自动布局与Masonry使用注意事项

1 理解自身内容尺寸约束与抗压抗拉

自身内容尺寸约束:一般来说，要确定一个视图的精确位置，至少需要4个布局约束（以确定水平位置x、垂直位置y、宽度w和高度h）。但是，某些用来展现内容的用户控件，例如文本控件UILabel、按钮UIButton、图片视图UIImageView等，它们具有自身内容尺寸（Intrinsic Content Size），此类用户控件会根据自身内容尺寸添加布局约束。也就是说，如果开发者没有显式给出其宽度或者高度约束，则其自动添加的自身内容约束将会起作用。因此看似“缺失”约束，实际上并非如此。

关于自身内容尺寸约束，简单来说就是某些用来展现内容的用户控件，它们会根据自身内容尺寸添加布局约束。

自身内容尺寸约束的抗挤压与抗拉抻效果。弹簧会有自身固有长度，当有外力作用时，弹簧会抵抗外力作用，尽量接近固有长度。
抗拉抻：当外力拉长弹簧时，弹簧长度大于固有长度，且产生向内收的力阻止外力拉抻，且尽量维持长度接近自身固有长度。 
抗挤压：当外力挤压弹簧时，弹簧长度小于固有长度，且产生向外顶的力阻止外力挤压，且尽量维持长度接近自身固有长度。

关于抗压抗拉，就是布局冲突需要牺牲某些控件的某些宽度或者高度约束时，抗压高的控件越不容易被压缩，抗拉高的控件越不容易被拉升。即自身布局对抗外界布局的能力。

样例：

一种常见的业务场景是用户修改地址，在输入新地址之前先读取用户之前的地址作为填充。UI实现是水平平行的UILabel和UITextField。
代码实现如下：

- (NSString *)aLongAddress
{
    return @"A long long long long long long long long long address";
}

- (NSString *)aShortAddress
{
    return @"A short address";
}

- (void)sampleCode
{
    UIView *layoutView = [UIView new];
    layoutView.frame = CGRectMake(0, 200, [UIScreen mainScreen].bounds.size.width, 100);
    layoutView.backgroundColor = [[UIColor alloc] initWithRed:0.5 green:0.5 blue:0.5 alpha:0.5];
    [self.view addSubview:layoutView];
    
    UILabel *address = [[UILabel alloc] init];
    [layoutView addSubview:address];
    address.text = @"地址:";
    address.backgroundColor = [UIColor blueColor];
    [address mas_makeConstraints:^(MASConstraintMaker *make) {
        make.centerY.equalTo(layoutView);
        make.left.equalTo(layoutView).offset(10);
    }];
    
    UITextField *addressTextField = [[UITextField alloc] init];
    [layoutView addSubview:addressTextField];
    addressTextField.returnKeyType = UIReturnKeyDone;
    addressTextField.font = [UIFont systemFontOfSize:15];
    addressTextField.clearButtonMode = UITextFieldViewModeWhileEditing;
    addressTextField.layer.borderWidth = 1 / [UIScreen mainScreen].scale;
    addressTextField.layer.borderColor =  [[[UIColor alloc] initWithRed:1 green:1 blue:0 alpha:1] CGColor];
    addressTextField.layer.cornerRadius = 3;
    addressTextField.text = [self aLongAddress];
    [addressTextField mas_makeConstraints:^(MASConstraintMaker *make) {
        make.height.equalTo(address);
        make.centerY.equalTo(address);
        make.right.equalTo(layoutView.mas_right).offset(-10);
        make.left.equalTo(address.mas_right).offset(10);
    }];
}

此处使用了UILabel的自身内容尺寸约束，当houseNumberTextField.text = [self aShortAddress]UI表现正常。

但，当houseNumberTextField.text = [self aLongAddress]时会出现address UILabel被挤压掉的情况，如下图所示：

原因是address Label的水平抗压缩没有设置。

在address Label创建的时候添加如下代码[address setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal]则显示正常。

