手把手教你Masonry的理解

我们先来看看是如何开始使用Masonry的，一般我们使用这个布局框架的时候，都会调用以下代码。。。。。

[self.view1 mas_makeConstraints:^(MASConstraintMaker *make) {
make.left.mas_equalTo(50);
make.right.mas_equalTo(-50);
make.top.mas_equalTo(50);
make.bottom.mas_equalTo(-50);
}];我们来分析下这段代码，UIView类型的使用了mas_makeConstraints这个方法，这个是写在分类里面的，有一个block参数，我们看一下里面的实现代码：

- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block {
self.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];
block(constraintMaker); return [constraintMaker install];
}

首先设置translatesAutoresizingMaskIntoConstraints属性为NO

第二行初始化了一个MASConstraintMaker类，那这个类里面做了什么操作呢，好吧，本着刨根问底的精神我们继续往下看

- (id)initWithView:(MAS_VIEW *)view {
self = [super init]; if (!self) return nil;

self.view = view;
self.constraints = NSMutableArray.new;
return self;
}

将当前的view赋给maker类，然后初始化constraints，这是一个约束数组。

我们仔细看block(constraintMaker);这一行代码。。。

神马意思呢？

首先这个block是(void(^)(MASConstraintMaker *))类型的一个参数，我们其实可以更改为其它名称，感觉用block就有点混淆。。。

- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))constraintMakerblock {
self.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];
constraintMakerblock(constraintMaker); return [constraintMaker install];
}比如可以更改为上面的形式...

这段代码的理解就是初始化了一个constraintMaker类，然后设置它的constraint属性，最后加载安装（install）

[self.view1 mas_makeConstraints:^(MASConstraintMaker *make) {
make.left.mas_equalTo(50);
make.right.mas_equalTo(-50);
make.top.mas_equalTo(50);
make.bottom.mas_equalTo(-50);
}];注意：这种写法很容易理解为这是一个block，但其实只是在执行一个函数，后面一大块只不过是函数的一个参数而已，只不过它的参数是一个函数。

它等价与下面的一种写法：

self.view1.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self.view1];
constraintMaker.left.mas_equalTo(50);
constraintMaker.right.mas_equalTo(-50);
constraintMaker.top.mas_equalTo(50);
constraintMaker.bottom.mas_equalTo(-50);
[constraintMaker install];那这样写的话就不是很方便了，那么有什么办法吗？？？

我们把它封装成一个函数？？？好的，封装成一个函数我们要做几件事情

-(void)addMakeConstraints
{
//初始化MASConstraintMaker
self.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];

//设置它的属性
// .........

//加载
[constraintMaker install];
}那么第二步该怎么做呢？

传参数？？？但是参数有很多呀？？怎么办？？

比如我设置一个字典，里面用key，value来表示（left:50,right:-50,top:50,bottom:-50）好像也可以，但总觉的不是很方便

-(void)addMakeConstraints:(NSMutableDictionary *)dictionary
{
//初始化MASConstraintMaker
self.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];

//设置它的属性
// .........
constraintMaker.left.mas_equalTo([dictionary[@"left"] intValue]);
constraintMaker.right.mas_equalTo([dictionary[@"right"] intValue]);
constraintMaker.top.mas_equalTo([dictionary[@"top"] intValue]);
constraintMaker.bottom.mas_equalTo([dictionary[@"bottom"] intValue]);

//加载
[constraintMaker install];
}NSMutableDictionary *constraint=[[NSMutableDictionary alloc]init];
[constraint setObject:@"50" forKey:@"left"];
[constraint setObject:@"-50" forKey:@"right"];
[constraint setObject:@"50" forKey:@"top"];
[constraint setObject:@"-50" forKey:@"bottom"];
[self.view1 addMakeConstraints:constraint];貌似也可以实现的额，那有没有什么更好地办法呢？我们看下第一种写法：

-(void)addMakeConstraints
{
//初始化MASConstraintMaker
self.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];

//设置它的属性
// .........

//加载
[constraintMaker install];
}其实我们中间就是缺了一段代码：那可以使用delegate吗

-(void)addMakeConstraints
{
//初始化MASConstraintMaker
self.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];

[self.del addConstraint:constraintMaker];

//加载
[constraintMaker install];
}

-(void)addConstraint:(MASConstraintMaker *)maker
{
maker.left.mas_equalTo(50);
maker.right.mas_equalTo(-50);
maker.top.mas_equalTo(50);
maker.bottom.mas_equalTo(-50);
}既然可以用代理实现，那么block肯定也是可以的

好的，我们改装成block版本

-(void)addMakeConstraints
{
//初始化MASConstraintMaker
self.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];

self.makerBlock(constraintMaker);

//加载
[constraintMaker install];
}[self.view1 addMakeConstraints];
self.view1.makerBlock=^(MASConstraintMaker *maker){

maker.left.mas_equalTo(50);
maker.right.mas_equalTo(-50);
maker.top.mas_equalTo(50);
maker.bottom.mas_equalTo(-50);

