與 Masonry 不同，SnapKit 充分利用了 Swift 的語言特性，用更優雅的方式實現了一套 DSL。而這一切的開始，源於 ConstraintDSL。
ConstraintDSL
ConstraintDSL 是一個協議：
public protocol ConstraintDSL {
var target: AnyObject? { get }
func setLabel(_ value: String?)
func label() -> String?
}
複製程式碼public
public 的作用是，即使兩部分程式碼處於兩個不同的 module，也能使用這個協議。在尋常的專案中，預設的許可權級別是 internal，即在同一個 module 內可以使用。如果是寫三方庫，則要用 public 來標記提供給外界的 API 了。
協議擴充套件
與 Objective-C 中的協議不同，Swift 中的協議可以通過擴充套件為協議新增預設的實現，如下所示，為 setLabel ,label 協議方法提供了預設的實現：
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
}
}
複製程式碼關聯物件
提供給 ConstraintDSL 協議的預設實現其實是實現了關聯物件的效果，關聯物件是通過擴充套件實現屬性的一種方式，set 方法使用的 API 為：
@available(iOS 3.1, *)
public func objc_setAssociatedObject(_ object: Any, _ key: UnsafeRawPointer, _ value: Any?, _ policy: objc_AssociationPolicy)
複製程式碼object 參數列示關聯的源物件，在這裡是 self.target as Any，因為 Swift 是一門強型別的語言，所以需要做一個型別轉換。
key 參數列示關聯物件的鍵，SnapKit 是宣告瞭一個 private var labelKey: UInt8 = 0 來作為 key，使用的時候需要配合 & 操作符一起使用。
value 參數列示與物件的關鍵鍵關聯的值。
policy 參數列示關聯的策略，是一個列舉，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
}
複製程式碼與 Objective-C 屬性的對應關係是一致的，在此不再贅述。
關聯引用的 get 的 API 如下：
@available(iOS 3.1, *)
public func objc_getAssociatedObject(_ object: Any, _ key: UnsafeRawPointer) -> Any?
複製程式碼所需的兩個引數與 set 中的一致。
ConstraintBasicAttributesDSL
ConstraintBasicAttributesDSL 是一個繼承於 ConstraintDSL 的協議，如下所示：
public protocol ConstraintBasicAttributesDSL : ConstraintDSL {
}
複製程式碼進而又通過擴充套件為其新增了一些計算屬性，會返回物件和其相關的佈局屬性的模型物件，屬於 ConstraintItem 型別，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)
}
}
複製程式碼ConstraintAttributesDSL
ConstraintAttributesDSL 是一個繼承於 ConstraintBasicAttributesDSL 的協議，如下所示：
public protocol ConstraintAttributesDSL : ConstraintBasicAttributesDSL {
}
複製程式碼進而又通過擴充套件為其新增了一些計算屬性，會返回物件和其相關的佈局屬性（baseline 和 margin 相關）的模型物件，屬於 ConstraintItem 型別，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)
}
}
複製程式碼ConstraintViewDSL
ConstraintViewDSL 是一個遵守了 ConstraintAttributesDSL 協議的結構體，定義如下：
public struct ConstraintViewDSL: ConstraintAttributesDSL {
...
}
複製程式碼ConstraintViewDSL 初始化方法只需要一個引數，配合內部的一個屬性 view 儲存需要佈局的檢視：
internal let view: ConstraintView
internal init(view: ConstraintView) {
self.view = view
}
複製程式碼ConstraintViewDSL 真正需要自己實現的代理中的屬性只有一個，就是 target，這是一個用來返回佈局對應的檢視的計算屬性，相關實現如下：
public var target: AnyObject? {
return self.view
}
複製程式碼ConstraintViewDSL 宣告瞭一些關鍵方法，內部都是通過呼叫 ConstraintMaker 類的相關方法實現的：
@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)
}
複製程式碼@discardableResult
有的時候，方法有一個返回值，但我們經常性的會忽視這個返回值，這時可以在方法前加上 @discardableResult 標記，可以在這種情況下不出現警告。
最後，宣告瞭一些計算屬性，作為一些設定 view 相關屬性的 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)
}
}
複製程式碼ConstraintLayoutGuideDSL
ConstraintLayoutGuideDSL 是一個遵守了 ConstraintAttributesDSL 協議的結構體，作用是替 UILayoutGuide 提供了一些 DSLs，與 ConstraintViewDSL 唯一的區別在於，target 屬性返回的物件不同：
public struct ConstraintLayoutGuideDSL: ConstraintAttributesDSL {
...
public var target: AnyObject? {
return self.guide
}
internal let guide: ConstraintLayoutGuide
internal init(guide: ConstraintLayoutGuide) {
self.guide = guide
}
}
複製程式碼ConstraintLayoutSupportDSL
ConstraintLayoutSupportDSL 是一個遵守了 ConstraintDSL 協議的結構體，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)
}
}
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