Design Pattern : Singleton

Design Pattern : Singleton

Definition :

「Ensure a class has only one instance and provide a global point of access to it」

Singleton通常使用於只能建立某類別的單一個物件，例如 取號碼機（某段時間的號碼唯一）、戶政事務所發身分證（身分字號唯一）， 目的是確保不會有多人同時建立或是同時修改這個物件。

我在先前有寫了一篇「Singleton in MutiThread AP」

裡面有三種利用Singleton建立取號機的方式，但是其實文章中”取號”的動作卻不是Thread-Safe； 雖然程式碼中可以確保Multi-thread中是取得同一個取號機，但是取號的動作其實還必須lock才能確保取號的動作是Thread-Safe，否則還是有可能取道同樣的號碼。 （文章就不修改，留下不經一事不長一智的紀錄 XD）

底下是利用Singleton取得每台汽車的CarUid， 可以把它想成車牌號碼，必須是唯一的。

1.  Singleton Class :
=> 使用Thread-safe, lazy singleton
=> 注意在實際取得CarUid時，有使用lock以確保Thread-safe

public class CarUidSingleton
{
    /// Car's Unique id
    private string Uid = "000000";
   
    private static readonly object block = new object();
   
    public static CarUidSingleton GetInstance
    {
        get
        {  return InnerClass.instance; }
    }
    public String GetUid()
    {
         lock(block)
         { 
            int _uid = 0;
            if (int.TryParse(this.Uid, out _uid))
            {
                 _uid++;
                 this.Uid = _uid.ToString().PadLeft(6, '0');
            }
            else
                 throw new Exception("The Uid is not correct!");
            return this.Uid;
        }
     }
     private class InnerClass
     {
         static InnerClass() { }
         internal static readonly CarUidSingleton instance = new CarUidSingleton();
     }
}

2.  主程式：
=> 剛好拿Async-Awiat和Task兩種方式來實做Multi-thread的測試程式…

        List<ICar> carList = new List<ICar>();
        for(int i=0; i<5; i++)
        {
           carList.Add(new Focus());
            carList.Add(new Civic());
            carList.Add(new Fit());
            carList.Add(new V60());
            carList.Add(new Accord());
        };

        Parallel.ForEach(carList, car =>
            {
             #region Async-Await
                 GetCarUidInfo_UseAsyncAwait(car); ///請參考3.
             #endregion

             #region Task
                 GetCarUidInfo_UseTask(car); ///請參考4.
             #endregion
            }
         );

3.  Async-Await:

private async static void GetCarUidInfo_UseAsyncAwait(ICar car)
{
     String uid = await GetAsyncCarUid();
     Console.WriteLine( String.Format("{0}({1})", car.Name, uid));
}

private async static Task<String> GetAsyncCarUid()
{
        Thread.Sleep(1500);
        return CarUidSingleton.GetInstance.GetUid();
}

4.  Task:

private static void GetCarUidInfo_UseTask(ICar car)
{
      Task<String> _task = new Task<String>(
             () => CarUidSingleton.GetInstance.GetUid()
       );
       _task.Start();
       _task.ContinueWith(TaskEnded, car);
}

private static void TaskEnded(Task<String> task, object car)
{
       String uid =  task.Result;
        Console.WriteLine(
           String.Format("{0}({1})", (car as ICar).Name, uid));
}

5.  結果：
Fit(000001)
Accord(000002)
Focus(000003)
Civic(000004)
V60(000005)
Fit(000006)
Accord(000007)
Accord(000008)
…
Focus(000024)
Civic(000025)

6.  延伸閱讀：
Singleton in MutiThread AP
Thread-safe use of a singleton's members