Masonry 布局冲突快速定位

MASonry 布局冲突快速定位

如果调试的时候，出现布局冲突，可以使用 MASAttachKeys 宏来给view添加key,提示信息就会提示出，是哪个view的哪个约束出冲突了！

用法如下

直接设置key

view1.mas_key = @"view1", view2.mas_key = @"view2";

或者
通过 MASAttachKeys(view1, view2);

给view1设置mas_key为view1以此类推

这样子在布局有冲突的时候，提示信息就会有 view1 view2 这样子的提示

实现原理如下

define MASAttachKeys(...)                                                    \
    NSDictionary *keyPairs = NSDictionaryOfVariableBindings(__VA_ARGS__);     \
    for (id key in keyPairs.allKeys) {                                        \
        id obj = keyPairs[key];                                               \
        NSAssert([obj respondsToSelector:@selector(setMas_key:)],             \
                 @"Cannot attach mas_key to %@", obj);                        \
        [obj setMas_key:key];                                                 \
    }

通过 NSDictionaryOfVariableBindings 宏来生成一个字典，比如：NSDictionaryOfVariableBindings(obj1,obj2.obj3),最后生成的字典是 {"obj1" : obj1, "obj2" : obj2, "obj3" : obj3}
然后通过 [obj setMas_key:key]; 完成赋值。

Demo

我们使用Monsary制造一个布局冲突玩玩。

UIView *v1 = [UIView new];
    MASAttachKeys(v1);
    [self.view addSubview:v1];
    [v1 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.left.top.offset(20);
        make.size.mas_equalTo(CGSizeMake(50, 50));
    }];
    
    
    [v1 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.left.top.offset(60);
        make.size.mas_equalTo(CGSizeMake(70, 90));
    }];
    

看看提示

BeautifyFaceDemo[6417:555809] [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. 
(
    "<MASLayoutConstraint:0x6000000b0c80 UIView:v1.left == UIView:0x7fb4f8612740.left + 20>",
    "<MASLayoutConstraint:0x6000000b1340 UIView:v1.left == UIView:0x7fb4f8612740.left + 60>"
)

Will attempt to recover by breaking constraint 
<MASLayoutConstraint:0x6000000b1340 UIView:v1.left == UIView:0x7fb4f8612740.left + 60>

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.
2017-09-12 11:06:18.641562+0800 BeautifyFaceDemo[6417:555809] [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. 
(
    "<MASLayoutConstraint:0x6000000b0f80 UIView:v1.top == UIView:0x7fb4f8612740.top + 20>",
    "<MASLayoutConstraint:0x6000000b13a0 UIView:v1.top == UIView:0x7fb4f8612740.top + 60>"
)

Will attempt to recover by breaking constraint 
<MASLayoutConstraint:0x6000000b13a0 UIView:v1.top == UIView:0x7fb4f8612740.top + 60>

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.
2017-09-12 11:06:18.642053+0800 BeautifyFaceDemo[6417:555809] [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. 
(
    "<MASLayoutConstraint:0x6000000b1040 UIView:v1.width == 50>",
    "<MASLayoutConstraint:0x6000000b1400 UIView:v1.width == 70>"
)

Will attempt to recover by breaking constraint 
<MASLayoutConstraint:0x6000000b1400 UIView:v1.width == 70>

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.
2017-09-12 11:06:18.642516+0800 BeautifyFaceDemo[6417:555809] [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. 
(
    "<MASLayoutConstraint:0x6000000b1100 UIView:v1.height == 50>",
    "<MASLayoutConstraint:0x6000000b1460 UIView:v1.height == 90>"
)

Will attempt to recover by breaking constraint 
<MASLayoutConstraint:0x6000000b1460 UIView:v1.height == 90>

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.

如上提示已经提示出 是 v1 的某个约束没写好。

为什么能打印出来？

之所以能打印出来，是因为

#import "NSLayoutConstraint+MASDebugAdditions.h"

这个类中，会附加一些调试信息，出现冲突的时候，系统会调用 NSLayoutConstraint 的 description方法，会紧跟着调用

+ (NSString *)descriptionForObject:(id)obj {
    if ([obj respondsToSelector:@selector(mas_key)] && [obj mas_key]) {
        return [NSString stringWithFormat:@"%@:%@", [obj class], [obj mas_key]];
    }
    return [NSString stringWithFormat:@"%@:%p", [obj class], obj];
}

然后把key 打印出来。

那么 NSLayoutConstraint 的 mas_key 在哪里赋值的呢，MASAttachKeys 只是给view设置了 mas_key ，

原因很简单，在 约束安装的时候，在 MASViewConstraint 的 install 的方法中

- (void)install {
    if (self.hasBeenInstalled) {
        return;
    }
    
    MAS_VIEW *firstLayoutItem = self.firstViewAttribute.item;
    NSLayoutAttribute firstLayoutAttribute = self.firstViewAttribute.layoutAttribute;
    MAS_VIEW *secondLayoutItem = self.secondViewAttribute.item;
    NSLayoutAttribute secondLayoutAttribute = self.secondViewAttribute.layoutAttribute;

    // alignment attributes must have a secondViewAttribute
    // therefore we assume that is refering to superview
    // eg make.left.equalTo(@10)
    if (!self.firstViewAttribute.isSizeAttribute && !self.secondViewAttribute) {
        secondLayoutItem = self.firstViewAttribute.view.superview;
        secondLayoutAttribute = firstLayoutAttribute;
    }
    
    MASLayoutConstraint *layoutConstraint
        = [MASLayoutConstraint constraintWithItem:firstLayoutItem
                                        attribute:firstLayoutAttribute
                                        relatedBy:self.layoutRelation
                                           toItem:secondLayoutItem
                                        attribute:secondLayoutAttribute
                                       multiplier:self.layoutMultiplier
                                         constant:self.layoutConstant];
    
    layoutConstraint.priority = self.layoutPriority;
    
    ///设置调试信息
    layoutConstraint.mas_key = self.mas_key;
    

注意事项

设置key必须在布局之前设置，否则无效！