Targets 是一套协议，为基本数据类型扩充了一些方法，更方便我们进行 AutoLayout。
ConstraintRelatableTarget
ConstraintRelatableTarget 是一个空的协议，如下：
public protocol ConstraintRelatableTarget {
}
按我个人理解，它是为了让所有遵守它的协议有了一个类似公共父类的作用，便于进行类型匹配。
extension Int: ConstraintRelatableTarget {
}
extension UInt: ConstraintRelatableTarget {
}
extension Float: ConstraintRelatableTarget {
}
extension Double: ConstraintRelatableTarget {
}
extension CGFloat: ConstraintRelatableTarget {
}
extension CGSize: ConstraintRelatableTarget {
}
extension CGPoint: ConstraintRelatableTarget {
}
extension ConstraintInsets: ConstraintRelatableTarget {
}
extension ConstraintItem: ConstraintRelatableTarget {
}
extension ConstraintView: ConstraintRelatableTarget {
}
@available(iOS 9.0, OSX 10.11, *)
extension ConstraintLayoutGuide: ConstraintRelatableTarget {
}
ConstraintConstantTarget
ConstraintConstantTarget 也是一个空的协议，遵守它的有 CGPoint、CGSize 和 ConstraintInsets 这三种数据类型，并通过协议扩展为它们增加了一个方法 constraintConstantTargetValueFor，用于根据 LayoutAttribute 获得组合数据类型中正确的值。
extension ConstraintConstantTarget {
internal func constraintConstantTargetValueFor(layoutAttribute: LayoutAttribute) -> CGFloat {
if let value = self as? CGFloat {
return value
}
if let value = self as? Float {
return CGFloat(value)
}
if let value = self as? Double {
return CGFloat(value)
}
if let value = self as? Int {
return CGFloat(value)
}
if let value = self as? UInt {
return CGFloat(value)
}
if let value = self as? CGSize {
if layoutAttribute == .width {
return value.width
} else if layoutAttribute == .height {
return value.height
} else {
return 0.0
}
}
if let value = self as? CGPoint {
#if os(iOS) || os(tvOS)
switch layoutAttribute {
case .left, .right, .leading, .trailing, .centerX, .leftMargin, .rightMargin, .leadingMargin, .trailingMargin, .centerXWithinMargins:
return value.x
case .top, .bottom, .centerY, .topMargin, .bottomMargin, .centerYWithinMargins, .lastBaseline, .firstBaseline:
return value.y
case .width, .height, .notAnAttribute:
return 0.0
}
#else
switch layoutAttribute {
case .left, .right, .leading, .trailing, .centerX:
return value.x
case .top, .bottom, .centerY, .lastBaseline, .firstBaseline:
return value.y
case .width, .height, .notAnAttribute:
return 0.0
}
#endif
}
if let value = self as? ConstraintInsets {
#if os(iOS) || os(tvOS)
switch layoutAttribute {
case .left, .leftMargin, .centerX, .centerXWithinMargins:
return value.left
case .top, .topMargin, .centerY, .centerYWithinMargins, .lastBaseline, .firstBaseline:
return value.top
case .right, .rightMargin:
return -value.right
case .bottom, .bottomMargin:
return -value.bottom
case .leading, .leadingMargin:
return (ConstraintConfig.interfaceLayoutDirection == .leftToRight) ? value.left : value.right
case .trailing, .trailingMargin:
return (ConstraintConfig.interfaceLayoutDirection == .leftToRight) ? -value.right : -value.left
case .width:
return -(value.left + value.right)
case .height:
return -(value.top + value.bottom)
case .notAnAttribute:
return 0.0
}
#else
switch layoutAttribute {
case .left, .centerX:
return value.left
case .top, .centerY, .lastBaseline, .firstBaseline:
return value.top
case .right:
return -value.right
case .bottom:
return -value.bottom
case .leading:
return (ConstraintConfig.interfaceLayoutDirection == .leftToRight) ? value.left : value.right
case .trailing:
return (ConstraintConfig.interfaceLayoutDirection == .leftToRight) ? -value.right : -value.left
case .width:
return -(value.left + value.right)
case .height:
return -(value.top + value.bottom)
case .notAnAttribute:
return 0.0
}
#endif
}
return 0.0
}
}
ConstraintPriorityTarget
ConstraintPriorityTarget 是一个协议，有一个计算属性的声明：
public protocol ConstraintPriorityTarget {
var constraintPriorityTargetValue: Float { get }
}
作用是把不同类型的值转换为 Float 类型，以便供 priority 使用。
extension Int: ConstraintPriorityTarget {
