获取 NSLayoutConstraints 关联视图

Question

I trying to loop through a views constraints.

I added to view1: top, trailing, leading and height constraints.

top, trailing and leading are to the main ViewControllers view.

if i loop through view1's constraints i only see the height constraint.

for constraint in view1.constraints {
    print(constraint)
}

NSLayoutConstraint:0x6180000968a0 UIView:0x7fae6b409dd0.height == 146 (active)

so i looped through its superviews constraints (the ViewControllers main view) and i got lots of constraints some of them are associated with view1.

for constraint in view1.superview?.constraints {
    print(constraint)
}

NSLayoutConstraint:0x618000096670 H:|-(0)-[UIView:0x7fae6b409dd0] (active, names: '|':UIView:0x7fae6b40a180 )

NSLayoutConstraint:0x6180000974d0 H:[UIView:0x7fae6b409dd0]-(0)-| (active, names: '|':UIView:0x7fae6b40a180 )

NSLayoutConstraint:0x618000097520 V:|-(0)-[UIView:0x7fae6b409dd0] (active, names: '|':UIView:0x7fae6b40a180 )

and i get a few more that i dont care about.

So my problem is that i want to loop through all of view1's superviews constraints and get only the ones that are associated with it.

In this example UIView:0x7fae6b409dd0 is view1.

But i cant figure out how to get that property.

Thanks

---

If i print out constraint.firstAnchor i get some more information but still cant get the associated view.

NSLayoutXAxisAnchor:0x608000265480 "UIView:0x7fae6b409dd0.leading">

NSLayoutXAxisAnchor:0x608000265480 "UIView:0x7fae6b409dd0.trailing">

NSLayoutXAxisAnchor:0x608000265480 "UIView:0x7fae6b409dd0.top">

Answer 1

You can use the firstItem and secondItem properties of NSLayoutConstraint to get the views related to the constraint. Note that secondItem is an Optional and must be unwrapped.

Then you can use the === operator to compare if it is the same object:

let constraints = view1.superview!.constraints
var count = 0

print("superview has \(constraints.count) constraints")

for constraint in constraints {
    if constraint.firstItem === view1 {
        count += 1
        print(constraint)
    } else if let secondItem = constraint.secondItem, secondItem === view1 {
        count += 1
        print(constraint)
    }
}

print("\(count) of them relate to view1")

Answer 2

一个简单的方法是比较约束对象中是否存在视图的哈希码：

[[NSString stringWithFormat:@"%@", constraint] containsString:[NSString stringWithFormat:@"%x", view.hash]]