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+---
+layout: single
+title: "Design Pattern 系列 (4) - Simple Factory Pattern"
+date: 2022-02-13 23:45:00 +0800
+excerpt: "將會變動的創建物件程式碼與不會變動的程式碼分開吧，增加程式碼可讀性及維護性"
+header:
+  overlay_image: /assets/images/vindemia-winery-wGk29doZtpQ-unsplash.jpg
+  teaser: /assets/images/vindemia-winery-wGk29doZtpQ-unsplash.jpg
+tags: [Simple Factory Pattern]
+categories: [Design Pattern]
+---
+
+> 您可於此 [design_pattern repo](https://github.com/nickhuang9527/design_pattern) 下載 Design Pattern 系列程式碼。
+
+今天我們想做一套飲料點餐系統，很快的我們畫出 UML 並寫出了下面的程式碼
+
+![design_pattern_4_simple_factory_pattern_uml_1](/blog/assets/images/design_pattern_4_simple_factory_pattern_uml_1.png)
+
+```swift
+public protocol Beverage {
+    func addSuger(level: Int)
+    func addIce(level: Int)
+    func shake()
+    func packageUp()
+}
+
+public extension Beverage {
+    func addSuger(level: Int) {
+        print("[\(self)] addSuger \(level)")
+    }
+
+    func addIce(level: Int) {
+        print("[\(self)] addIce \(level)")
+    }
+
+    func shake() {
+        print("[\(self)] shake")
+    }
+
+    func packageUp() {
+        print("[\(self)] packageUp")
+    }
+}
+
+public class BlackTea: Beverage {
+
+}
+
+public class GreenTea: Beverage {
+
+}
+
+public class BeverageShop {
+    public init() {}
+
+    public func order(beverageName: String) -> Beverage? {
+        var beverage: Beverage?
+
+        switch beverageName {
+        case "black tea":
+            beverage = BlackTea()
+        case "green tea":
+            beverage = GreenTea()
+        default:
+            break
+        }
+
+        beverage?.addSuger(level: 5)
+        beverage?.addIce(level: 5)
+        beverage?.shake()
+        beverage?.packageUp()
+
+        return beverage
+    }
+}
+
+let beverageShop = BeverageShop()
+let blackTea = beverageShop.order(beverageName: "black tea")
+let greenTea = beverageShop.order(beverageName: "green tea")
+```
+
+隨著飲料店越來越多新飲品，我們也需要修改 order 方法，但這樣容易影響不會變動的程式碼，於是我們需要找出 `需要變動` 以及 `不需變動` 的程式碼，把它們分隔開來
+
+需要變動的程式碼
+
+```swift
+switch beverageName {
+case "black tea":
+    beverage = BlackTea()
+case "green tea":
+    beverage = GreenTea()
+// case "milk tea":
+    // beverage = MilkTea()
+default:
+    break
+}
+```
+
+不需變動的程式碼
+
+```swift
+beverage?.addSuger(level: 5)
+beverage?.addIce(level: 5)
+beverage?.shake()
+beverage?.packageUp()
+```
+
+找出後該如何做呢，這時候需要用到 `簡單工廠模式` 來將其分離
+
+## Simple Factory Pattern 簡單工廠模式
+
+其實就是定義一個工廠類別來專門處理創建物件的邏輯
+那就讓我們修改一下上面的 UML 及程式碼吧
+
+![design_pattern_4_simple_factory_pattern_uml_2](/blog/assets/images/design_pattern_4_simple_factory_pattern_uml_2.png)
+
+```swift
+class BeverageFactory {
+    class func createBeverage(beverageName: String) -> Beverage? {
+        var beverage: Beverage?
+
+        switch beverageName {
+        case "black tea":
+            beverage = BlackTea()
+        case "green tea":
+            beverage = GreenTea()
+        default:
+            break
+        }
+
+        return beverage
+    }
+}
+
+public class BeverageShop {
+    public init() {}
+
+    public func order(beverageName: String) -> Beverage? {
+        let beverage: Beverage? = BeverageFactory.createBeverage(beverageName: beverageName)
+
+        beverage?.addSuger(level: 5)
+        beverage?.addIce(level: 5)
+        beverage?.shake()
+        beverage?.packageUp()
+
+        return beverage
+    }
+}
+```
+
+透過簡單工廠模式，我們就將 `需要變動` 以及 `不需變動` 的程式碼成功分隔開來，當要修改菜單時，只需修改 `BeverageFactory` 即可，不會影響到其他程式碼。
+
+> 簡單工廠其實不是設計模式，反而比較像是一種編成習慣
+>
+> 有些開發者的確是把這個編成習慣誤認為 `工廠模式` (Factory Pattern)
+>
+> 不要因為簡單工廠不是一個 `真正的` 模式，就忽略了它的用法。
+>
+> -- Head First Design Pattern Ch.4 P.117
+
+最後來看一下 `簡單工廠模式` 的 UML 圖
+![design_pattern_4_simple_factory_pattern_uml_3](/blog/assets/images/design_pattern_4_simple_factory_pattern_uml_3.png)
+
+## 總結
+
+簡單工廠雖然不是 23 個設計模式之中的一種，但它非常簡單，且能訓練我們將變動及不會變動的程式碼分離的習慣
+來看一下我們在簡單工廠用到了哪些 [Design Principle](https://nickhuang9527.github.io/blog/design%20pattern/design-pattern-1-design-principle/)
+
+- Encapsulate What Varies
+- Single Responsibility Principle
+
+下一篇正式進入 23 個 Design Pattern 的第一個 Factory Method Pattern 工廠方法模式
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