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《重构与模式》 Swift 版之创建方法

正如上个月在几个会议上我所讲的那样，《重构与模式》这本书被多次提及，特别是在我喜欢的研讨会上。最终我还是读了一小部分（至少是在 WWDC 大会发布后的疯狂情绪高涨之前），并且我希望能够通过将书中的模式（pattern）记录下来，以供将来参考。我还发现，通过把书中的 Java 代码翻译成 Swift，我能更好地记住这些知识点。

第一个模式就是 创建方法 。

重构之前

先假设你有一个 model，比如是借款，并且里面有一些不同的初始化构造器（initializer）。在这个例子中，每个初始化构造器都有存在的理由——每个初始化构造器代表了对应了不同的借款类型：

struct Loan {
    let commitment: NSDecimalNumber
    let riskRating: Float
    let maturity: Int
    let expiry: NSDate?
    let capitalStrategy: String?
    let outstanding: NSDecimalNumber?
    
    init(commitment: NSDecimalNumber, riskRating: Float, maturity: Int, expiry: NSDate?, capitalStrategy: String?, outstanding: NSDecimalNumber?) {
        self.commitment = commitment
        self.riskRating = riskRating
        self.maturity = maturity
        self.expiry = expiry
        self.capitalStrategy = capitalStrategy
        self.outstanding = outstanding
    }
    
    init(commitment: NSDecimalNumber, riskRating: Float, maturity: Int) {
        self.init(commitment: commitment, riskRating: riskRating, maturity: maturity, expiry: nil, capitalStrategy: nil, outstanding: nil)
    }
    
    init(commitment: NSDecimalNumber, riskRating: Float, maturity: Int, expiry: NSDate) {
        self.init(commitment: commitment, riskRating: riskRating, maturity: maturity, expiry: expiry, capitalStrategy: nil, outstanding: nil)
    }
    
    init(commitment: NSDecimalNumber, outstanding: NSDecimalNumber, riskRating: Float, maturity: Int, expiry: NSDate) {
        self.init(commitment: commitment, riskRating: riskRating, maturity: maturity, expiry: expiry, capitalStrategy: nil, outstanding: outstanding)
    }
    
    init(capitalStrategy: String, commitment: NSDecimalNumber, riskRating: Float, maturity: Int, expiry: NSDate) {
        self.init(commitment: commitment, riskRating: riskRating, maturity: maturity, expiry: expiry, capitalStrategy: capitalStrategy, outstanding: nil)
    }
    
    init(capitalStrategy: String, commitment: NSDecimalNumber, outstanding: NSDecimalNumber, riskRating: Float, maturity: Int, expiry: NSDate) {
        self.init(commitment: commitment, riskRating: riskRating, maturity: maturity, expiry: expiry, capitalStrategy: capitalStrategy, outstanding: outstanding)
    }
}

看看这代码，你会完全没有思路（除非你知道所有的借贷类型），不知道该用哪个初始化构造器。当然了，一些刚刚接触这代码的新人可能还对项目的业务不是很清楚，所以他们可能会用错初始化构造器，甚至会对这些初始化构造器的作用意图感到疑惑。

是时候来重构了！

重构之后

用创建方法（Creator Method）重构后的代码如下：

struct Loan {
    let commitment: NSDecimalNumber
    let riskRating: Float
    let maturity: Int
    let expiry: NSDate?
    let capitalStrategy: String?
    let outstanding: NSDecimalNumber?
    
  // 原来的初始化构造器（initializer）现在可以是私有的
    private init(commitment: NSDecimalNumber, riskRating: Float, maturity: Int, expiry: NSDate?, capitalStrategy: String?, outstanding: NSDecimalNumber?) {
        self.commitment = commitment
        self.riskRating = riskRating
        self.maturity = maturity
        self.expiry = expiry
        self.capitalStrategy = capitalStrategy
        self.outstanding = outstanding
    }
    
    static func createTermLoan(commitment: NSDecimalNumber, riskRating: Float, maturity: Int) -> Loan {
        return Loan(commitment: commitment, riskRating: riskRating, maturity: maturity, expiry: nil, capitalStrategy: nil, outstanding: nil)
    }
    
    static func createTermLoan(capitalStrategy: String, commitment: NSDecimalNumber, outstanding: NSDecimalNumber, riskRating: Float, maturity: Int, expiry: NSDate) -> Loan {
        return Loan(commitment: commitment, riskRating: riskRating, maturity: maturity, expiry: expiry, capitalStrategy: capitalStrategy, outstanding: outstanding)
    }
    
    static func createRevolverLoan(commitment: NSDecimalNumber, outstanding: NSDecimalNumber, riskRating: Float, maturity: Int, expiry: NSDate) -> Loan {
        return Loan(commitment: commitment, riskRating: riskRating, maturity: maturity, expiry: expiry, capitalStrategy: nil, outstanding: outstanding)
    }
    
    static func createRevolverLoan(capitalStrategy: String, commitment: NSDecimalNumber, riskRating: Float, maturity: Int, expiry: NSDate) -> Loan {
        return Loan(commitment: commitment, riskRating: riskRating, maturity: maturity, expiry: expiry, capitalStrategy: capitalStrategy, outstanding: nil)
    }
    
    static func createRCTL(commitment: NSDecimalNumber, riskRating: Float, maturity: Int, expiry: NSDate) -> Loan {
        return Loan(commitment: commitment, riskRating: riskRating, maturity: maturity, expiry: expiry, capitalStrategy: nil, outstanding: nil)
    }
}