XXX.translatesAutoresizingMaskIntoConstraints = NO;
[self addConstraints:@[
[NSLayoutConstraint constraintWithItem:XXX
attribute:NSLayoutAttributeLeft
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeLeft
multiplier:1.0
constant:0.0],
[NSLayoutConstraint constraintWithItem:XXX
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeTop
multiplier:1.0
constant:0.0],
[NSLayoutConstraint constraintWithItem:XXX
attribute:NSLayoutAttributeRight
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeRight
multiplier:1.0
constant:0.0],
[NSLayoutConstraint constraintWithItem:XXX
attribute:NSLayoutAttributeBottom
relatedBy:NSLayoutRelationEqual
toItem:self
attribute:NSLayoutAttributeBottom
multiplier:1.0
constant:0.0],
]];
XXX表示需要添加约束的view.
为什么 translatesAutoresizingMaskIntoConstraints 使用约束布局时候，就要设置为 NO？
translatesAutoresizingMaskIntoConstraints 的本意是将 frame 布局 自动转化为 约束布局，转化的结果是为这个视图自动添加所有需要的约束，如果我们这时给视图添加自己创建的约束就一定会约束冲突。
为了避免上面说的约束冲突，我们在代码创建约束布局的控件时直接指定这个视图不能用frame布局（即translatesAutoresizingMaskIntoConstraints=NO.
以下内容来自网络:
[NSLayoutConstraint constraintWithItem:(id)item
attribute:(NSLayoutAttribute)attribute
relatedBy:(NSLayoutRelation)relation
toItem:(id)otherItem
attribute:(NSLayoutAttribute)otherAttribute
multiplier:(CGFloat)multiplier
constant:(CGFloat)constant]
1.参数说明:
第一个参数:指定约束左边的视图view1
第二个参数:指定view1的属性attr1
第三个参数:指定左右两边的视图的关系relation
第四个参数:指定约束右边的视图view2
第五个参数:指定view2的属性attr2
第六个参数:指定一个与view2属性相乘的乘数multiplier
第七个参数:指定一个与view2属性相加的浮点数constant
依据的公式是：view1.attr1 = view2.attr2*multiplier +constant
2.NSLayoutAttribute的类型：
NSLayoutAttributeLeft视图的左边
NSLayoutAttributeRight视图的右边
NSLayoutAttributeTop视图的上边
NSLayoutAttributeBottom视图的下边
NSLayoutAttributeLeading视图的前边
NSLayoutAttributeTrailing视图的后边
NSLayoutAttributeWidth视图的宽度
NSLayoutAttributeHeight视图的高度
NSLayoutAttributeCenterX视图的中点的X值
NSLayoutAttributeCenterY视图中点的Y值
NSLayoutAttributeBaseline视图的基准线
NSLayoutAttributeNotAnAttribute无属性
3.NSLayoutRelation的类型：
NSLayoutRelationLessThanOrEqual视图关系小于或等于
NSLayoutRelationEqual视图关系等于
NSLayoutRelationGreaterThanOrEqual视图关系大于或等于
//leftView的上= self.view的上+20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:leftView
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeTop
multiplier:1.
constant:20]];
// leftView的左= self.view的左+ 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:leftView
attribute:NSLayoutAttributeLeft
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeLeft
multiplier:1.
constant:20]];
// rightView的上= leftView的上
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:rightView
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeTop
multiplier:1.
constant:0]];
// rightView的左= leftView的右+ 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:rightView
attribute:NSLayoutAttributeLeft
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeRight
multiplier:1.
constant:20]];
// rightView的右= self.view的右- 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:rightView
attribute:NSLayoutAttributeRight
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeRight
multiplier:1.
constant:-20]];
// rightView的高= leftView的高
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:rightView
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeHeight
multiplier:1.
constant:0]];
// rightView的宽= leftView的宽
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:rightView
attribute:NSLayoutAttributeWidth
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeWidth
multiplier:1.0
constant:0]];
//bottomView的左= self.view的左+ 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:bottomView
attribute:NSLayoutAttributeLeft
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeLeft
multiplier:1
constant:20]];
//bottomView的右= self.view的右- 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:bottomView
attribute:NSLayoutAttributeRight
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeRight
multiplier:1
constant:-20]];
//bottomView的上= leftView的下+ 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:bottomView
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeBottom
multiplier:1
constant:20]];
//bottomView的高= leftView的高
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:bottomView
attribute:NSLayoutAttributeHeight
relatedBy:NSLayoutRelationEqual
toItem:leftView
attribute:NSLayoutAttributeHeight
multiplier:1
constant:0]];
//bottomView的下= self.view的下+ 20
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:bottomView
attribute:NSLayoutAttributeBottom
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeBottom
multiplier:1
constant:-20]];
