SnapKit 源码解读（二）：DSLs

与 Masonry 不同，SnapKit 充分利用了 Swift 的语言特性，用更优雅的方式实现了一套 DSL。而这一切的开始，源于 ConstraintDSL。

ConstraintDSL

ConstraintDSL 是一个协议：

public protocol ConstraintDSL {
    
    var target: AnyObject? { get }
    
    func setLabel(_ value: String?)
    func label() -> String?
    
}

public

public 的作用是，即使两部分代码处于两个不同的 module，也能使用这个协议。在寻常的项目中，默认的权限级别是 internal，即在同一个 module 内可以使用。如果是写三方库，则要用 public 来标记提供给外界的 API 了。

协议扩展

与 Objective-C 中的协议不同，Swift 中的协议可以通过扩展为协议添加默认的实现，如下所示，为 setLabel ,label 协议方法提供了默认的实现：

extension ConstraintDSL {
    
    public func setLabel(_ value: String?) {
        objc_setAssociatedObject(self.target as Any, &labelKey, value, .OBJC_ASSOCIATION_COPY_NONATOMIC)
    }
    public func label() -> String? {
        return objc_getAssociatedObject(self.target as Any, &labelKey) as? String
    }
    
}

关联对象

提供给 ConstraintDSL 协议的默认实现其实是实现了关联对象的效果，关联对象是通过扩展实现属性的一种方式，set 方法使用的 API 为：

@available(iOS 3.1, *)
public func objc_setAssociatedObject(_ object: Any, _ key: UnsafeRawPointer, _ value: Any?, _ policy: objc_AssociationPolicy)

object 参数表示关联的源对象，在这里是 self.target as Any，因为 Swift 是一门强类型的语言，所以需要做一个类型转换。

key 参数表示关联对象的键，SnapKit 是声明了一个 private var labelKey: UInt8 = 0 来作为 key，使用的时候需要配合 & 操作符一起使用。

value 参数表示与对象的关键键关联的值。

policy 参数表示关联的策略，是一个枚举，取值如下：

public enum objc_AssociationPolicy : UInt {

    
    /**< Specifies a weak reference to the associated object. */
    case OBJC_ASSOCIATION_ASSIGN

    /**< Specifies a strong reference to the associated object. 
     *   The association is not made atomically. */
    case OBJC_ASSOCIATION_RETAIN_NONATOMIC

    
    /**< Specifies that the associated object is copied. 
     *   The association is not made atomically. */
    case OBJC_ASSOCIATION_COPY_NONATOMIC

    
    /**< Specifies a strong reference to the associated object.
     *   The association is made atomically. */
    case OBJC_ASSOCIATION_RETAIN

    
    /**< Specifies that the associated object is copied.
     *   The association is made atomically. */
    case OBJC_ASSOCIATION_COPY
}

与 Objective-C 属性的对应关系是一致的，在此不再赘述。

关联引用的 get 的 API 如下：

@available(iOS 3.1, *)
public func objc_getAssociatedObject(_ object: Any, _ key: UnsafeRawPointer) -> Any?

所需的两个参数与 set 中的一致。

ConstraintBasicAttributesDSL

ConstraintBasicAttributesDSL 是一个继承于 ConstraintDSL 的协议，如下所示：

public protocol ConstraintBasicAttributesDSL : ConstraintDSL {
}

进而又通过扩展为其添加了一些计算属性，会返回对象和其相关的布局属性的模型对象，属于 ConstraintItem 类型，如下所示：

extension ConstraintBasicAttributesDSL {
    
    // MARK: Basics
    
    public var left: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.left)
    }
    
    public var top: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.top)
    }
    
    public var right: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.right)
    }
    
    public var bottom: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.bottom)
    }
    
    public var leading: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.leading)
    }
    
    public var trailing: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.trailing)
    }
    
    public var width: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.width)
    }
    
    public var height: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.height)
    }
    
    public var centerX: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.centerX)
    }
    
    public var centerY: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.centerY)
    }
    
    public var edges: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.edges)
    }
    
    public var size: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.size)
    }
    
    public var center: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.center)
    }
    
}

ConstraintAttributesDSL

ConstraintAttributesDSL 是一个继承于 ConstraintBasicAttributesDSL 的协议，如下所示：

public protocol ConstraintAttributesDSL : ConstraintBasicAttributesDSL {
}

进而又通过扩展为其添加了一些计算属性，会返回对象和其相关的布局属性（baseline 和 margin 相关）的模型对象，属于 ConstraintItem 类型，如下所示：

extension ConstraintAttributesDSL {
    
    // MARK: Baselines
    
    @available(*, deprecated:3.0, message:"Use .lastBaseline instead")
    public var baseline: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.lastBaseline)
    }
    
    @available(iOS 8.0, OSX 10.11, *)
    public var lastBaseline: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.lastBaseline)
    }
    
