学习AutoLayout(VFL中“-”的使用)

上篇文章说的tableViewCell的问题，其实不是，是我VFL的语法写错了，下面请看下这段代码：

//设置autoLayout
NSDictionary *viewsDictionary = @{@"titleLabel":self.titleLabel, @"subLabel":self.subLabel, @"bgScrollView":self.bgScrollView};
//约束1 横向 Horizontal
[self.contentView addConstraints:
 [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-10-[titleLabel]|"
                                         options:0
                                         metrics:nil
                                           views:viewsDictionary]];
[self.contentView addConstraints:
 [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-10-[subLabel]|"
                                         options:0
                                         metrics:nil
                                           views:viewsDictionary]];
[self.contentView addConstraints:
 [NSLayoutConstraint constraintsWithVisualFormat:@"H:|[bgScrollView]|"
                                         options:0
                                         metrics:nil
                                           views:viewsDictionary]];
    
//约束2 纵向 Vertical
NSString *formatV = @"V:|-2-[titleLabel(28)]-[subLabel(15)]-4-[bgScrollView]|";
[self.contentView addConstraints:
 [NSLayoutConstraint constraintsWithVisualFormat:formatV
                                         options:0
                                         metrics:nil
                                           views:viewsDictionary]];

第一句代码是和
NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings(view);一样的，但是用NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings(view);会有括号里面传属性crash的问题，原因就是self.。
解释下代码实现的效果
titleLabel距离父视图左边10点，右边和父视图对齐。
subLabel距离父视图左边10点，右边和父视图对齐。
bgScrollView和父视图等宽。
titleLabel距离父视图上边2点高度为28，下边和subLabel上边对齐，subLabel高度为15，和bgScrollView距离4点，bgScrollView和父视图下边对齐。
这段代码看起来没有问题，但是会报错。

    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:0x7ff2ec18d060 V:|-(2)-[UILabel:0x7ff2ec1b9d10'\U4e94\U9053\U53e3\U5546\U5708']   (Names: '|':UITableViewCellContentView:0x7ff2ec192230 )>",
    "<NSLayoutConstraint:0x7ff2ec19f560 V:[UILabel:0x7ff2ec1b9d10'\U4e94\U9053\U53e3\U5546\U5708'(28)]>",
    "<NSLayoutConstraint:0x7ff2ec19f5b0 V:[UILabel:0x7ff2ec1b9d10'\U4e94\U9053\U53e3\U5546\U5708']-(NSSpace(8))-[UILabel:0x7ff2ec191cb0'\U534e\U6e05\U5609\U56ed | \U84dd\U65d7\U8425 | \U9038\U6210\U4e1c\U82d1\n']>",
    "<NSLayoutConstraint:0x7ff2ec19f600 V:[UILabel:0x7ff2ec191cb0'\U534e\U6e05\U5609\U56ed | \U84dd\U65d7\U8425 | \U9038\U6210\U4e1c\U82d1\n'(15)]>",
    "<NSLayoutConstraint:0x7ff2ec19f650 V:[UILabel:0x7ff2ec191cb0'\U534e\U6e05\U5609\U56ed | \U84dd\U65d7\U8425 | \U9038\U6210\U4e1c\U82d1\n']-(4)-[UIScrollView:0x7ff2ec19e6e0]>",
    "<NSLayoutConstraint:0x7ff2ec19f6a0 V:[UIScrollView:0x7ff2ec19e6e0]-(0)-|   (Names: '|':UITableViewCellContentView:0x7ff2ec192230 )>",
    "<NSLayoutConstraint:0x7ff2e9f02660 'UIView-Encapsulated-Layout-Height' V:[UITableViewCellContentView:0x7ff2ec192230(49.5)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x7ff2ec19f5b0 V:[UILabel:0x7ff2ec1b9d10'xxxxx']-(NSSpace(8))-[UILabel:0x7ff2ec191cb0'xxxxx | xxxxx | xxxxx
']>

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.

错误是这样的：

没有办法同时满足所有约束.
下面列出的约束中,至少有一个是你不想要的.试着这么做:(1)查看每个约束,然后想明白哪个你不需要(2)找到你不想添加约束的代码,修改下.
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) 

是这一个约束冲突了：

"<NSLayoutConstraint:0x7ff2e9f02660 'UIView-Encapsulated-Layout-Height' V:[UITableViewCellContentView:0x7ff2ec192230(49.5)]>"

UITableViewCellContentView压缩的高度冲突了？
最后找到了@"V:|-2-[titleLabel(28)]-[subLabel(15)]-4-[bgScrollView]|"这里。
原因在于[titleLabel(28)]-[subLabel(15)]中间的-，起初我认为的-10-这样才会有效果，单独的-相当于-0-，这样错了。
-在[titleLabel(28)]-[subLabel(15)]是默认8点距离，相当于-8-，在|-[titleLabel]-|，默认10点距离，相当于|-10-[titleLabel]-10-|。有资料说是20点，但我自己试的是10点，不过这不是问题。以后用每次都会加上-8-或者|-10-[titleLabel]-10-|，这样也好维护。
上面的代码改成[titleLabel(28)][subLabel(15)]就解决问题了，原因是本身的约束已经满足确定位置的条件，bgScrollView和下边对齐，但是再加上-，就多出8点高度，这样就冲突了。
或者我这样写 @"V:|-2-[titleLabel(28)]-[subLabel(15)]-4-[bgScrollView(213)]"也不会报错，但是UI的位置会因为-不正确。
所以写VFL之前一定要充分学习研究VFL语法，比如scrollview、label、imageView就需要注意点，这样可以避免很多不必要的问题。
当然一开始写VFL一般都会遇到各种奇葩问题的，重要的是不能慌不能急，尝试各种解决方法，尤其>=、<=、@用起来效果会很好，也能避免很多问题。
比如：|-10-[view(40)]|，这样写会有问题吗？有的，view距离父视图左边10点，宽度40，和父视图右边对齐。这样一看就冲突了，设置了左边距和宽度，再设置右边距为0就冲突了。
可以这么写|-10-[view(40)]或者|-10-[view(40)]->=0-|或者|-10-[view]|。