SnapKit源码分析

Snapkit版本：5.6.0

1. 给谁做约束

ConstraintView：对iOS而言是UIView，对macOS而言是NSView

#if os(iOS) || os(tvOS)
    public typealias ConstraintView = UIView
#else
    public typealias ConstraintView = NSView
#endif

给ConstraintView扩展了snp属性，snp为ConstraintViewDSL结构体

public extension ConstraintView {
    var snp: ConstraintViewDSL {
        return ConstraintViewDSL(view: self)
    }
}

ConstraintViewDSL

在ConstraintViewDSL中提供了prepareConstraints、makeConstraints等我们经常调用的方法。

public struct ConstraintViewDSL: ConstraintAttributesDSL {
    
    @discardableResult
    public func prepareConstraints(_ closure: (_ make: ConstraintMaker) -> Void) -> [Constraint] {
        return ConstraintMaker.prepareConstraints(item: self.view, closure: closure)
    }
    
    public func makeConstraints(_ closure: (_ make: ConstraintMaker) -> Void) {
        ConstraintMaker.makeConstraints(item: self.view, closure: closure)
    }
    
    public func remakeConstraints(_ closure: (_ make: ConstraintMaker) -> Void) {
        ConstraintMaker.remakeConstraints(item: self.view, closure: closure)
    }
    
    public func updateConstraints(_ closure: (_ make: ConstraintMaker) -> Void) {
        ConstraintMaker.updateConstraints(item: self.view, closure: closure)
    }
  //....
  
        internal init(view: ConstraintView) {
        self.view = view
    }
}

（1）ConstraintViewDSL遵循ConstraintAttributesDSL协议，ConstraintAttributesDSL主要是增加了iOS 8.0和OSX 10.11之后的新的属性；

（2）ConstraintAttributesDSL遵循ConstraintBasicAttributesDSL协议，ConstraintBasicAttributesDSL主要是一些如left、top、right、size等基础的布局属性。

（3）通过internal init(view: ConstraintView)方法将要设置约束的view赋值给self.view

2. 分析设置约束的过程

通过分析ConstraintViewDSL的makeConstraints方法，了解设置约束的过程

public func makeConstraints(_ closure: (_ make: ConstraintMaker) -> Void) {
  ConstraintMaker.makeConstraints(item: self.view, closure: closure)
}

这里通过调用ConstraintMaker的makeConstraints来实现，通过prepareConstraints构造Constraint后，进行逐个添加和激活

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

    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 {
            guard let constraint = description.constraint else {
                continue
            }
            constraints.append(constraint)
        }
        return constraints
    }
    

扩充属性

（1）ConstraintMaker：就是我们常写的makeConstraints回调中make的类型。

    LayoutConstraintItem：是遵循AnyObject的一个协议，扩展了prepare、superview、constraints、add、remove、constraintsSet属性和方法

    因为ConstraintView扩展了这个协议，所以可以直接传ConstraintView类型

（2）ConstraintMaker 包含left、top、centerX等基本属性，且返回ConstraintMakerExtendable，使得其能链式调用

public class ConstraintMaker {
    
    public var left: ConstraintMakerExtendable {
        return self.makeExtendableWithAttributes(.left)
    }
    
    public var top: ConstraintMakerExtendable {
        return self.makeExtendableWithAttributes(.top)
    }
    
    public var bottom: ConstraintMakerExtendable {
        return self.makeExtendableWithAttributes(.bottom)
    }
    
    public var right: ConstraintMakerExtendable {
        return self.makeExtendableWithAttributes(.right)
    }
    
    public var leading: ConstraintMakerExtendable {
        return self.makeExtendableWithAttributes(.leading)
    }
    //...
}

通过ConstraintMaker的makeExtendableWithAttributes方法，不断新增描述中的属性（description.attributes）

其中attributes遵循OptionSet, ExpressibleByIntegerLiteral协议。

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

扩充值

（3）ConstraintMakerExtendable遵循ConstraintMakerRelatable协议，扩充了equalTo、equalToSuperview、lessThanOrEqualTo、greaterThanOrEqualTo等方法。
这些方法最终都会调用ConstraintMakerRelatable的relatedTo方法，将约束描述补充，并返回ConstraintMakerEditable类型。

    internal func relatedTo(_ other: ConstraintRelatableTarget, relation: ConstraintRelation, file: String, line: UInt) -> ConstraintMakerEditable {
        let related: ConstraintItem
        let constant: ConstraintConstantTarget
        
        if let other = other as? ConstraintItem {
            guard other.attributes == ConstraintAttributes.none ||
                  other.attributes.layoutAttributes.count <= 1 ||
                  other.attributes.layoutAttributes == self.description.attributes.layoutAttributes ||
                  other.attributes == .edges && self.description.attributes == .margins ||
                  other.attributes == .margins && self.description.attributes == .edges ||
                  other.attributes == .directionalEdges && self.description.attributes == .directionalMargins ||
                  other.attributes == .directionalMargins && self.description.attributes == .directionalEdges else {
                fatalError("Cannot constraint to multiple non identical attributes. (\(file), \(line))");
            }
            
