Creating Game Manager using State Machine and Singleton pattern in Unity3d


Game Manager is quite broad topic and there are many ways to implement it.

From this article you will learn how to get the basic manager structure with use of simple State Machine and Singleton pattern.

Concept of GameManager – one object to control them all

Before we go any further we should consider what do we expect from our Game Manager and so it should:

  • be easily extendible,
  • have only one instance allowed,
  • persist across all the scenes – shouldn’t be destroyed when new scene is loaded,
  • contain list of available game states – so those can be accessed from other classes via ex.:  GameState.IntroScreen,
  • store name of current game state – as above to enable accessing it externally with: GameManager.gameState,
  • allow changing game state – again to expose it publicly via: GameManager.SetGameState(GameState gameState),
  • trigger event when state changes – which will allow other objects’ modules to respond to that change with own handlers,

Additionally it  should:

  • persist game progress – allowing user to continue after game re-opens after crash or close,
  • save of current game progress – allowing manual saving progress of current level (game),
  • restore desired game state – allowing manual load saved game.

List of requirements might be much longer but because it is quite broad topic for now I will try to focus on those listed above. Game Manager can be extended later on.

Singleton class – one instance and one instance only

First things first. Using Singleton Pattern lets make sure there is only once instance of Game Manager allowed at any given point in time.

There are quite few different ways to implement singleton and I will not go over all of them but here is the one that works great for me. It does allow only one instance and protects from unwanted usage displaying errors if constructor is being called in regular way.

Now if you try to create new Singleton instance using as you would create instance of regular object:

Unity will throw an error. You will see alert similar to the following:

If you look at the code again you will notice that class constructor method has been followed by  protected keyword, which prevents from calling it.

The only way you can access singleton object is by referring to its public static  property called  Instance

Game Manager – long live and indestructible

Section updated on  the 6th of January, 2014
As “KarnEdge” spotted (see comments below) code published here before caused to throw an error. If you are interested feel free to check out the update to this code that will explain the problem in a bit more detail.

Within UnityEngine namespace Object has own static method OnDestroyOnLoad  available. It will store reference to the object while others will get destroyed while loading new scene. Lets modify our code a bit (it will make it easier to extend later on) and lets rename file to GameManager.cs too.

Now Game Manager object should persist when we change, load new scene in game. Notice that we also inherited from MonoBehaviour  to make sure OnDestroyOnLoad  exists in current context. Later that will also allow attaching GameManager to an empty object, when we decide to create some public properties for easier management.

Game States – publicly exposed

We could build dynamic State Machine with own customised interface for Unity Inspector allowing to change states on the fly assigning conditional behaviours for state transitions. We could… but we will NOT!

Goal is to create small robust code that can be easily re-used in other games, in fact… other small puzzle games targeted to deployed on web and mobile. Will described approach work for bigger projects? Yes! It will require more work though, which matter of fact should be easy as extendibility is one of the acceptance criteria for this project. So to keep all of that in mind lets create list of Game States adding it above Game Manager class definition, which will make it exposed.

We also need gameState variable inside of Game Manager, so paste it above its constructor.

Game State setter – exposed too

Creating gameState property we decided to publish it with read-only access, making setting private. Aim was to use different method as a property setter and so now we can add it to our Game Manager class.

 Delegate – as an elegant way of handling stuff

If you have some C# background not much has to be said but “delegate” topic might be quite new if you have experience in other languages and you might want to read up about it.

In lame man terms delegate is quite similar to a variable with the exemption it is made for functions. It allows other objects to perform += amd -= operations on delegate variable adding or removing own methods that will be triggered when delegate property function is called.

So what it does for us? It allows to handle triggered events by adding component handlers  method to a public delegate property. Lets see that in action. Firstly lets declare delegate above Game Manager class.

Next we need to create delegate property for Game Manager.

And last thing is calling  OnStateChange() method from within the body of  SetGameState() method.

Job done! Your GameManager.cs code (after cleanup) should look like this.


See it in action – example please!

Testing our simple Game Manager is quite simple. Create new stage, and attach below GameManagerTest.cs file to any object placed on that scene. And yes… you can attach it to the Main Camera. It is just a quick test.

You should see those three expected logs in the console.

Want some improvements?

In the next tutorial I will show how to utilise Singleton pattern for MonoBehaviour which becomes very useful especially if you are planning to make it even more extendable.


December 11th, 2013 by

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8 Comments on "Creating Game Manager using State Machine and Singleton pattern in Unity3d"

1 year 3 months ago

Thanks for sharing your knowledge.
Please clarify in GameManagerTest what you typically do in HandleOnStateChange() to start using the new state?

1 year 4 months ago

Note that calling OnStateChange() on line 25 in GameManager.cs will throw a NullReferenceException if no handler is subscribed to the event. Place an if(OnStateChange!=null) before the call just to be sure :)

1 year 4 months ago

Since the instance has to be created with AddComponent, it doesn’t finish until after Awake which means the Example test would throw a NRE error.

1 year 4 months ago

Unity doesn’t seem to like your line 18 of GameManager.cs:
You are trying to create a MonoBehaviour using the ‘new’ keyword. This is not allowed. MonoBehaviours can only be added using AddComponent(). Alternatively, your script can inherit from ScriptableObject or no base class at all.