ios – 在约束依赖于框架的自定义视图中使用自动布局

- (void)updateConstraints {
    [self.superview addConstraint:[NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.superview attribute:NSLayoutAttributeCenterX multiplier:1 constant:0]];

    [self.superview addConstraint:[NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributetop relatedBy:NSLayoutRelationEqual toItem:self.superview attribute:NSLayoutAttributetop multiplier:1 constant:0]];
    [self.superview addConstraint:[NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.superview attribute:NSLayoutAttributeBottom multiplier:1 constant:0]];

    // some more constraints,you get the point

    self.bottomSpacingConstraint = [NSLayoutConstraint constraintWithItem:self.imageView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self attribute:NSLayoutAttributeBottom multiplier:1 constant:-(0.2 * CGRectGetHeight(self.bounds))];
    [self addConstraint:self.bottomSpacingConstraint];

    [super updateConstraints];
}

问题是self.bounds返回CGRectZero的等价物.我做了我的研究并根据这个objc.io article,这是预期的,因为在调用layoutSubviews之前框架不会被设置.它也提到了

To force the system to update the layout of a view tree immediately,you can call layoutIfNeeded/layoutSubtreeIfNeeded (on iOS and OS X respectively). This can be helpful if your next steps rely on the views’ frame being up to date.

但是,当我添加

[self setNeedsLayout];
[self layoutIfNeeded];

在updateConstraints中设置self.bottomSpacingConstraint之前,我仍然得到一个CGRectZero返回框架.根据objc.io文章(和本SO answer),这些方法应该触发布局并更新框架.

任何人都可以对如何使这一切工作有所启发吗？我对解决方案感兴趣,并解释了导致调用布局相关方法的原因(例如,看起来在layoutSubviews中更改现有约束的常量会导致调用setNeedsUpdateConstraints,然后触发updateConstraints并导致约束要多次添加).