Singleton pattern in X++

The singleton pattern is one of the simplest design patterns: it involves only one class which is responsible to instantiate itself, to make sure it creates not more than one instance; in the same time it provides a global point of access to that instance. In this case the same instance can be used from everywhere, being impossible to invoke directly the constructor each time.

Generally, singleton classes use a static variable to hold a reference their own instance, but as Axapta does not support static variables we are left with no other option to use the global caching mechanism to accomplish the similar functionality. Due to the nature of Axapta global caching, we need to add the instance reference to both the client- and server-side caches separately.

We should override the new () method of our singleton class and use the private access modifier to ensure that it cannot be instantiated directly. For accessing the instance we will create static method which will act as static accessor method used typically in normal Singleton pattern implementation.

Implementation

//Accessor method

public static SingletonClass instance()

{

    SingletonClass   singleton;

    SysGlobalCache  globalCache = infolog.objectOnServer() ? appl.globalCache() : infolog.globalCache();

    ;

    if (globalCache.isSet(classStr(SingletonClass), 0))

        singleton = globalCache.get(classStr(SingletonClass), 0);

    else

    {

        singleton = new SingletonClass();

        infoLog.globalCache().set(classStr(SingletonClass), 0, singleton);

        appl.globalCache().set(classStr(SingletonClass), 0, singleton);

    }

    return singleton;

}

//Avoid instantiation publically

private void new()

{

}