public var constraintPriorityTargetValue: Float {
return Float(self)
}
}
extension UInt: ConstraintPriorityTarget {
public var constraintPriorityTargetValue: Float {
return Float(self)
}
}
extension Float: ConstraintPriorityTarget {
public var constraintPriorityTargetValue: Float {
return self
}
}
extension Double: ConstraintPriorityTarget {
public var constraintPriorityTargetValue: Float {
return Float(self)
}
}
extension CGFloat: ConstraintPriorityTarget {
public var constraintPriorityTargetValue: Float {
return Float(self)
}
}
ConstraintMultiplierTarget
ConstraintMultiplierTarget 是一个协议，有一个计算属性的声明：
public protocol ConstraintMultiplierTarget {
var constraintMultiplierTargetValue: CGFloat { get }
}
作用是把不同类型的值转换为 CGFloat 类型，以便供 multiplier 使用。
extension Int: ConstraintMultiplierTarget {
public var constraintMultiplierTargetValue: CGFloat {
return CGFloat(self)
}
}
extension UInt: ConstraintMultiplierTarget {
public var constraintMultiplierTargetValue: CGFloat {
return CGFloat(self)
}
}
extension Float: ConstraintMultiplierTarget {
public var constraintMultiplierTargetValue: CGFloat {
return CGFloat(self)
}
}
extension Double: ConstraintMultiplierTarget {
public var constraintMultiplierTargetValue: CGFloat {
return CGFloat(self)
}
}
extension CGFloat: ConstraintMultiplierTarget {
public var constraintMultiplierTargetValue: CGFloat {
return self
}
}
ConstraintOffsetTarget
ConstraintOffsetTarget 是一个协议，继承自 ConstraintConstantTarget：
public protocol ConstraintOffsetTarget: ConstraintConstantTarget {
}
通过协议扩展为遵守该协议的类型增加了新的方法，作用是把不同类型的值转换为 CGFloat 类型，以便供 offset 使用。
extension Int: ConstraintOffsetTarget {
}
extension UInt: ConstraintOffsetTarget {
}
extension Float: ConstraintOffsetTarget {
}
extension Double: ConstraintOffsetTarget {
}
extension CGFloat: ConstraintOffsetTarget {
}
extension ConstraintOffsetTarget {
internal var constraintOffsetTargetValue: CGFloat {
let offset: CGFloat
if let amount = self as? Float {
offset = CGFloat(amount)
} else if let amount = self as? Double {
offset = CGFloat(amount)
} else if let amount = self as? CGFloat {
offset = CGFloat(amount)
} else if let amount = self as? Int {
offset = CGFloat(amount)
} else if let amount = self as? UInt {
offset = CGFloat(amount)
} else {
offset = 0.0
}
return offset
}
}
ConstraintInsetTarget
ConstraintOffsetTarget 是一个协议，继承自 ConstraintConstantTarget：
public protocol ConstraintInsetTarget: ConstraintConstantTarget {
}
通过协议扩展为遵守该协议的类型增加了新的方法，作用是把不同类型的值转换为 ConstraintInsets 类型，以便供 inset 使用。
extension Int: ConstraintInsetTarget {
}
extension UInt: ConstraintInsetTarget {
}
extension Float: ConstraintInsetTarget {
}
extension Double: ConstraintInsetTarget {
}
extension CGFloat: ConstraintInsetTarget {
}
extension ConstraintInsets: ConstraintInsetTarget {
}
extension ConstraintInsetTarget {
internal var constraintInsetTargetValue: ConstraintInsets {
if let amount = self as? ConstraintInsets {
return amount
} else if let amount = self as? Float {
return ConstraintInsets(top: CGFloat(amount), left: CGFloat(amount), bottom: CGFloat(amount), right: CGFloat(amount))
} else if let amount = self as? Double {
return ConstraintInsets(top: CGFloat(amount), left: CGFloat(amount), bottom: CGFloat(amount), right: CGFloat(amount))
} else if let amount = self as? CGFloat {
return ConstraintInsets(top: amount, left: amount, bottom: amount, right: amount)
} else if let amount = self as? Int {
return ConstraintInsets(top: CGFloat(amount), left: CGFloat(amount), bottom: CGFloat(amount), right: CGFloat(amount))
} else if let amount = self as? UInt {
return ConstraintInsets(top: CGFloat(amount), left: CGFloat(amount), bottom: CGFloat(amount), right: CGFloat(amount))
} else {
return ConstraintInsets(top: 0, left: 0, bottom: 0, right: 0)
}
}
}
原文地址：SnapKit 源码解读（四）：Targets
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