設計模式 Swift（簡單工廠）

閱讀大話設計模式心得，使用 Swift Ver. 3.0 (Jun 6, 2016)

Swift API Doc

簡單計算機程式

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let numA        = 10
let numB        = 20
var resultNum   = 0
let strOperate  = "+"

switch(strOperate){
    case "+":
        resultNum = numA + numB
    case "-":
        resultNum = numA - numB
    case "*":
        resultNum = numA * numB
    case "/":
        if numB != 0 {
            resultNum = numA / numB    
        } else {
            print("Error")    
        }
    default:
        print("default")
}
print("Result = \(resultNum)")

物件導向程式

希望程式 1.可維護 2.可複用 3.可擴展 4.靈活性好

=> 透過程式重構、封裝、繼承、多型、降低偶合度

功能封裝

把業務邏輯(實際計算過程)和介面邏輯(GUI等顯示畫面)分開

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/* 使用封裝特性
   業務邏輯 -> 計算類別，此類別可重複使用 */
class Operator {
    class func GetResult(_ numA:Int,_ numB:Int,_ strOperate:String) -> Int {
        var resultNum   = 0
        switch(strOperate){
            case "+":
                resultNum = numA + numB
            case "-":
                resultNum = numA - numB
            case "*":
                resultNum = numA * numB
            case "/":
                if numB != 0 {
                    resultNum = numA / numB    
                } else {
                    print("Error")    
                }
            default:
                print("default")
        }    
        return resultNum
    }
}

let numA        = 10
let numB        = 20
let strOperate  = "+"
print("Result = \(Operator.GetResult(numA,numB,strOperate))")

耦合度

新加功能 : 要加上 sqrt 開根號功能
一般想法 : 直接在 switch 中加一個新的判斷
產生問題 :
1.程式碼要整個重新編譯(+-/)，即使 +-/ 沒有更動
2.有可能修改到原有+-*/的程式碼，導致程式出錯

class Operation {
    private var _numA = 0
    private var _numB = 0

    var NumA: Int 
    {
        get { return _numA      }
        set { _numA = newValue  }
    }

    var NumB: Int 
    {
        get { return _numB      }
        set { _numB = newValue  }
    }    

    func GetResult() -> Int {
        let resultNum = 0
        return resultNum
    }
}

class OperationAdd : Operation {
    override func GetResult() -> Int { return NumA + NumB }
}

class OperationSub : Operation {
    override func GetResult() -> Int { return NumA - NumB }
}

class OperationMul : Operation {
    override func GetResult() -> Int { return NumA * NumB }
}

class OperationDiv : Operation {
    override func GetResult() -> Int { return NumA / NumB }
}