autolayout - sizeClass - 2

对于单个约束的文档详述。

/* Create constraints explicitly.  Constraints are of the form "view1.attr1 = view2.attr2 * multiplier + constant" 
 If your equation does not have a second view and attribute, use nil and NSLayoutAttributeNotAnAttribute.
 */
+(instancetype)constraintWithItem:(id)view1 attribute:(NSLayoutAttribute)attr1 relatedBy:(NSLayoutRelation)relation toItem:(id)view2 attribute:(NSLayoutAttribute)attr2 multiplier:(CGFloat)multiplier constant:(CGFloat)c;

+ (instancetype)constraintWithItem:(id)view1 attribute:(NSLayoutAttribute)attr1 relatedBy:(NSLayoutRelation)relation toItem:(id)view2 attribute:(NSLayoutAttribute)attr2 multiplier:(CGFloat)multiplier constant:(CGFloat)c
Description	
Creates a constraint that defines the relationship between the specified attributes of the given views.
Constraints represent linear equations of the form view1.attr1 <relation> multiplier × view2.attr2 + c. If the constraint you wish to express does not have a second view and attribute, use nil and NSLayoutAttributeNotAnAttribute.
Parameters	
view1 : The view for the left side of the constraint.
attr1 : The attribute of the view for the left side of the constraint.
relation : The relationship between the left side of the constraint and the right side of the constraint.
view2 : The view for the right side of the constraint.
attr2 : The attribute of the view for the right side of the constraint.
multiplier : The constant multiplied with the attribute on the right side of the constraint as part of getting the modified attribute.
c : The constant added to the multiplied attribute value on the right side of the constraint to yield the final modified attribute.
Returns	
A constraint object relating the two provided views with the specified relation, attributes, multiplier, and constant.

对于NSLayoutAttribute的参数如下：每一个参数意义就不说了，自己查。

typedef NS_ENUM(NSInteger, NSLayoutAttribute) {
    NSLayoutAttributeLeft = 1,
    NSLayoutAttributeRight,
    NSLayoutAttributeTop,
    NSLayoutAttributeBottom,
    NSLayoutAttributeLeading,
    NSLayoutAttributeTrailing,
    NSLayoutAttributeWidth,
    NSLayoutAttributeHeight,
    NSLayoutAttributeCenterX,
    NSLayoutAttributeCenterY,
    NSLayoutAttributeBaseline,
    NSLayoutAttributeLastBaseline = NSLayoutAttributeBaseline,
    NSLayoutAttributeFirstBaseline NS_ENUM_***AILABLE_IOS(8_0),
    
    
    NSLayoutAttributeLeftMargin NS_ENUM_***AILABLE_IOS(8_0),
    NSLayoutAttributeRightMargin NS_ENUM_***AILABLE_IOS(8_0),
    NSLayoutAttributeTopMargin NS_ENUM_***AILABLE_IOS(8_0),
    NSLayoutAttributeBottomMargin NS_ENUM_***AILABLE_IOS(8_0),
    NSLayoutAttributeLeadingMargin NS_ENUM_***AILABLE_IOS(8_0),
    NSLayoutAttributeTrailingMargin NS_ENUM_***AILABLE_IOS(8_0),
    NSLayoutAttributeCenterXWithinMargins NS_ENUM_***AILABLE_IOS(8_0),
    NSLayoutAttributeCenterYWithinMargins NS_ENUM_***AILABLE_IOS(8_0),
    
    NSLayoutAttributeNotAnAttribute = 0
};

对于约束关系，relations只有大于等于，小于等于，等于这三个枚举值。

typedef NS_ENUM(NSInteger, NSLayoutRelation) {
    NSLayoutRelationLessThanOrEqual = -1,
    NSLayoutRelationEqual = 0,
    NSLayoutRelationGreaterThanOrEqual = 1,
};

对于参数constant：c :
The constant added to the multiplied attribute value on the right side of the constraint to yield the final modified attribute.
实质上赋完参之后会发现其实就是一句VFL罢了，参照VFL的效果和格式就可以明白上面参数的意义了。
下面用前面的大牛的例子来详细解析一下：