    @available(iOS 8.0, OSX 10.11, *)
    public var firstBaseline: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.firstBaseline)
    }
    
    // MARK: Margins
    
    @available(iOS 8.0, *)
    public var leftMargin: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.leftMargin)
    }
    
    @available(iOS 8.0, *)
    public var topMargin: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.topMargin)
    }
    
    @available(iOS 8.0, *)
    public var rightMargin: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.rightMargin)
    }
    
    @available(iOS 8.0, *)
    public var bottomMargin: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.bottomMargin)
    }
    
    @available(iOS 8.0, *)
    public var leadingMargin: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.leadingMargin)
    }
    
    @available(iOS 8.0, *)
    public var trailingMargin: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.trailingMargin)
    }
    
    @available(iOS 8.0, *)
    public var centerXWithinMargins: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.centerXWithinMargins)
    }
    
    @available(iOS 8.0, *)
    public var centerYWithinMargins: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.centerYWithinMargins)
    }
    
    @available(iOS 8.0, *)
    public var margins: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.margins)
    }
    
    @available(iOS 8.0, *)
    public var centerWithinMargins: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.centerWithinMargins)
    }
    
}

ConstraintViewDSL

ConstraintViewDSL 是一个遵守了 ConstraintAttributesDSL 协议的结构体，定义如下：

public struct ConstraintViewDSL: ConstraintAttributesDSL {
    
    ...
    
}

ConstraintViewDSL 初始化方法只需要一个参数，配合内部的一个属性 view 存储需要布局的视图：

internal let view: ConstraintView
    
internal init(view: ConstraintView) {
    self.view = view
        
}

ConstraintViewDSL 真正需要自己实现的代理中的属性只有一个，就是 target，这是一个用来返回布局对应的视图的计算属性，相关实现如下：

public var target: AnyObject? {
    return self.view
}

ConstraintViewDSL 声明了一些关键方法，内部都是通过调用 ConstraintMaker 类的相关方法实现的：

@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)
}
    
public func removeConstraints() {
    ConstraintMaker.removeConstraints(item: self.view)
}

@discardableResult

有的时候，方法有一个返回值，但我们经常性的会忽视这个返回值，这时可以在方法前加上 @discardableResult 标记，可以在这种情况下不出现警告。

最后，声明了一些计算属性，作为一些设置 view 相关属性的 shortcut：

public var contentHuggingHorizontalPriority: Float {
    get {
        return self.view.contentHuggingPriority(for: .horizontal).rawValue
    }
    set {
        self.view.setContentHuggingPriority(LayoutPriority(rawValue: newValue), for: .horizontal)
    }
}
    
public var contentHuggingVerticalPriority: Float {
    get {
        return self.view.contentHuggingPriority(for: .vertical).rawValue
    }
    set {
        self.view.setContentHuggingPriority(LayoutPriority(rawValue: newValue), for: .vertical)
    }
}
    
public var contentCompressionResistanceHorizontalPriority: Float {
    get {
        return self.view.contentCompressionResistancePriority(for: .horizontal).rawValue
    }
    set {
        self.view.setContentCompressionResistancePriority(LayoutPriority(rawValue: newValue), for: .horizontal)
    }
}
    
public var contentCompressionResistanceVerticalPriority: Float {
    get {
        return self.view.contentCompressionResistancePriority(for: .vertical).rawValue
    }
    set {
        self.view.setContentCompressionResistancePriority(LayoutPriority(rawValue: newValue), for: .vertical)
    }
}

ConstraintLayoutGuideDSL

ConstraintLayoutGuideDSL 是一个遵守了 ConstraintAttributesDSL 协议的结构体，作用是替 UILayoutGuide 提供了一些 DSLs，与 ConstraintViewDSL 唯一的区别在于，target 属性返回的对象不同：

public struct ConstraintLayoutGuideDSL: ConstraintAttributesDSL {
    
    ...
    
    public var target: AnyObject? {
        return self.guide
    }
    
    internal let guide: ConstraintLayoutGuide
    
    internal init(guide: ConstraintLayoutGuide) {
        self.guide = guide
        
    }
    
}

ConstraintLayoutSupportDSL

ConstraintLayoutSupportDSL 是一个遵守了 ConstraintDSL 协议的结构体，作用是替 ConstraintLayoutSupport 提供了一些 DSLs：

@available(iOS 8.0, *)
public struct ConstraintLayoutSupportDSL: ConstraintDSL {
    
    public var target: AnyObject? {
        return self.support
    }
    
    internal let support: ConstraintLayoutSupport
    
    internal init(support: ConstraintLayoutSupport) {
        self.support = support
        
    }
    
    public var top: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.top)
    }
    
    public var bottom: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.bottom)
    }
    
    public var height: ConstraintItem {
        return ConstraintItem(target: self.target, attributes: ConstraintAttributes.height)
    }
}

原文地址：SnapKit 源码解读（二）：DSLs

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