第三方 Masonry约束的使用

给Autolayout披上一层漂亮的外衣之后，将其称为Masonry，但Masonry的本质还是Autolayout。可以理解为Masonry是对Autolayout的封装.

对于一个约束。他实际表示的是一个不等或者相等关系

用Masonry创建一个完整的约束应该是这样的

//view1的左边距离父View左边10个点：
[view1 mas_makeConstraints:^(MASConstraintMaker *make) {make.left.equalTo(view1.superview.mas_left).multipliedBy(1).offset(10);
}];

Attribute

MASConstraintMaker

上面的表达式中，我们可以看到，make是MASConstraintMaker类型。MASConstraintMaker给我们提供了22种Attribute类型

//Basic Attribute
@property (nonatomic, strong, readonly) MASConstraint *left;
@property (nonatomic, strong, readonly) MASConstraint *top;
@property (nonatomic, strong, readonly) MASConstraint *right;
@property (nonatomic, strong, readonly) MASConstraint *bottom;
@property (nonatomic, strong, readonly) MASConstraint *leading;
@property (nonatomic, strong, readonly) MASConstraint *trailing;
@property (nonatomic, strong, readonly) MASConstraint *width;
@property (nonatomic, strong, readonly) MASConstraint *height;
@property (nonatomic, strong, readonly) MASConstraint *centerX;
@property (nonatomic, strong, readonly) MASConstraint *centerY;
@property (nonatomic, strong, readonly) MASConstraint *baseline;//Margin Attribute
@property (nonatomic, strong, readonly) MASConstraint *leftMargin;
@property (nonatomic, strong, readonly) MASConstraint *rightMargin;
@property (nonatomic, strong, readonly) MASConstraint *topMargin;
@property (nonatomic, strong, readonly) MASConstraint *bottomMargin;
@property (nonatomic, strong, readonly) MASConstraint *leadingMargin;
@property (nonatomic, strong, readonly) MASConstraint *trailingMargin;
@property (nonatomic, strong, readonly) MASConstraint *centerXWithinMargins;
@property (nonatomic, strong, readonly) MASConstraint *centerYWithinMargins;//Convenient Attribute
@property (nonatomic, strong, readonly) MASConstraint *edges;
@property (nonatomic, strong, readonly) MASConstraint *size;
@property (nonatomic, strong, readonly) MASConstraint *center;

Attribute总体来说分为三大类

Basic Attribute: 基本属性，支持到iOS6，一般使用得比较多
Margin Attribute: 边缘相关属性，支持到iOS8。由于版本要求比较高，一般用得比较少。
Convenient Attribute: 便捷属性，为了使用方便而特意新增的属性。Autolayout本身没有对应的相关属性

Convenient Attribute实际是基本属性的组合。比如：edges表示left, right, top, bottom。
下面的两个代码实际的意义是一样的

//Convenient Attribute
make.edges.insets(edge);//Basic Attribute
make.left.right.top.bottom.insets(edge);

MASConstraint

前面我们看到MASConstraintMaker中所有的Attribute都是MASConstraint类型。对于多个Attribute一起写的表达式：

make.left.right.top.bottom.insets(edge);

make.left返回的已经是MASConstraint类型，也就是说right这个Attribute是MASConstraint的属性。
MASConstraint给我们提供了19种Attribute：

//Basic Attribute
@property (nonatomic, strong, readonly) MASConstraint *left;
@property (nonatomic, strong, readonly) MASConstraint *top;
@property (nonatomic, strong, readonly) MASConstraint *right;
@property (nonatomic, strong, readonly) MASConstraint *bottom;
@property (nonatomic, strong, readonly) MASConstraint *leading;
@property (nonatomic, strong, readonly) MASConstraint *trailing;
@property (nonatomic, strong, readonly) MASConstraint *width;
@property (nonatomic, strong, readonly) MASConstraint *height;
@property (nonatomic, strong, readonly) MASConstraint *centerX;
@property (nonatomic, strong, readonly) MASConstraint *centerY;
@property (nonatomic, strong, readonly) MASConstraint *baseline;//Margin Attribute
@property (nonatomic, strong, readonly) MASConstraint *leftMargin;
@property (nonatomic, strong, readonly) MASConstraint *rightMargin;
@property (nonatomic, strong, readonly) MASConstraint *topMargin;
@property (nonatomic, strong, readonly) MASConstraint *bottomMargin;
@property (nonatomic, strong, readonly) MASConstraint *leadingMargin;
@property (nonatomic, strong, readonly) MASConstraint *trailingMargin;
@property (nonatomic, strong, readonly) MASConstraint *centerXWithinMargins;
@property (nonatomic, strong, readonly) MASConstraint *centerYWithinMargins;

