设计模式 - 创建型 - 单例模式

类型 Type

• 创建型 Creational

含义 Intent

The intent of the singleton pattern is to ensure that a class only has one instance, and to provide a global point of access to it.

例子 Real Example

Logger，通常来说应用中只会有一个，以避免意想不到的结果。

代码 Code

class Logger
{
    private static readonly Lazy<Logger> lazyLogger = new Lazy<Logger>(() => new Logger());

    public static Logger Instance => lazyLogger.Value;

    protected Logger()
    {
    }

    // <summary>
    // SingletonOperation
    // </summary>
    public void Log(string message)
    {
        Console.WriteLine($"Log: {message}");
    }
}

使用时机 Time to Use

• When there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point

• When the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code

用例 User Case

• 管理数据库连接池 connection pool

• 缓存 cache

• 管理应用配置 manage application configuration

• 通用资源管理 general resource management

模式好坏 Pattern Consequence

• 违反了单一职责 Violates the SRP

  • 负责对象的创建 Control Creation

  • 负责对象的生命周期管理 manage own lifecycle

• 严格的控制客户对其的访问 Strict Control over how and when clients access it

• Avoids polluting the namespace with global variables

• Subclassing allows configuring the application with an instance of the class you need at runtime

• Multipe instances can be allowed without having to alter the client

• 很多时候我们不用自己实现，让 IoC 容器管理吧