ios6 - iOS Autolayout: Issue with UILabels in a resizing parent view

Question

I've got a view that contains only a UILabel. This label contains multiline text. The parent has a variable width that can be resized with a pan gesture. My problem is that when I do this resizing the UILabel does not recalculate its height such that all of the content is still visible, it simply cuts it off.

I've managed to fix it with a bit of a hack but it is horribly slow to run:

- (void)layoutSubviews {

    CGSize labelSize = [self.labelDescription sizeThatFits:CGSizeMake(self.frame.size.width, FLT_MAX)];

    if (self.constraintHeight) {
        [self removeConstraint:self.constraintHeight];
    }

    self.constraintHeight = [NSLayoutConstraint constraintWithItem:self.labelDescription attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:labelSize.height];

    [self addConstraint:self.constraintHeight];

    [super layoutSubviews];
}

Shouldn't this happen automatically with autolayout?

EDIT

The structure of my view is:

UIScrollView
---> UIView
     ---> UILabel

Here are the constraints on the UIScrollView:

<NSLayoutConstraint:0x120c4860 H:|-(>=32)-[DescriptionView:0x85c81c0]   (Names: '|':UIScrollView:0x85db650 )>,
<NSLayoutConstraint:0x120c48a0 H:|-(32@900)-[DescriptionView:0x85c81c0] priority:900   (Names: '|':UIScrollView:0x85db650 )>,
<NSLayoutConstraint:0x120c48e0 H:[DescriptionView:0x85c81c0(<=576)]>,
<NSLayoutConstraint:0x120c4920 H:[DescriptionView:0x85c81c0]-(>=32)-|   (Names: '|':UIScrollView:0x85db650 )>,
<NSLayoutConstraint:0x120c4960 H:[DescriptionView:0x85c81c0]-(32@900)-| priority:900   (Names: '|':UIScrollView:0x85db650 )>,
<NSLayoutConstraint:0x8301450 DescriptionView:0x85c81c0.centerX == UIScrollView:0x85db650.centerX>,

Here are the constraints on the UIView:

<NSLayoutConstraint:0x85c4580 V:|-(0)-[UILabel:0x85bc7b0]   (Names: '|':DescriptionView:0x85c81c0 )>,
<NSLayoutConstraint:0x85c45c0 H:|-(0)-[UILabel:0x85bc7b0]   (Names: '|':DescriptionView:0x85c81c0 )>,
<NSLayoutConstraint:0x85c9f80 UILabel:0x85bc7b0.trailing == DescriptionView:0x85c81c0.trailing>,
<NSLayoutConstraint:0x85c9fc0 UILabel:0x85bc7b0.centerY == DescriptionView:0x85c81c0.centerY>

The UILabel itself has no constraints on it, aside from pinning it to the edges of its parent

Answer 1

Okay, I finally nailed it. The solution is to set the preferredMaxLayoutWidth in viewDidLayoutSubviews, but only after the first round of layout. You can arrange this simply by dispatching asynchronously back onto the main thread. So:

- (void)viewDidLayoutSubviews {
    dispatch_async(dispatch_get_main_queue(), ^{
        self.theLabel.preferredMaxLayoutWidth = self.theLabel.bounds.size.width;
    });
}

That way, you don't set preferredMaxLayoutWidth until after the label's width has been properly set by its superview-related constraints.

Working example project here:

https://github.com/mattneub/Programming-iOS-Book-Examples/blob/master/ch23p673selfSizingLabel4/p565p579selfSizingLabel/ViewController.m

EDIT: Another approach! Subclass UILabel and override layoutSubviews:

- (void) layoutSubviews {
    [super layoutSubviews];
    self.preferredMaxLayoutWidth = self.bounds.size.width;
}

The result is a self-sizing label - it automatically changes its height to accommodate its contents no matter how its width changes (assuming its width is changed by constraints / layout).