关于Autolayout的调试
本文算是截取了Blog部分，主要是记录调试的方法，防止下次碰到同样问题而重新查找。
刚开始使用Autolayout遇到下面的警告人容易让人气馁。经常不知所措而放弃了使用Autolayout。
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)
(...........)
Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.正如输出中所述，Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger，现在介绍下使用UIViewAlertForUnsatisfiableConstraints的调试方法。
在UIViewAlertForUnsatisfiableConstraints添加symbolic breakpoint：
- 打开
断点导航（cmd+8） - 点击左下角的+按钮
- 选择Add Symbolic Breakpoint
- 在Symbol添加UIViewAlertForUnsatisfiableConstraints
再次调试的时候就可以通过LLDB来调试了，然并卵，如果你不知道LLDB的话。
所以交给你一个小技巧，添加po [[UIWindow keyWindow] _autolayoutTrace]（OC项目）或expr -l objc++ -O -- [[UIWindow keyWindow] _autolayoutTrace]（Swift项目）。
这样就可以直接看到输出：
(lldb) po [[UIWindow keyWindow] _autolayoutTrace]
UIWindow:0x7f9481c93360
| •UIView:0x7f9481c9d680
| | *UIView:0x7f9481c9d990- AMBIGUOUS LAYOUT for UIView:0x7f9481c9d990.minX{id: 13}, UIView:0x7f9481c9d990.minY{id: 16}
| | *_UILayoutGuide:0x7f9481c9e160- AMBIGUOUS LAYOUT for _UILayoutGuide:0x7f9481c9e160.minY{id: 17}
| | *_UILayoutGuide:0x7f9481c9ebb0- AMBIGUOUS LAYOUT for _UILayoutGuide:0x7f9481c9ebb0.minY{id: 27}其中AMBIGUOUS相关的视图就是约束有问题的。0x7f9481c9d990就是有问题视图的首地址。
当然进一步的调试需要LLDB的命令。比如
打印视图对象：
(lldb) po 0x7f9481c9d990
<UIView: 0x7f9481c9d990; frame = (0 0; 768 359); autoresize = RM+BM; layer = <CALayer: 0x7fc82d338960>>改变颜色：
(lldb) expr ((UIView *)0x174197010).backgroundColor = [UIColor redColor]
(UICachedDeviceRGBColor *) $4 = 0x0000000174469cc0## 标题 ##