细看一下，MASConstraint中的Attribute和MASConstraintMaker完全一样。只是MASConstraintMaker中多了3种Convenient Attribute。

两者Attribute的一致，大大的提升了使用的方便性。使用过程中我们不用再去区分当前属性是MASConstraint还是MASConstraintMaker类型。(事实上没研究他的类型之前，我都不知道他们分别属于2种不同类的属性)

UIView

我们可以看到在.equalTo(view1.superview.mas_left)里面，superView也有Attribute。我们来看看UIView中有哪些Attribute：

// Basic Attribute
@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;// Margin Attribute
@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;

可以看出，在UIView中的Attribute和MASConstraint中的几乎一模一样，只是每一个Attribute加了一个mas_前缀。

由于UIView是系统的类，对其扩展属性和方法一般都需要添加自己的前缀，避免跟原有属性和方法冲突。不过他们的意义跟MASConstraint中的Attribute是相同的

Relationship

约束表示的是2个item之间的关系，在Autolayout中一共定义了3种关系：=, >=, <=，对应到Masonry中:

- (MASConstraint * (^)(id attr))equalTo;
- (MASConstraint * (^)(id attr))greaterThanOrEqualTo;
- (MASConstraint * (^)(id attr))lessThanOrEqualTo;

相等关系我们一般用的多。那么不相等关系我们什么时候用呢？

假如我有一个Label。Label的长度不能超出父View，如果label中的文字比较短，我希望是文字有多长，Label就有多长。
由于label具有IntrinsicContentSize属性。所以默认情况下，他是文字有多长，Label就有多长。所以我们只需要设置Label的长度小于父View即可

[label mas_makeConstraints:^(MASConstraintMaker *make) {    make.left.offset(0);    make.centerY.offset(0);    make.width.lessThanOrEqualTo(label.superview);}];

multiplier

multiplier表示Attribute前面的乘数。Masonry提供了2种添加multiplier的方法

//    Sets the NSLayoutConstraint multiplier property
- (MASConstraint * (^)(CGFloat multiplier))multipliedBy;//    Sets the NSLayoutConstraint multiplier to 1.0/dividedBy
- (MASConstraint * (^)(CGFloat divider))dividedBy;

multipliedBy: 直接设置乘数
dividedBy: 设置乘数的倒数 multiplier = 1.0/dividedBy
一般宽或者高的约束使用multiplier比较多

Constant

Masonry提供了4种设置constant的方法

//Modifies the NSLayoutConstraint constant,only affects MASConstraints in which the first item's NSLayoutAttribute is one of the following NSLayoutAttributeTop, NSLayoutAttributeLeft, NSLayoutAttributeBottom, NSLayoutAttributeRight
- (MASConstraint * (^)(MASEdgeInsets insets))insets;//Modifies the NSLayoutConstraint constant,only affects MASConstraints in which the first item's NSLayoutAttribute is one of the following NSLayoutAttributeWidth, NSLayoutAttributeHeight
- (MASConstraint * (^)(CGSize offset))sizeOffset;//Modifies the NSLayoutConstraint constant, only affects MASConstraints in which the first item's NSLayoutAttribute is one of the following NSLayoutAttributeCenterX, NSLayoutAttributeCenterY
- (MASConstraint * (^)(CGPoint offset))centerOffset;//Modifies the NSLayoutConstraint constant
- (MASConstraint * (^)(CGFloat offset))offset;

insets: 用来设置left, right, top, bottom。接受MASEdgeInsets类型值

sizeOffset: 用来设置width, height。接受CGSize类型的值
centerOffset: 用来设置centerX, centerY。接受CGPoint类型的值
offset: 可以用来设置所有的东西。接受CGFloat类型的值

小技巧

如果等式2边的Attribute是一样的，我们可以省略等式右边的Attribute
如果是等于关系，并且右边的view是父View。连equalTo也可以省略
如果equalTo里面传的是NSValue类型，效果跟设置offset是一样的
如果offset为0，其实也是可以省略的...