};在这里，我们是作为一个属性来使用block的，那么我们还可以直接用参数作为block来实现的。

也就是说把这段代码封装成一个block参数

-(void)addMakeConstraints:(void(^)(MASConstraintMaker *maker))maker
{ //初始化MASConstraintMaker
self.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];

maker(constraintMaker);
//加载 [constraintMaker install];
} [self.view1 addMakeConstraints:^(MASConstraintMaker *make)
{
make.left.mas_equalTo(50);
make.right.mas_equalTo(-50);
make.top.mas_equalTo(50);
make.bottom.mas_equalTo(-50);
}];至此，这段代码已经分析完毕，哈哈，总结就一句话：初始化MASConstraintMaker类，然后设置属性，最后加载，不同的就是将设置属性作为代码块调到实现的类里面来了而已。

好的 我们看下View+MASAdditions类

/**
* following properties return a new MASViewAttribute with current view and appropriate NSLayoutAttribute */@property (nonatomic, strong, readonly) MASViewAttribute *mas_left;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_top;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_right;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_bottom;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_leading;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_trailing;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_width;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_height;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_centerX;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_centerY;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_baseline;
@property (nonatomic, strong, readonly) MASViewAttribute *(^mas_attribute)(NSLayoutAttribute attr);#if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 80000) || (__TV_OS_VERSION_MIN_REQUIRED >= 9000) || (__MAC_OS_X_VERSION_MIN_REQUIRED >= 101100)@property (nonatomic, strong, readonly) MASViewAttribute *mas_firstBaseline;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_lastBaseline;#endif#if TARGET_OS_IPHONE || TARGET_OS_TV@property (nonatomic, strong, readonly) MASViewAttribute *mas_leftMargin;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_rightMargin;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_topMargin;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_bottomMargin;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_leadingMargin;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_trailingMargin;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_centerXWithinMargins;
@property (nonatomic, strong, readonly) MASViewAttribute *mas_centerYWithinMargins;这一段是对MASViewAttribute的声明，看它的名字就知道这一段是对NSLayoutAttribute的封装，我们知道NSLayoutAttribute只是一些属性的枚举，那么封装有什么意义呢？

好吧，我们看下这段代码：

[self.view2 mas_makeConstraints:^(MASConstraintMaker *make) {
make.top.mas_equalTo(self.view1.mas_bottom).offset(10);
}];有个self.view1.mas_bottom,我们知道要设置这个属性的话，要使用原生的方法：

NSLayoutConstraint *constaint=[NSLayoutConstraint constraintWithItem:self
attribute:att
relatedBy:NSLayoutRelationEqual
toItem:attribute.view
attribute:attribute.attribute
multiplier:1.0
constant:space];

在这里有个toItem，那要设置toItem就必须知道当前是哪个view，如果只有NSLayoutAttribute属性的话，这段代码中得toItem是设置不了的。

所以MASViewAttribute便添加了view这个属性，方便这个地方的设置MAS---View---Attribute，可以这么理解，哈哈！

/**
* Creates a MASConstraintMaker with the callee view.
* Any constraints defined are added to the view or the appropriate superview once the block has finished executing
*
* @param block scope within which you can build up the constraints which you wish to apply to the view.
*
* @return Array of created MASConstraints */- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *make))constraintMakerblock;/**
* Creates a MASConstraintMaker with the callee view.
* Any constraints defined are added to the view or the appropriate superview once the block has finished executing.
* If an existing constraint exists then it will be updated instead.
*
* @param block scope within which you can build up the constraints which you wish to apply to the view.
*
* @return Array of created/updated MASConstraints */- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block;/**
* Creates a MASConstraintMaker with the callee view.
* Any constraints defined are added to the view or the appropriate superview once the block has finished executing.
* All constraints previously installed for the view will be removed.
*
* @param block scope within which you can build up the constraints which you wish to apply to the view.
*
* @return Array of created/updated MASConstraints */- (NSArray *)mas_remakeConstraints:(void(^)(MASConstraintMaker *make))block;这几个方法分别是添加约束，更新约束以及注销约束，前面已经将mas_makeConstraints这个方法讲的很详细了，后面两个方法类似。

只不过多了两个变量来控制：constraintMaker.updateExisting = YES;constraintMaker.removeExisting = YES;

好的，我们接着向下看：

- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))constraintMakerblock {
self.translatesAutoresizingMaskIntoConstraints = NO;
MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];
constraintMakerblock(constraintMaker); return [constraintMaker install];
}第一步：将maker的view设置为当前view

第二步：设置属性

第三步：加载

我们看下第二步：

make.left.mas_equalTo(50);
make.right.mas_equalTo(-50);
make.top.mas_equalTo(50);
make.bottom.mas_equalTo(-50);第二步用的链式结构，如果不懂得话可以看看我之前的文章。

make.left返回的MASConstraint类型

- (MASConstraint * (^)(id attr))mas_equalTo;
- (MASConstraint * (^)(id attr))mas_greaterThanOrEqualTo;
- (MASConstraint * (^)(id attr))mas_lessThanOrEqualTo;- (MASConstraint * (^)(id))mas_equalTo {
return ^id(id attribute) {
return self.equalToWithRelation(attribute, NSLayoutRelationEqual);
};
}然后执行mas_equalTo的方法：

它的返回值是：MASConstraint * (^)(id) 一个block。