# 12. Flyweight

###### tags: `DesignPatterns`

## 關於 Flyweight 本篇將討論以下幾個問題

> ### 1. 關於 Flyweight
> ### 2. UML
> ### 3. 將 UML 轉為程式碼
> ### 4. 情境

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## 測試環境：
>OS：Windows 10
>IDE：Visual Studio 2019

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## 1. 關於 Flyweight
> Use sharing to support large numbers of fine-grained objects efficiently.
>
> by [Gang of Four](https://en.wikipedia.org/wiki/Design_Patterns)

- 用來有效率的共享大量細粒度的對象

Flyweight(享元)屬於結構型(Structural Patterns)，當遇到**系統中頻繁創建且只會異動狀態而不改變物件本身**的小物件時，可藉由 Flyweight 來緩存物件，提供不同呼叫端重複使用，達到節省記憶體的目的。

優點：
- 藉由共用相似物件來達到節省記憶體

缺點：
- 使用 Flyweight 的物件存於緩存中，就算沒使用也不會被 GC 回收

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## 2. UML

Class 間關聯：
- FlyweightFactory 可包含 Flyweight
- Client 關聯 FlyweightFactory & ConcreteFlyweight
- ConcreteFlyweight 繼承 Flyweight

Class：
- Client：呼叫端
- FlyweightFactory：以工廠方法實作享元工廠
- Flyweight：享元的抽象類別或介面
- ConcreteFlyweight：享元實作

---
## 3. 將 UML 轉為程式碼
享元介面

/// <summary>
/// 享元介面
/// </summary>
public interface IFlyweight
{
    void Operation(int extrinsicState);
}

享元工廠

/// <summary>
/// 享元工廠
/// </summary>
public class FlyweightFactory
{
    private Hashtable flyweights = new Hashtable();

    public FlyweightFactory()
    {
        flyweights.Add("One", new ConcreteFlyweight());
        flyweights.Add("Two", new ConcreteFlyweight());
        flyweights.Add("Three", new ConcreteFlyweight());
    }

    public IFlyweight GetFlyweight(string key)
    {
        return ((IFlyweight)flyweights[key]);
    }
}

享元實作

/// <summary>
/// 享元實作
/// </summary>
public class ConcreteFlyweight : IFlyweight
{
    public void Operation(int extrinsicState)
    {
        Console.WriteLine($"ConcreteFlyweight: {extrinsicState}");
    }
}

1. 建立享元工廠`factory`
2. 取得享元工廠中已建立實體
3. 操作享元物件

static void Main(string[] args)
{
    int extrinsicstate = 0;

    Default.FlyweightFactory factory = new Default.FlyweightFactory();

    Default.IFlyweight flyweight_1 = factory.GetFlyweight("One");
    flyweight_1.Operation(++extrinsicstate);

    Default.IFlyweight flyweight_2 = factory.GetFlyweight("Two");
    flyweight_2.Operation(++extrinsicstate);

    Default.IFlyweight flyweight_3 = factory.GetFlyweight("Three");
    flyweight_3.Operation(++extrinsicstate);

    Console.ReadLine();
}

執行結果

ConcreteFlyweight: 1
ConcreteFlyweight: 2
ConcreteFlyweight: 3

---
## 4. 情境
我們接到了一個提供線上商城客戶結帳時轉換匯率的需求
- 線上商城客戶很多，每個客戶都要 new 匯率 class 很吃記憶體
- 匯率固定提供部分貨幣類別，介面相同

貨幣類別 Enum

/// <summary>
/// 貨幣類別
/// </summary>
public enum CurrencyType
{
    TWD,
    JPY,
    USD,
    EUR,
}

實作匯率工廠，並在建構子提供對應匯率

/// <summary>
/// 匯率工廠
/// </summary>
public class ExchangeRatesFactory
{
    private Hashtable flyweights = new Hashtable();

    public ExchangeRatesFactory()
    {
        flyweights.Add(CurrencyType.TWD, new ExchangeRates((decimal)1));
        flyweights.Add(CurrencyType.JPY, new ExchangeRates((decimal)0.273));
        flyweights.Add(CurrencyType.USD, new ExchangeRates((decimal)28.235));
        flyweights.Add(CurrencyType.EUR, new ExchangeRates((decimal)34.35));
    }

    public IExchangeRates GetConverter(CurrencyType currencyType)
    {
        return ((IExchangeRates)flyweights[currencyType]);
    }
}

轉換介面

/// <summary>
/// 轉換介面
/// </summary>
public interface IExchangeRates
{
    public decimal Converter(int amount);
}

傳換實作

/// <summary>
/// 傳換實作
/// </summary>
public class ExchangeRates : IExchangeRates
{
    private decimal _CashRate { get; }
    public ExchangeRates (decimal cashRate)
    {
        _CashRate = cashRate;
    }
    public decimal Converter(int amount)
    {
        var newAmount = amount / _CashRate;
        return decimal.Parse(newAmount.ToString("#0.00"));
    }
}

1. 建立享元工廠`factory`
2. 取得享元物件(匯率轉換)
3. 透過享元物件計算金額

static void Main(string[] args)
{
    Situation.ExchangeRatesFactory factory = new Situation.ExchangeRatesFactory();
    int amount = 100;

    Situation.IExchangeRates converter_JPY = factory.GetConverter(Situation.CurrencyType.JPY);
    var jpy = converter_JPY.Converter(amount);

    Console.WriteLine($"TWD 轉換至 JPY：{amount} → {jpy}");

    Situation.IExchangeRates converter_USD = factory.GetConverter(Situation.CurrencyType.USD);
    var usd = converter_USD.Converter(amount);

    Console.WriteLine($"TWD 轉換至 USD：{amount} → {usd}");

    Situation.IExchangeRates converter_EUR = factory.GetConverter(Situation.CurrencyType.EUR);
    var eur = converter_EUR.Converter(amount);

    Console.WriteLine($"TWD 轉換至 EUR：{amount} → {eur}");

    Console.ReadLine();
}

執行結果

TWD 轉換至 JPY：100 → 366.30
TWD 轉換至 USD：100 → 3.54
TWD 轉換至 EUR：100 → 2.91

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## 完整程式碼
GitHub：[Structural_06_Flyweight](https://github.com/darionnnnnn/blog/tree/master/Blog/Structural_06_Flyweight)

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## 總結
### Flyweight 屬於結構型，重點是重複使用而並非創建，所以創建的部分使用工廠來處理。 Flyweight 的物件是全部呼叫端共用，若是可修改內部狀態，則可能會造成 A 處理到一半時狀態被 B 修改，而造成 A 的結果與預期不同，這問題也許開發測試期間未必會發現，等到實際上線時才發現已經亂成一團了。

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## 參考資料
1. [Design Patterns](https://en.wikipedia.org/wiki/Design_Patterns)
2. [大話設計模式](https://www.tenlong.com.tw/products/9789866761799)
2. [dofactory](https://www.dofactory.com/)
3. [Refactoring.Guru](https://refactoring.guru/)

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## 新手上路，若有錯誤還請告知，謝謝