Swift 3 Custom Operator

Swift 3 出來之後，之前參考別人寫 autolayout + custom operator 部分也跟著不能運作。

當然光靠內建 convert 是行不通的，最好的方式還是去看改版到底改了什麼。

於是就翻了 Xcode Release Notes …

The syntax for adding new operators has changed significantly. The new language model uses a more semantic model based on named precedence groups rather than magic numbers. This only affects declaring new operators such as >>> and not adding new overloads of existing operators such as ==. (SE-0077)

custom operator 語法有顯著的改變。

新語言模組是使用更有語義的模組，並基於 precedence groups 而非 magic numbers

這只會影響到 custom operator，而不會影響既有的 operator(overloads)

詳細請看 proposal : SE-0077

首先先回顧一下 Swift 2 之前 custom operator 定義跟使用方式，以及剛剛提到的 magic numbers

可參考 NSHIPSTER:swift-operators

定義部分：

• 決定：前(prefix) 中(infix) 後置(postfix)
  範例：infix
• 符號
  範例：<~
• 結合律：左至右 右至左
  範例：left
• 優先度：越大越先做
  範例：125

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  // 定義 custom operator // 中置, 結合律：左至右,優先度 125 infix operator <~ { associativity left precedence 125 // magic numbers 150 } // 定義 custom operator 功能 public func <~ (left: NSLayoutConstraint, right: UILayoutPriority) -> NSLayoutConstraint { left.priority = right return left }

回過來看 Swift 3 該怎麼改，首先先回顧原本的 magic numbers 125

對照優先度： 120 < 125 < 130

再對照符號： && < <~ < ==

再對照 Swift 2： Conjunctive < target < Comparative

最後是 Swift 3： LogicalConjunctionPrecedence < target < ComparisonPrecedence

於是我們知道目標是處於 LogicalConjunctionPrecedence < target < ComparisonPrecedence 之間

這時才來看他所謂的更有語義的模組 precedence groups

首先需要先定義 custom precedencegroup 並給上名稱 precedencegroup YumeComparisonAssignmentPrecedence

因為目標是 infix，所以一樣要定義結合律 associativity: left

接著是有語義的比大小

• lowerThan: ComparisonPrecedence
• higherThan: LogicalConjunctionPrecedence

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precedencegroup YumeComparisonAssignmentPrecedence {
    associativity: left
    lowerThan: ComparisonPrecedence
    higherThan: LogicalConjunctionPrecedence
}

然後定義你的 custom operator 要使用哪一種 Precedence

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infix operator <~ : YumeComparisonAssignmentPrecedence

最後是實現功能

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extension NSLayoutConstraint {
    static public func <~ (left: NSLayoutConstraint, right: UILayoutPriority) -> NSLayoutConstraint {
        left.priority = right
        return left
    }
}

小抱怨：預設 Precedence 必須到 Standard library changes 查找，有點麻煩。

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precedencegroup BitwiseShiftPrecedence {
    higherThan: MultiplicationPrecedence
}

precedencegroup FunctionArrowPrecedence {
    associativity: right
}

precedencegroup MultiplicationPrecedence {
    associativity: left
    higherThan: AdditionPrecedence
}

precedencegroup TernaryPrecedence {
    associativity: right
    higherThan: AssignmentPrecedence
}

precedencegroup DefaultPrecedence {
    higherThan: TernaryPrecedence
}

precedencegroup LogicalDisjunctionPrecedence {
    associativity: left
    higherThan: TernaryPrecedence
}

precedencegroup LogicalConjunctionPrecedence {
    associativity: left
    higherThan: LogicalDisjunctionPrecedence
}

precedencegroup ComparisonPrecedence {
    higherThan: LogicalConjunctionPrecedence
}

precedencegroup NilCoalescingPrecedence {
    associativity: right
    higherThan: ComparisonPrecedence
}

precedencegroup AdditionPrecedence {
    associativity: left
    higherThan: RangeFormationPrecedence
}

precedencegroup CastingPrecedence {
    higherThan: NilCoalescingPrecedence
}

precedencegroup AssignmentPrecedence {
    associativity: right
    assignment: true
    higherThan: FunctionArrowPrecedence
}

precedencegroup RangeFormationPrecedence {
    higherThan: CastingPrecedence
}