另，在某些情况下存在view被拉升，极有可能是没有设置抗拉升，此处不一一列举。

附，抗压抗拉相关API如下：

- (UILayoutPriority)contentHuggingPriorityForAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);
- (void)setContentHuggingPriority:(UILayoutPriority)priority forAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);

- (UILayoutPriority)contentCompressionResistancePriorityForAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);
- (void)setContentCompressionResistancePriority:(UILayoutPriority)priority forAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);

2 NSLayoutConstraint只能修改constant

NSLayoutConstraint即自动布局的约束类，它是自动布局的关键之一。该类有如下属性我们需要重点关注。

NS_CLASS_AVAILABLE_IOS(6_0)
@interface NSLayoutConstraint : NSObject

// other code

@property UILayoutPriority priority;
@property BOOL shouldBeArchived;

/* accessors
 firstItem.firstAttribute {==,<=,>=} secondItem.secondAttribute * multiplier + constant
 */
@property (readonly, assign) id firstItem;
@property (readonly) NSLayoutAttribute firstAttribute;
@property (readonly) NSLayoutRelation relation;
@property (nullable, readonly, assign) id secondItem;
@property (readonly) NSLayoutAttribute secondAttribute;
@property (readonly) CGFloat multiplier;

/* Unlike the other properties, the constant may be modified after constraint creation.  Setting the constant on an existing constraint performs much better than removing the constraint and adding a new one that's just like the old but for having a new constant.
 */
@property CGFloat constant;

/* The receiver may be activated or deactivated by manipulating this property.  Only active constraints affect the calculated layout.  Attempting to activate a constraint whose items have no common ancestor will cause an exception to be thrown.  Defaults to NO for newly created constraints. */
@property (getter=isActive) BOOL active NS_AVAILABLE(10_10, 8_0);

// other code

@end

布局公式：firstItem.firstAttribute {==,<=,>=} secondItem.secondAttribute * multiplier + constant

解释：firstItem与secondItem分别是界面中受约束的视图与被参照的视图。

注意：当使用代码来修改约束时，只能修改约束的常量值constant。一旦创建了约束，其他只读属性都是无法修改的，特别要注意的是比例系数multiplier也是只读的。

Masonry是基于NSLayoutConstraint等类的封装，也正是如此，我们在调用- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *))block的时候也只能更新NSLayoutConstraint中的@property CGFloat constant。

在MASViewConstraint找到如下代码可以佐证：

- (void)install {

    // other code
    
    MASLayoutConstraint *existingConstraint = nil;
    if (self.updateExisting) { //如果是update，则去匹配对应的existingConstraint
        existingConstraint = [self layoutConstraintSimilarTo:layoutConstraint];
    }
    if (existingConstraint) { //找到了existingConstraint，最终也只更新了existingConstraint.constant
        // just update the constant
        existingConstraint.constant = layoutConstraint.constant;
        self.layoutConstraint = existingConstraint;
    } else { //没有找到existingConstraint，添加一个新的约束
        [self.installedView addConstraint:layoutConstraint];
        self.layoutConstraint = layoutConstraint;
        [firstLayoutItem.mas_installedConstraints addObject:self];
    }
}

// 除了constant，其它都一样的约束是Similar约束
- (MASLayoutConstraint *)layoutConstraintSimilarTo:(MASLayoutConstraint *)layoutConstraint {
    // check if any constraints are the same apart from the only mutable property constant

    // go through constraints in reverse as we do not want to match auto-resizing or interface builder constraints
    // and they are likely to be added first.
    for (NSLayoutConstraint *existingConstraint in self.installedView.constraints.reverseObjectEnumerator) {
        if (![existingConstraint isKindOfClass:MASLayoutConstraint.class]) continue;
        if (existingConstraint.firstItem != layoutConstraint.firstItem) continue;
        if (existingConstraint.secondItem != layoutConstraint.secondItem) continue;
        if (existingConstraint.firstAttribute != layoutConstraint.firstAttribute) continue;
        if (existingConstraint.secondAttribute != layoutConstraint.secondAttribute) continue;
        if (existingConstraint.relation != layoutConstraint.relation) continue;
        if (existingConstraint.multiplier != layoutConstraint.multiplier) continue;
        if (existingConstraint.priority != layoutConstraint.priority) continue;

        return (id)existingConstraint;
    }
    return nil;
}

样例：

@interface ViewController ()

