# Singleton Pattern

Singleton is a Programming Design Pattern:

> In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system [wikipedia](https://en.wikipedia.org/wiki/Singleton_pattern)

An implementation of the singleton pattern must:

* ensure that only one instance of the singleton class ever exists
* provide global access to that instance

  [wikipedia](https://en.wikipedia.org/wiki/Singleton_pattern)

## Singleton in Unity:

In Unity, Singletons are often used for management of aspects a game such as: state-manager, data-manager, audio-manager, etc.

In Unity, the Singleton Pattern should ensure:

* only one instance of the object exists 
* in each scene, it's necessary to destroy any instances that are created as duplicates
* A global, static variable should be initialized that creates an object-reference to the singleton object.
* Code to create a Singleton should be located in Awake( );

### StateManager Singleton

```java
public class StateManager : MonoBehaviour {

    public static StateManager instanceRef;  //this will be a reference to the only class object instance allowed to exist

    //Called before any GameObjects Start() method is called
    //Allows pre-initialization: Called once when a script is loaded
    void Awake(){
        if(instanceRef==null){  //test to see if instanceRef has been assigned to an object yet.
            instanceRef=this;  //if not, assign it to this current object instance
            Debug.Log ("create me");
            DontDestroyOnLoad(gameObject);  //make sure the gameObject that this is attached to is not destroyed when jumping to a new scene
        }
        else{  //an instance of this class had previously been created.
            Debug.Log ("destroy me");
            DestroyImmediate(gameObject);  //destroy gameObject and the script object instance, to prevent duplication
        }
    }

    //additional StateManager code
```