            related = other
            constant = 0.0
        } else if let other = other as? ConstraintView {
            related = ConstraintItem(target: other, attributes: ConstraintAttributes.none)
            constant = 0.0
        } else if let other = other as? ConstraintConstantTarget {
            related = ConstraintItem(target: nil, attributes: ConstraintAttributes.none)
            constant = other
        } else if #available(iOS 9.0, OSX 10.11, *), let other = other as? ConstraintLayoutGuide {
            related = ConstraintItem(target: other, attributes: ConstraintAttributes.none)
            constant = 0.0
        } else {
            fatalError("Invalid constraint. (\(file), \(line))")
        }
        
        let editable = ConstraintMakerEditable(self.description)
        editable.description.sourceLocation = (file, line)
        editable.description.relation = relation
        editable.description.related = related
        editable.description.constant = constant
        return editable
    }

扩充计算

ConstraintMakerEditable类型包含multipliedBy、offset、dividedBy、inset等方法，支持对值做相应计算。

public class ConstraintMakerEditable: ConstraintMakerPrioritizable {

    @discardableResult
    public func multipliedBy(_ amount: ConstraintMultiplierTarget) -> ConstraintMakerEditable {
        self.description.multiplier = amount
        return self
    }
    
    @discardableResult
    public func dividedBy(_ amount: ConstraintMultiplierTarget) -> ConstraintMakerEditable {
        return self.multipliedBy(1.0 / amount.constraintMultiplierTargetValue)
    }
    
    @discardableResult
    public func offset(_ amount: ConstraintOffsetTarget) -> ConstraintMakerEditable {
        self.description.constant = amount.constraintOffsetTargetValue
        return self
    }
    
    @discardableResult
    public func inset(_ amount: ConstraintInsetTarget) -> ConstraintMakerEditable {
        self.description.constant = amount.constraintInsetTargetValue
        return self
    }
    
    #if os(iOS) || os(tvOS)
    @discardableResult
    @available(iOS 11.0, tvOS 11.0, *)
    public func inset(_ amount: ConstraintDirectionalInsetTarget) -> ConstraintMakerEditable {
        self.description.constant = amount.constraintDirectionalInsetTargetValue
        return self
    }
    #endif
}

ConstraintMakerEditable继承自ConstraintMakerPrioritizable

扩充优先级

ConstraintMakerPrioritizable包含了优先级相关的方法priority、priorityRequired、priorityHigh、priorityMedium、priorityLow

ConstraintMakerPrioritizable继承自ConstraintMakerFinalizable

完整描述

ConstraintMakerFinalizable
一个只有一个类型为 ConstraintDescription 的属性的类，正如它的类名，有一个 ConstraintMakerFinalizable 实例，就得到了对于一个约束的完整描述。

过程

            blackView.snp.makeConstraints { make in
                make.center.equalTo(view)
                make.size.equalTo(CGSize(width: 100, height: 100))
            }

（1）回到ConstraintMaker的prepareConstraints方法，根据需要对属性、值、计算和优先级做一系列处理后，我们可以得到通过closure(maker)使maker.descriptions包含所有的约束描述，将每条描述再转换成Constraint类型（真实需要的约束）的约束信息，并返回[Constraint]类型

（2）对[Constraint]的每个Constraint执行 internal func activateIfNeeded(updatingExisting: Bool = false) 方法

通过NSLayoutConstraint的 open class func activate(_ constraints: [NSLayoutConstraint])让每个约束（即Constraint的layoutConstraints属性）激活

LayoutConstraint继承自UIKit.NSLayoutConstraint或者AppKit.NSLayoutConstraint

public final class Constraint {
    public var layoutConstraints: [LayoutConstraint]
}

public class LayoutConstraint : NSLayoutConstraint {
    public var label: String? {
        get {
            return self.identifier
        }
        set {
            self.identifier = newValue
        }
    }
    internal weak var constraint: Constraint? = nil
}


/* Convenience method that activates each constraint in the contained array, in the same manner as setting active=YES. This is often more efficient than activating each constraint individually. */
    @available(macOS 10.10, *)
    open class func activate(_ constraints: [NSLayoutConstraint])

让LayoutConstraintItem（也就是对应的ConstraintView）通过internal func add(constraints: [Constraint]) 方法将LayoutConstraintItem的constraintsSet添加上所有约束

其中constraintsSet是与LayoutConstraintItem相关联的

   private var constraintsSet: NSMutableSet {
        let constraintsSet: NSMutableSet
        
        if let existing = objc_getAssociatedObject(self, &constraintsKey) as? NSMutableSet {
            constraintsSet = existing
        } else {
            constraintsSet = NSMutableSet()
            objc_setAssociatedObject(self, &constraintsKey, constraintsSet, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
        return constraintsSet
    }

private var constraintsKey: UInt8 = 0

至此，SnapKit完成了约束的添加和约束与对象关联，以方便对约束的更新。