//autolayout实现控件的平面中心水平和垂直居中：

UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[button setTitle:@"测试按钮" forState:UIControlStateNormal];
[button sizeToFit];
[button setBackgroundColor:[UIColor yellowColor]];
button.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:button];

NSLayoutConstraint *constraint;

//水平居中
constraint = [NSLayoutConstraint
              constraintWithItem:button
              attribute:NSLayoutAttributeCenterX
              relatedBy:NSLayoutRelationEqual
              toItem:self.view
              attribute:NSLayoutAttributeCenterX
              multiplier:1.0f
              constant:0.0f];
[self.view addConstraint:constraint];

//垂直居中
constraint = [NSLayoutConstraint
              constraintWithItem:button
              attribute:NSLayoutAttributeCenterY
              relatedBy:NSLayoutRelationEqual
              toItem:self.view
              attribute:NSLayoutAttributeCenterY
              multiplier:1.0f
              constant:0.0f];
[self.view addConstraint:constraint];

相信例子不难大家都能懂，根据上面的文档：Constraints are of the form "view1.attr1 = view2.attr2 * multiplier + constant"

代入公式可以得到：button.attributeCenterX LayoutRelationEqual self.view.attributeCenterX * multiplier + constant ;
其中：LayoutRelationEqul = '=' ，multiplier = 1.0f，constant = 0.0f 得到：
button.CenterX = self.view.CenterX
相当好理解吧！
同里在上篇文章中的例子：我们尝试代入：

//上边距
    constraint = [NSLayoutConstraint
                  constraintWithItem:button
                  attribute:NSLayoutAttributeTop
                  relatedBy:NSLayoutRelationEqual
                  toItem:self.view
                  attribute:NSLayoutAttributeTop
                  multiplier:1.0f
                  constant:50.0f];
    [self.view addConstraint:constraint];
    
    //左边距
    constraint = [NSLayoutConstraint
                  constraintWithItem:button
                  attribute:NSLayoutAttributeLeading
                  relatedBy:NSLayoutRelationEqual
                  toItem:self.view
                  attribute:NSLayoutAttributeLeading
                  multiplier:1.0f
                  constant:100.0f];
    [self.view addConstraint:constraint];
    
    //右边距
    constraint = [NSLayoutConstraint
                  constraintWithItem:button
                  attribute:NSLayoutAttributeTrailing
                  relatedBy:NSLayoutRelationEqual
                  toItem:self.view
                  attribute:NSLayoutAttributeTrailing
                  multiplier:1.0f
                  constant:-100.0f];
    [self.view addConstraint:constraint];
    
    //下边距
    constraint = [NSLayoutConstraint
                  constraintWithItem:button
                  attribute:NSLayoutAttributeBottom
                  relatedBy:NSLayoutRelationEqual
                  toItem:self.view
                  attribute:NSLayoutAttributeBottom
                  multiplier:1.0f
                  constant:-350.0f];
    [self.view addConstraint:constraint];

得到公式如下：

//上边距 ： button.Top = self.view.Top + 50.0;
//下边距 ： button.Bottom = self.view.Bottom - 350;
//左边距 ： button.Left = self.view.Left + 100;
//右边距 ： button.Right = self.view.Right - 100;
相当好理解吧！

再来一个例子，看看约束怎么限定一个控件的width和height。

/* 直接设定的控件尺寸 */
    UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    [button setTitle:@"测试按钮" forState:UIControlStateNormal];
    [button sizeToFit];
    [button setBackgroundColor:[UIColor yellowColor]];
    button.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:button];
    
    NSLayoutConstraint *constraint;
    
    //设置宽度
    constraint = [NSLayoutConstraint constraintWithItem:button attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0f constant:130.0f];
    
    [self.view addConstraint:constraint];
    
    //设置高度
    constraint = [NSLayoutConstraint constraintWithItem:button attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0f constant:70.0f];
    
    [self.view addConstraint:constraint];
    
    NSLog(@"button autoChanged frame is :%@", NSStringFromCGRect(button.bounds));

同样的道理按照上面的公式去套，得到公式如下：

//宽度 ： button.width = 130;
//高度 ： button.height = 70;
非常直观吧！
同时将button的bounds输出可以看到，还是sizeToFit的值哦。

待续...
如果以上引用大牛的例子造成任何不好的影响，请评论区通知小弟一声，马上修改。