iOS的自动布局：问题与调整大小的父视图UILabels [英] iOS Autolayout: Issue with UILabels in a resizing parent view

问题描述

我有一个只包含一个UILabel视图。这个标签包含多行文字。父具有可变宽度可与平移手势被调整。我的问题是，当我这样做调整大小的UILabel不会重新计算其高度，这样所有的内容仍然是可见的，它只是削减其关闭。

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.

我已经成功地与一个黑客位的修复它，但它是可怕的缓慢运行：

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];
}

不应该用这种自动版式自动发生？

Shouldn't this happen automatically with autolayout?

修改

我的观点的结构是：

UIScrollView
---> UIView
     ---> UILabel

下面是对的UIScrollView的约束：

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>,

下面是对的UIView的约束：

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>

该的UILabel本身上有没有限制，除了它钉到其父的边缘

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

推荐答案

我提出一个bug与苹果之后修复了这个问题。问题是多行文本需要两通的方法来布局，这一切依赖于财产 preferredMaxLayoutWidth

I've fixed this issue after raising a bug with Apple. The issue that multiline text requires a two-pass approach to layout and it all relies on the property preferredMaxLayoutWidth

下面是相关code需要被加入到包含多行标签视图控制器：

Here is the relevant code that needs to be added to a view controller that contains a multiline label:

- (void)viewWillLayoutSubviews
{
    // Clear the preferred max layout width in case the text of the label is a single line taking less width than what would be taken from the constraints of the left and right edges to the label's superview
    [self.label setPreferredMaxLayoutWidth:0.];
}

- (void)viewDidLayoutSubviews
{
    // Now that you know what the constraints gave you for the label's width, use that for the preferredMaxLayoutWidth—so you get the correct height for the layout
    [self.label setPreferredMaxLayoutWidth:[self.label bounds].size.width];

    // And then layout again with the label's correct height.
    [self.view layoutSubviews];
}

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