@property (nonatomic, strong) UILabel *lbl;
@property (nonatomic, strong) UIButton *btn;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    self.btn = [UIButton buttonWithType:UIButtonTypeCustom];
    self.btn.backgroundColor = [UIColor blueColor];
    [self.btn setTitle:@"按钮" forState:UIControlStateNormal];
    [self.btn addTarget:self action:@selector(onTest:) forControlEvents:UIControlEventTouchDown];
    [self.view addSubview:self.btn];
    [self.btn mas_updateConstraints:^(MASConstraintMaker *make) {
        make.top.equalTo(self.view).offset(200);
        make.centerX.equalTo(self.view);
        make.size.mas_equalTo(CGSizeMake(100, 33));
    }];
    
    self.lbl = [[UILabel alloc] init];
    self.lbl.text = @"一个label";
    self.lbl.backgroundColor = [UIColor redColor];
    self.lbl.textAlignment = NSTextAlignmentCenter;
    [self.view addSubview:self.lbl];
    [self.lbl mas_updateConstraints:^(MASConstraintMaker *make) {
        make.top.equalTo(self.view).offset(300);
        make.centerX.equalTo(self.view);
        make.size.equalTo(self.btn);
    }];
}

- (void)onTest:(id)sender
{
    [self.lbl mas_updateConstraints:^(MASConstraintMaker *make) {
        make.size.mas_equalTo(CGSizeMake(200, 100));
    }];
}

@end

当按钮被按下时，控制台出现如下警告

2016-08-03 18:49:13.110 layout[47924:2886276] 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:0x7ffecb632470 UIButton:0x7ffecb4f28e0.width == 100>",
    "<MASLayoutConstraint:0x7ffecb637550 UILabel:0x7ffecb637030.width == UIButton:0x7ffecb4f28e0.width>",
    "<MASLayoutConstraint:0x7ffecb71fc10 UILabel:0x7ffecb637030.width == 200>"
)

Will attempt to recover by breaking constraint 
<MASLayoutConstraint:0x7ffecb71fc10 UILabel:0x7ffecb637030.width == 200>

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.
2016-08-03 18:49:13.111 layout[47924:2886276] 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:0x7ffecb612bc0 UIButton:0x7ffecb4f28e0.height == 33>",
    "<MASLayoutConstraint:0x7ffecb625300 UILabel:0x7ffecb637030.height == UIButton:0x7ffecb4f28e0.height>",
    "<MASLayoutConstraint:0x7ffecb486f10 UILabel:0x7ffecb637030.height == 100>"
)

Will attempt to recover by breaking constraint 
<MASLayoutConstraint:0x7ffecb486f10 UILabel:0x7ffecb637030.height == 100>

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.

原因是，lbl创建时其size约束是make.size.equalTo(self.btn)，但btn被点击时，企图去update size约束为make.size.mas_equalTo(CGSizeMake(200, 100))，然而无法找到existingConstraint，因此实际上是额外添加了一个约束make.size.mas_equalTo(CGSizeMake(200, 100))出现了布局冲突。

这件事可以这么看，NSLayoutConstraint只能修改constant决定了mas_updateConstraints的实现方式为：找到既有约束就去改变constant找不到既有约束就添加新约束。

3 被Masonry布局的view一定要与比较view有共同的祖先view

这句话比较拗口，其中涉及三类view，解释如下。

1. 被Masonry布局的view：执行了- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *make))block、- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block 、- (NSArray *)mas_remakeConstraints:(void(^)(MASConstraintMaker *make))block等函数的view。
2. 比较view：以上3函数block块里面出现的view。
3. 共同的祖先view：【1】和【2】的共同祖先view。

样例1：

- (void)sampleCode
{
    UIView *v0 = [UIView new];
    [self.view addSubview:v0];

