UnitySingletonExample - A Singleton Pattern utilizing Unitys Entity-Component system

[Go to https://stackoverflow.com/documentation/unity3d/2137/singletons-in-unity/30885/singleton-pattern-without-static-fields#t=201705281620064376864 for open discussion and questions]

The core idea is to use GameObjects to represent singletons which has multiple advantages:

• Keeps complexity to a minimum but supports concepts like dependency injection
• Singletons have a normal Unity lifecycle as part of the Entity-Component system
• Singletons can be lazy loaded and cached locally where regulary needed (e.g. in update loops)
• No static fields needed
• No need to modify existing MonoBehaviours / Components to use them as Singletons
• Easy to reset (just destroy the Singletons GameObject), will be lazy loaded again on next usage
• Easy to inject mocks (just initialize it with the mock before using it)
• Inspection and configuration using normal Unity editor and can happen already on editor time

Test.cs (which uses the example singleton):

public class Test : MonoBehaviour {
    void Start() {
        ExampleSingleton singleton = ExampleSingleton.instance;
        Assert.IsNotNull(singleton); // automatic initialization on first usage
        Assert.AreEqual("abc", singleton.myVar1);
        singleton.myVar1 = "123";
        // multiple calls to instance() return the same object:
        Assert.AreEqual(singleton, ExampleSingleton.instance); 
        Assert.AreEqual("123", ExampleSingleton.instance.myVar1);
    }
}

ExampleSingleton.cs (which contains an example and the actual Singleton class):

public class ExampleSingleton : MonoBehaviour {
    public static ExampleSingleton instance { get { return Singleton.get<ExampleSingleton>(); } }
    public string myVar1 = "abc";
    public void Start() { Assert.AreEqual(this, instance, "Singleton more than once in scene"); } 
}

/// <summary> Helper that turns any MonBehaviour or other Component into a Singleton </summary>
public static class Singleton {
    public static T get<T>() where T : Component {
        return GetOrAddGo("Singletons").GetOrAddChild("" + typeof(T)).GetOrAddComponent<T>();
    }
    private static GameObject GetOrAddGo(string goName) {
        var go = GameObject.Find(goName);
        if (go == null) { return new GameObject(goName); }
        return go;
    }
}

public static class GameObjectExtensionMethods { 
    public static GameObject GetOrAddChild(this GameObject parentGo, string childName) {
        var childGo = parentGo.transform.FindChild(childName);
        if (childGo != null) { return childGo.gameObject; } // child found, return it
        var newChild = new GameObject(childName);        // no child found, create it
        newChild.transform.SetParent(parentGo.transform, false); // add it to parent
        return newChild;
    }

    public static T GetOrAddComponent<T>(this GameObject parentGo) where T : Component {
        var comp = parentGo.GetComponent<T>();
        if (comp == null) { return parentGo.AddComponent<T>(); }
        return comp;
    }
}

The two extension methods for GameObject are helpful in other situations as well, if you don't need them move them inside the Singleton class and make them private.