自动布局问题

Question

在视图控制器的根视图上有以下自动布局约束：

(lldb) po [superview constraints]
<__NSArrayM 0x9e65ba0>(
<NSLayoutConstraint:0x9e67d10 H:[HeaderView:0x9e6a1b0]-(0)-|   (Names: '|':UIView:0x9e69040 )>,
<NSLayoutConstraint:0x9e67ce0 H:|-(0)-[HeaderView:0x9e6a1b0]   (Names: '|':UIView:0x9e69040 )>,
<NSLayoutConstraint:0x9e67cb0 V:|-(0)-[HeaderView:0x9e6a1b0]   (Names: '|':UIView:0x9e69040 )>,
<NSLayoutConstraint:0x9e67c80 H:|-(0)-[RadialGradientView:0x9e685c0]   (Names: '|':UIView:0x9e69040 )>,
<NSLayoutConstraint:0x9e67c50 V:[RadialGradientView:0x9e685c0]-(0)-|   (Names: '|':UIView:0x9e69040 )>,
<NSLayoutConstraint:0x9e67c20 H:[RadialGradientView:0x9e685c0]-(0)-|   (Names: '|':UIView:0x9e69040 )>,
<NSLayoutConstraint:0x9e67bf0 V:[HeaderView:0x9e6a1b0]-(0)-[RadialGradientView:0x9e685c0]>,
<NSLayoutConstraint:0x10c89830 HeaderView:0x9e6a1b0.height == UIView:0x9e69040.height + 35>
)

当我添加高度约束(最后一个)时，我得到一个自动收费模糊布局错误：

2013-10-16 16:18:43.121 Application[26038:a0b] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSLayoutConstraint:0xb016da0 V:|-(0)-[HeaderView:0xb0192a0]   (Names: '|':UIView:0xb018130 )>",
    "<NSLayoutConstraint:0xb016d40 V:[RadialGradientView:0xb0176b0]-(0)-|   (Names: '|':UIView:0xb018130 )>",
    "<NSLayoutConstraint:0xb016ce0 V:[HeaderView:0xb0192a0]-(0)-[RadialGradientView:0xb0176b0]>",
    "<NSLayoutConstraint:0xb00c3f0 HeaderView:0xb0192a0.height == UIView:0xb018130.height + 35>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0xb016ce0 V:[HeaderView:0xb0192a0]-(0)-[RadialGradientView:0xb0176b0]>

Break on objc_exception_throw to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

RadialGradientView是UIView的子类，它只覆盖drawRect:方法。HeaderView也是UIView的子类，具有以下代码：

@implementation HeaderView
{
    NSLayoutConstraint  * heightConstraint;
}

static UINib * headerViewNib = nil;

- (id)awakeAfterUsingCoder:(NSCoder *)aDecoder
{
    if (![[self subviews] count])
    {
        if (headerViewNib == nil)
            headerViewNib = [UINib frameworkNibWithNibName: @"HeaderView"];

        SMDFHeaderView * headerView = [[headerViewNib instantiateWithOwner: nil options: nil] lastObject];

        [headerView setTranslatesAutoresizingMaskIntoConstraints: NO];
        [self removeConstraints: [self constraints]];

        return headerView;
    }
    return self;
}

- (void)layoutSubviews
{
    if (!heightConstraint)
        [self setNeedsUpdateConstraints];

    [super layoutSubviews];
}

- (void)updateConstraints
{
    if (!heightConstraint)
    {
        UIView * superview = [self superview];
        heightConstraint = [NSLayoutConstraint constraintWithItem: self attribute: NSLayoutAttributeHeight relatedBy: NSLayoutRelationEqual toItem: superview attribute:NSLayoutAttributeHeight multiplier: 1.0 constant: 35.0];
        [superview addConstraint: heightConstraint];
    }

    [super updateConstraints];
}

/* Tried that, but it does not change anything.
- (CGSize)intrinsicContentSize
{
    return CGSizeMake([[super superview] frame].size.width, 35);
}*/

@end

PS :这个问题似乎与nib替换技巧无关，即使我评论了awakeAfterUsingCoder方法，它也会做同样的事情。

Answer 1

您误解了这一行的工作方式：

heightConstraint = [NSLayoutConstraint constraintWithItem: self attribute: NSLayoutAttributeHeight relatedBy: NSLayoutRelationEqual toItem: superview attribute:NSLayoutAttributeHeight multiplier: 1.0 constant: 35.0];

这使得self等于superview的高度，乘以1，再加上35。这意味着您要求子视图比superview高35分，在其他地方也要求子视图与superview相匹配。那是行不通的。

要将视图约束到特定的高度，您需要使用如下内容：

heightConstraint = [NSLayoutConstraint constraintWithItem: self attribute: NSLayoutAttributeHeight relatedBy: NSLayoutRelationEqual toItem: nil attribute:NSLayoutAttributenNotAnAttribute multiplier: 1.0 constant: 35.0];

这只是将视图限制在特定的高度上。