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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