    UIView *v1 = [UIView new];
    [v0 addSubview:v1];
    [v1 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.size.mas_equalTo(CGSizeMake(10, 10));
    }];

    UIView *v2 = [UIView new];
    [v0 addSubview:v2];
    [v2 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.size.equalTo(v1);
    }];
}

针对如下代码块来说

UIView *v2 = [UIView new];
[v0 addSubview:v2];
[v2 mas_makeConstraints:^(MASConstraintMaker *make) {
    make.size.equalTo(v1);
}];

v2是被Masonry布局的view，v1是比较view，v0是共同的祖先view。

样例2：

@implementation AutoLayoutViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    [self useMasonryWithoutSuperView];
}

- (void)useMasonryWithoutSuperView
{
    UIView *masView = [UIView new];
    [masView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.center.equalTo(self.view);
    }];
}

@end

以上代码执行时会crash，crash log如下：

2016-08-04 00:52:47.542 CommonTest[1731:22953] *** Assertion failure in -[MASViewConstraint install], /Users/shuncheng/SourceCode/SampleCode/AutoLayout/Pods/Masonry/Masonry/MASViewConstraint.m:338
2016-08-04 00:52:47.548 CommonTest[1731:22953] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'couldn't find a common superview for <UIView: 0x7fa59bd30dd0; frame = (0 0; 0 0); layer = <CALayer: 0x7fa59bd2f3c0>> and <UIView: 0x7fa59bd30c60; frame = (0 0; 414 736); autoresize = W+H; layer = <CALayer: 0x7fa59bd24780>>'

crash的原因显而易见，即，masView(被Masonry布局的view)与self.view(比较view)没有共同祖先view，因为masView没有父view，所以它和self.view必然没有共同祖先view。

被Masonry布局的view没有添加到superview上其实比较容易被发现，最怕的是出现如样例3一样的鬼畜情况。

样例3：

@implementation AutoLayoutViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    [self sampleCode];
}

- (void)sampleCode
{
    AutoLayoutViewController * __weak weakSelf = self;
    [fooNetworkModel fetchData:^{
        AutoLayoutViewController * self = weakSelf;
        [AutoLayoutViewController showSampleViewAtView:self.view];
    }];
}

+ (void)showSampleViewAtView:(UIView *)view
{
    UIView *v1 = [UIView new];
    [view addSubview:v1];
    [v1 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.size.mas_equalTo(CGSizeMake(10, 10));
    }];
    
    UIView *v2 = [UIView new];
    [view addSubview:v2];
    [v2 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.size.equalTo(v1);
    }];
}

@end

以上代码通常不会出错，但是一种异常情况是：在AutoLayoutViewController析构后，网络数据返回，此时AutoLayoutViewController * self = weakSelf则self == nil。执行[AutoLayoutViewController showSampleViewAtView:nil]，则会出现【样例2】一样的crash。

原因是：v1和v2都没有添加到view上去（因为view为空）所以make.size.equalTo(v1)会出错（v1和v2没有共同的父view）。由此也引申到weakSelf的副作用，即必须要确保weakSelf是nil时，执行逻辑完全没有问题（目前已经两次被坑）。

4 不要被update迷惑

这里说的update有两层含义：

1. UIView的方法- (void)updateConstraints NS_AVAILABLE_IOS(6_0)
2. Masonry的方法- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block

这里先来讨论一下UIView的- (void)updateConstraints方法。

- (void)updateConstraints方法是用来更新view约束的，它有一个常见的使用场景——批量更新约束。比如你的多个约束是由多个不同的property决定，每次设置property都会直接更新局部约束。这样效率不高。不如直接override- (void)updateConstraints方法，在方面里面对property进行判断，每次设置property的时候调用一下- (void)setNeedsUpdateConstraints。伪代码如下：