下面所有代码实际效果是一样的：

// 完整的
make.left.equalTo(view1.superview.mas_left).offset(0);//省略Attribute的
make.left.equalTo(view1.superview).offset(0);//省略equalTo的
make.left.offset(0);//使用equalTo替代offset的
make.left.equalTo(@0);//终极大招，省略所有的... 可惜会有warning
make.left;

对于这个警告我们可以将返回值转为空消除：

(void)make.left;

设置或更新约束

对于约束的设置，Masonry提供了3种方法，分别为设置约束、更新约束、重写设置约束

// 设置约束
- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *make))block;// 更新约束
- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block;// 重新设置约束
- (NSArray *)mas_remakeConstraints:(void(^)(MASConstraintMaker *make))block;

mas_makeConstraints: 初次设置约束使用。
mas_updateConstraints: 更新约束时使用。如果找不着这条约束，会新增，相当于mas_makeConstraints。
mas_remakeConstraints: 重新设置约束。先将view上所有约束移除，再新增约束

注意：mas_updateConstraints只能更新已有约束。如果第一次使用的是left, right设置的相对宽度。更新的时候想换成使用width。不能使用mas_updateConstraints，因为已有约束里面没有width的约束，新增width之后会跟原有left, right约束冲突。此时应该使用mas_remakeConstraints

批量设置约束

假设有View1，view2，view3三个View，我们想要他们的宽高都等于CGSizeMake(100, 50)。我们可以对他们进行批量设置：

NSValue *sizeValue = [NSValue valueWithCGSize:CGSizeMake(100, 50)];
[@[view1,view2,view3] mas_makeConstraints:^(MASConstraintMaker *make) {make.size.equalTo(sizeValue);
}];

由于我们还要设置view的top,left等位置约束。那可不可以在设置位置的mas_makeConstraints里面批量设置宽高呢？实际是可以的！

//advance set
[view1 mas_makeConstraints:^(MASConstraintMaker *make) {(void)make.top.left;make.size.equalTo(@[view2,view3,sizeValue]);
}];

不过需要注意的是。设置约束的时候，view一定是已经被addSubview的，否则会抛异常。所以我们一般在最后一个view上加批量约束

Priority

我们知道约束是有优先级的，Masonry给我们提供了4个设置优先级的接口:

 //    Sets the NSLayoutConstraint priority to a float or MASLayoutPriority
- (MASConstraint * (^)(MASLayoutPriority priority))priority;//    Sets the NSLayoutConstraint priority to MASLayoutPriorityLow
- (MASConstraint * (^)())priorityLow;//    Sets the NSLayoutConstraint priority to MASLayoutPriorityMedium
- (MASConstraint * (^)())priorityMedium;//    Sets the NSLayoutConstraint priority to MASLayoutPriorityHigh
- (MASConstraint * (^)())priorityHigh;

priority: 可以设置任意的优先级，接受的参数是0-1000的数字
priorityLow: 设置低优先级，优先级为250
priorityMedium: 设置中优先级，优先级为500
priorityHigh: 设置高优先级，优先级为750

需要注意的是，使用priorityLow、priorityMedium、priorityHigh的时候。不是.priorityHigh，而是.priorityHigh()

Shorthand

在写代码的时候，可能你会感觉有的东西要加mas_前缀，有的东西又不用加，代码风格不统一，而且加mas_前缀还麻烦。

前面介绍过加mas_前缀主要是在扩展系统类的时候为了避免与原有类冲突，这是Apple推荐的做法。不过目前来说，即使不加mas_前缀，也不会有什么问题。所以Masonry提供了不加mas_前缀的方法，只需要你定义几个宏即可。

1, MAS_SHORTHAND 定义MAS_SHORTHAND宏之后。可以使用UIView，NSArray中不带mas_前缀的makeConstraints,updateConstraints,remakeConstraints。以及UIView中不带mas_前缀的Attribute 2, MAS_SHORTHAND_GLOBALS 默认的equalTo方法只接受id类型的对象。有时候我们想传入一个CGFloat, CGSize, UIEdgeInsets等。还需要将其转化成NSValue对象，比较麻烦。Masonry也考虑到了这种情况。只需要定义MAS_SHORTHAND_GLOBALS宏。就可以直接对equalTo传入基础类型。Masonry自动转化成NSValue对象

注意:#define MAS_SHORTHAND和#define MAS_SHORTHAND_GLOBALS这两个宏要在导入Masonry之前定义