using UnityEngine;

/// <summary>
/// A PartialMonoSingleton is like a MonoSingleton except simpler: It doesn't create itself if Instance is called and it doesn't exist.
/// Instead it will return null.
/// It still uses up the Awake and OnDestroy events so subclasses need to implement SubclassAwake or SubclassOnDestroy if they want to use those
/// </summary>
/// <typeparam name="T"></typeparam>
public abstract class PartialMonoSingleton<T> : MonoBehaviour where T : PartialMonoSingleton<T> {
	// Query this to see if the singleton has been instantiated anywhere
	// NOTE: Static field in generic type - the value will be different for each inheritance of MonoSingleton

	public static bool Exists = false;

	public static T Instance = null;

	// Assign if this is the first instance, else just show a warning that we have more than one
	public void Awake() {
		if (Instance == null) {
			Instance = this as T;
			if (Instance != null) {
				Exists = true;
				Instance.SubclassAwake();
			}
			else {
				Debug.LogError("PartialSingleton could not assign the instance of this class (" + typeof(T) + ")");
			}
		}
		else {
			Debug.LogWarning("Multiple instances of the PartialSingleton class " + typeof(T) + " created.");
		}
	}

	protected void OnDestroy() {
		if (Instance != null) Instance.SubclassOnDestroy();
		Instance = null;
		Exists = false;
	}

	// Substitute for Awake() in implementing classes
	protected virtual void SubclassAwake() { }

	// Substitute for OnDestroy() in implementing classes
	protected virtual void SubclassOnDestroy() { }
}