutConstraint
constraintsWithVisualFormat:@"V:|-(>=0)-[subview(height)]-(>=0)-|"
options:NSLayoutFormatAlignAllCenterX
metrics:@{@"height": @(height)}
views:@{@"subview":
self.subview}]];
To achieve the desired effect:
a: The subview needs to acquire separate width and height constraints that
reference itself.
b: The contentView needs to acquire constraints that center the subview within
it.
The autolayout docs say:
The notation prefers good visualization over completeness of
expressibility.
There are constraints that cannot be expressed in visual format syntax,
although most of the constraints that are useful in real user interfaces can be.
I would have intuitively thought that this was doable.
Others
(http://stackoverflow.com/questions/12873372/centering-a-view-in-its-superview-using-visual-format-language)
seem divided on the question.
I personally cannot get a VFL only solution to work on OS X.
Notes:
1. I can achieve it simply in IB.
2. I can achieve it using explicit constraints like so:
- (void)addCenteredSubview:(NSView *)subview
{
// if translatesAutoresizingMaskIntoConstraints = YES then constraints will
be automatically added
// when the view is added to a supview. we require to constrain manually
set make sure the
// translation is off.
subview.translatesAutoresizingMaskIntoConstraints = NO;
// with the above off we will need to apply width + height contstraints
CGFloat width = subview.frame.size.width;
CGFloat height = subview.frame.size.height;
// add width and height constraints to the subview
[subview addConstraint:[NSLayoutConstraint
constraintWithItem:subview
attribute:NSLayoutAttributeWidth
relatedBy:NSLayoutRelationEqual
toItem:nil
attribute:NSLayoutAttributeNotAnAttribute
multiplier:1.0
constant:width]];
[subview addConstraint:[NSLayoutConstraint
constraintWithItem:subview
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:nil
attribute:NSLayoutAttributeNotAnAttribute
multiplier:1.0
constant:height]];
// the subview needs to be part of the view hierarchy before constraints
// can be applied that relate the superview and the subview
[self addSubview:subview];
[self addConstraint:[NSLayoutConstraint
constraintWithItem:subview
attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeCenterX
multiplier:1.0
constant:0]];
[self addConstraint:[NSLayoutConstraint
constraintWithItem:subview
attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeCenterY
multiplier:1.0
constant:0]];
}
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