Snapkit - right.width.center 和 right, width, center 有区别吗？

Question

make.right.width.center equalTo(superview).offset(-20)

对

make.right.equalTo(superview).offset(-20)
make.width.equalTo(superview).offset(-20)
make.center.equalTo(superview).offset(-20) 

• 定位任何对象（标签、图像等）时是否有不同。我正在查看另一个开发人员的代码并想到以下两件事之一： 1. 这是分组定位，非常有效。然后，我想，伙计，这可能是右侧的宽度和中心锚点吗？..有点困惑哈哈。任何帮助表示赞赏。谢谢。

Answer 1

是的，效果一样

 greenView.snp.makeConstraints { make in
            make.right.width.centerX.centerY.equalTo(view).offset(-20)
            make.height.equalTo(130)
        }

等于

   greenView.snp.makeConstraints { make in
            make.right.equalTo(view).offset(-20)
            make.width.equalTo(view).offset(-20)
            make.center.equalTo(view).offset(-20)
            make.height.equalTo(130)
        }

---

可以从源码中阅读

这个过程分为两部分，

设置数据，然后激活布局约束

---

• make.right.width.centerX.centerY.equalTo(view).offset(-20)的设置数据部分

make.right 创建一个ConstraintMakerExtendable

public var right: ConstraintMakerExtendable {
        return self.makeExtendableWithAttributes(.right)
    }

通话

internal func makeExtendableWithAttributes(_ attributes: ConstraintAttributes) -> ConstraintMakerExtendable {
        let description = ConstraintDescription(item: self.item, attributes: attributes)
        self.descriptions.append(description)
        return ConstraintMakerExtendable(description)
    }

make.right.width，表示ConstraintMakerExtendable的description.attributes加一个宽度ConstraintAttributes

public var width: ConstraintMakerExtendable {
        self.description.attributes += .width
        return self
    }

最后的.offset(-20)，表示ConstraintMakerExtendable的偏移量已设置

@discardableResult
    public func offset(_ amount: ConstraintOffsetTarget) -> ConstraintMakerEditable {
        self.description.constant = amount.constraintOffsetTargetValue
        return self
    }

---

• make.right.width.centerX.centerY.equalTo(view).offset(-20)的活动布局约束部分

makeConstraints{ make in }的实现

internal static func makeConstraints(item: LayoutConstraintItem, closure: (_ make: ConstraintMaker) -> Void) {
        // prepare  the layout constraints 
        let constraints = prepareConstraints(item: item, closure: closure)
        //  active the layout constraints
        for constraint in constraints {
            constraint.activateIfNeeded(updatingExisting: false)
        }
    }

准备布局约束

重点是description.constraint

 internal static func prepareConstraints(item: LayoutConstraintItem, closure: (_ make: ConstraintMaker) -> Void) -> [Constraint] {
        let maker = ConstraintMaker(item: item)
        closure(maker)
        var constraints: [Constraint] = []
        for description in maker.descriptions {
             // the key point
            guard let constraint = description.constraint else {
                continue
            }
            constraints.append(constraint)
        }
        return constraints
    }

description.constraint 初始化布局约束

class Constraint 有一个属性layoutConstraints，它属于[LayoutConstraint]。

LayoutConstraint 继承自 NSLayoutConstraint

这里是描述的偏移量

internal lazy var constraint: Constraint? = {
        guard let relation = self.relation,
              let related = self.related,
              let sourceLocation = self.sourceLocation else {
            return nil
        }
        let from = ConstraintItem(target: self.item, attributes: self.attributes)
        
        return Constraint(
            from: from,
            to: related,
            relation: relation,
            sourceLocation: sourceLocation,
            label: self.label,
            multiplier: self.multiplier,
            constant: self.constant,      // here  it is
            priority: self.priority
        )
    }()

---

• 所以这个案子很简单，

  make.right.equalTo(superview).offset(-20)
  make.width.equalTo(superview).offset(-20)
  make.center.equalTo(superview).offset(-20) 
  

三个ConstraintMakerExtendable具有相同的偏移描述