.NET 設計模式實戰：享元模式深度解析與文件字元共享案例

模式簡介

享元模式運用共享技術有效支援大量細粒度物件。核心概念區分：

• 內部狀態：物件內部固定、可共享的部分（不隨環境改變）
• 外部狀態：隨環境變化、不可共享的部分（由用戶端傳入）

將外部狀態移出物件，透過共享極少數內部狀態物件，大幅減少實例數量。

適用場景

1. 應用程式使用大量物件，造成儲存開銷過大
2. 物件大部分狀態為外部狀態，移除後可用少量共享物件取代大量群組

結構角色

• FlyweightFactory：建立/管理 Flyweight，提供 GetFlyweight(key)，若存在則回傳既有實例
• Flyweight：抽象介面，定義 Operation(extrinsicState)
• ConcreteFlyweight：實作 Flyweight，儲存內部狀態
• UnsharedConcreteFlyweight：不需共享的子類別

基礎實作

// FlyweightFactory
public class FlyweightFactory {
    Hashtable flyweights = new Hashtable();
    public FlyweightFactory() {
        flyweights.Add("A", new ConcreteFlyweight());
        flyweights.Add("B", new ConcreteFlyweight());
        flyweights.Add("C", new ConcreteFlyweight());
    }
    public Flyweight GetFlyweight(string key) => flyweights[key] as Flyweight;
}

// Flyweight 抽象與實作
public abstract class Flyweight { public abstract void Operation(int extrinsicState); }
public class ConcreteFlyweight : Flyweight {
    public override void Operation(int s) => Console.WriteLine($"{GetType().Name}:{s}");
}
public class UnsharedConcreteFlyweight : Flyweight {
    public override void Operation(int s) => Console.WriteLine($"{GetType().Name}:{s}");
}

// Client
static void Main() {
    int state = 20;
    var factory = new FlyweightFactory();
    factory.GetFlyweight("A").Operation(--state);
    factory.GetFlyweight("B").Operation(--state);
    factory.GetFlyweight("C").Operation(--state);
    new UnsharedConcreteFlyweight().Operation(--state);
}

實戰案例：文件編輯器字元共享

文件 Document 中大量字元重複，僅字型大小（外部狀態）不同。結構：

• CharacterFactory：以 Dictionary<char, Character> 快取，GetCharacter 採延遲初始化
• Character 抽象類別：內部狀態含 symbol、width、height、ascent、descent、pointSize
• CharacterA~CharacterZ：各字元具體類別，Display(pointSize) 顯示

// CharacterFactory 關鍵片段
public Character GetCharacter(char key) {
    if (_characters.ContainsKey(key)) return _characters[key];
    Character c = key switch { 'A' => new CharacterA(), 'B' => new CharacterB(), /*...*/ 'Z' => new CharacterZ() };
    _characters.Add(key, c);
    return c;
}

// Client
string doc = "AAZZBBZB";
var factory = new CharacterFactory();
int pt = 10;
foreach (char c in doc) {
    factory.GetCharacter(c).Display(++pt);
}

總結

享元模式透過內外部狀態分離與工廠快取，極大降低大量相似物件的記憶體佔用，適用於字元、圖示、棋子等大量重複細粒度物件場景。