iOS 快速定位约束冲突

[LayoutConstraints] 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. 
(
    "<NSLayoutConstraint:0x6000005942d0 UIView:0x7ffc572062a0.height == 231   (active)>",
    "<NSLayoutConstraint:0x600000595680 V:|-(15)-[UIView:0x7ffc572062a0]   (active, names: '|':UIView:0x7ffc57206130 )>",
    "<NSLayoutConstraint:0x6000005955e0 V:[UIView:0x7ffc572062a0]-(20)-|   (active, names: '|':UIView:0x7ffc57206130 )>",
    "<NSLayoutConstraint:0x600000595540 UIView:0x7ffc57205fc0.height == 276   (active)>",
    "<NSLayoutConstraint:0x6000005954a0 V:[UIView:0x7ffc57206130]-(15)-|   (active, names: '|':UIView:0x7ffc57205fc0 )>",
    "<NSLayoutConstraint:0x600000595400 V:|-(15)-[UIView:0x7ffc57206130]   (active, names: '|':UIView:0x7ffc57205fc0 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x6000005942d0 UIView:0x7ffc572062a0.height == 231   (active)>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKitCore/UIView.h> may also be helpful.

快速定位约束冲突
一般简单界面冲突，我们很容易根据提示找到对应的 View, 但是当你视图层次很深（比如使用 CollectionView 多种 Cell 类型时），你就很难发现是哪个控件产生的冲突。

我们通过 lldb（控制台） 对象地址能快速找到对应控件。

没有断点怎么在 lldb 执行命令
在我们发现冲突的时候，界面可能并没有设置断点，或者不好设置断点来执行 lldb 命令操纵

此时我们只需要点击调试图层按钮,就可以进行命令执行了，不仅可以查看图层，打印视图信息，查看某个视图地址，还可以在 lldb 执行命令。 image.png

根据系统要删除的约束的控件地址，比如转化地址对象并输出属性

 (lldb) po ((UIView *)0x7fb43d20a6e0).subviews
<__NSArrayM 0x6000033baaf0>(
<UILabel: 0x7fb43d20a850; frame = (22 35; 42 21); text = 'Label'; opaque = NO; autoresize = RM+BM; userInteractionEnabled = NO; layer = <_UILabelLayer: 0x600001efcb40>>
)

通过打印子视图就可以看见 view 下面包含Label控件，从而方便定位。

tip: 建议这种情况，终端使用 Objective-C 语法。

根据地址转对象，打印子视图层级
我们通过打印图层层级就可以很容易判断冲突控件属于哪个 View

po [((UIView *)0x7fa5382061c0)  recursiveDescription]

结果

<UIView: 0x7fa5382061c0; frame = (0 15; 345 211); autoresize = RM+BM; layer = <CALayer: 0x600001e1d600>>
   | <UILabel: 0x7fa538206330; frame = (22 35; 42 21); text = 'Label'; opaque = NO; autoresize = RM+BM; userInteractionEnabled = NO; layer = <_UILabelLayer: 0x600003d19770>>

这里可以看到冲突控件下含有一个 带UILabel的UIView,就方便定为到目标视图，然后再去排查和解决冲突。

设置约束冲突断点
如果你记不住，还有一劳永逸的方法，直接捕获冲突断点，且设置打印 UIWindow 图层。

在发生冲突的时候直接借助对象地址定位到控件。

步骤：

1. 选中 show BreakPoint navigator
2. 选择左下角 + 添加 Symbolic BreakPoint
3. 在Symbo填入 UIViewAlertForUnsatisfiableConstraints
4. Action增加打印图层命令 po [[UIWindow keyWindow] _autolayoutTrace]
5. 建议勾选断点自动执行，否则每次发生冲突就会触发断点，且此时没有图层打印和定位冲突代码的能力。

注意：Xcode11及以上版本创建的 Swift 项目，由于 ScenceDelegate 的发布，取消了 keyWindow。使用 po [[[UIApplication sharedApplication] windows].firstObject recursiveDescription]或者po [[[UIApplication sharedApplication] windows].firstObject _autolayoutTrace]

Obj-C project

po [[UIWindow keyWindow] _autolayoutTrace]

Swift project

expr -l objc++ -O -- [[UIWindow keyWindow] _autolayoutTrace]

如下图：

image.png
如下图，直接通过搜索地址就可以发现问题控件 image.png
Then I paused execution image.png
and I changed problematic view's background color with the command (replacing 0x7f88a8cc2050 with the memory address of your object of course)...

Obj-C

expr ((UIView *)0x7f88a8cc2050).backgroundColor = [UIColor redColor]

Swift 3.0

expr -l Swift -- import UIKit
expr -l Swift -- unsafeBitCast(0x7f88a8cc2050, to: UIView.self).backgroundColor = UIColor.red

... and the result It was awesome!

image.png

Simply amazing!