[IOS]通过约束来改变控件位置

1.约束构造器：

 NSLayoutConstraint(item: ViewObject1, 
  attribute: NSLayoutAttribute1, 
  relatedBy: NSLayoutRelation, 
  toItem: ViewObject2, 
  attribute: NSLayoutAttribute2, 
  multiplier: CGFloatMultiplier, 
  constant: CGFloatConstant
)
ViewObject1 - the view from which we are constraining
NSLayoutAttribute1 - An NSLayoutAttribute, ex. Left, Right, Leading, Trailing, see full list here
NSLayoutRelation - An NSLaoyoutRelation, determines the relation between 2 attributes. Example Leading Edge of View Equals Leading Edge of SuperView. The possible values are  LessThanOrEqual, Equal, GreaterThanOrEqual. see full details here
ViewObject2 - is the second vew to which we are constraining
NSLayoutAttribute2 - The constraint addribute for the second view, ex Leading, Trailing, Width, etc
CGFloatMultiplier - The multiplier applied to the second attribute participating in the constraint.
CGFloatConstant - The value of the constraint. Ex. 20 points from the superview

例子：

NSLayoutConstraint *center = [NSLayoutConstraint constraintWithItem:_datePicker attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:_networkEnabled attribute:NSLayoutAttributeCenterX multiplier:1 constant:0];

2.查看第二个依赖控件：

[_setTimeTvTopConstraint secondItem]

3.创建两条约束，指向不同控件：

4.在代码中建立约束的链接：

@property (weak, nonatomic) IBOutlet NSLayoutConstraint *setTimeTvTopConstraint;
@property (weak, nonatomic) IBOutlet NSLayoutConstraint *setTimeTvTopPwdConstraint;

5.只需要在Remain Time 控件隐藏时，调整

_setTimeTvTopPwdConstraint.constant = 22;

即可自动上移。 

注意：在storyboard中定义的约束，relation firstItem secondItem都是只读属性，修改不了，固定了。如果不想重新控件画图，暂时发现只能通过这种约束方式来移动。

6.通过不同情况来添加或删除约束：

例如某些情况下靠边，某些情况下居中：

先设置靠边约束，然后设置居中约束，但是把installed前的勾取消

 

 

把约束链接到代码中，然后可以通过代码来控制不同情况的展示了：

if (!_timeRadioGroup.hidden) {
        NSLog(@"radio group:%i",_timeRadioGroup.hidden);
        [_align2ShowBtnConstraint setActive:YES];
        [_datePickerCenterConstraint setActive:NO];
    }

setActive可以决定哪个约束活动。