优化前：

@implementation AutoLayoutView

- (void)setFactor1:(NSInteger)factor1
{
    _factor1 = factor1;
    
    if (_factor1满足条件) {
        更新约束1
    }
}

- (void)setFactor2:(NSInteger)factor2
{
    _factor2 = factor2;
    
    if (_factor2满足条件) {
        更新约束2
    }
}

- (void)setFactor3:(NSInteger)factor3
{
    _factor3 = factor3;
    
    if (_factor3满足条件) {
        更新约束3
    }
}

@end

优化后：

@implementation AutoLayoutView

- (void)setFactor1:(NSInteger)factor1
{
    _factor1 = factor1;
    
    [self setNeedsUpdateConstraints];
}

- (void)setFactor2:(NSInteger)factor2
{
    _factor2 = factor2;
    
    [self setNeedsUpdateConstraints];
}

- (void)setFactor3:(NSInteger)factor3
{
    _factor3 = factor3;
    
    [self setNeedsUpdateConstraints];
}

- (void)updateConstraints
{
    if (self.factor1满足) {
        更新约束1
    }
    
    if (self.factor2满足) {
        更新约束2
    }
    
    if (self.factor3满足) {
        更新约束3
    }
    
    [super updateConstraints];
}

@end

注意：一种有误区的写法是在- (void)updateConstraints方法中进行初次constraint设置，这是不被推荐的。推荐的写法是在init或者viewDidLoad中进行view的初次constraint设置。

Masonry的方法- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block我们在第二节已经讨论过了。刚接触自动布局和Masonry的同学很容易跟着感觉在- (void)updateConstraints函数里面调用Masonry的- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block。实际上两者并没有必然联系。大多数情况在- (void)updateConstraints里面调用- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block很有可能产生布局冲突。

样例

// 头文件
typedef NS_ENUM(NSUInteger, AutoLayoutType) {
    HorizontalLayout,
    VerticalLayout,
};

@interface AutoLayoutView : UIView

@property (nonatomic, strong) UILabel *name;
@property (nonatomic, strong) UILabel *address;

@property (nonatomic, assign) AutoLayoutType layoutType;

@end

// 实现文件
@implementation AutoLayoutView

- (instancetype)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame]) {
        _name = [[UILabel alloc] init];
        [self addSubview:_name];
        
        _address = [[UILabel alloc] init];
        [self addSubview:_address];
        
        [_name mas_updateConstraints:^(MASConstraintMaker *make) {
            make.left.top.equalTo(self);
        }];
    }
    return self;
}

- (void)updateConstraints
{
    if (self.layoutType == HorizontalLayout) {
       // // 此处误用mas_updateConstraints
        [self.address mas_updateConstraints:^(MASConstraintMaker *make) {
            make.top.equalTo(self.name);
            make.left.equalTo(self.name.mas_right).offset(10);
        }];
    } else {
        // 此处误用mas_updateConstraints
        [self.address mas_updateConstraints:^(MASConstraintMaker *make) {
            make.left.equalTo(self.name);
            make.top.equalTo(self.name.mas_bottom).offset(10);
        }];
    }
    
    [super updateConstraints];
}

- (void)setLayoutType:(AutoLayoutType)layoutType
{
    _layoutType = layoutType;
    
    [self setNeedsUpdateConstraints];
}

@end

// 外部调用代码
- (void)sampleCode
{
    AutoLayoutView *view = [[AutoLayoutView alloc] init];
    view.name.text = @"name";
    view.address.text = @"address";
    [self.view addSubview:view];
    [view mas_makeConstraints:^(MASConstraintMaker *make) {
        make.center.equalTo(self.view);
        make.size.mas_equalTo(CGSizeMake(200, 300));
    }];
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        view.layoutType = VerticalLayout; //修改布局方式后，出现布局冲突
    });
}

5 总结

本文梳理了一下自动布局和Masonry使用的误区。在基本概念没搞清的情况下，很容易犯错。总结起来就如下4点：

1. 理解自身内容尺寸约束与抗压抗拉
2. NSLayoutConstraint只能修改constant和- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block实现细节之间的关系
3. 被Masonry布局的view一定要与比较view有共同的祖先view
4. 区分UIView的- (void)updateConstraints方法和- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block

6 参考资料

WWDC-Mysteries of Auto Layout, Part 2