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Understanding Unity Engine Objects

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We already discussed Game Objects and Components as two of the fundamental building blocks of the Unity Engine. Today, we’ll discuss their programmatic representation.

This is Unity for Software Engineers, a series for folks familiar with software development best practices seeking an accelerated introduction to Unity as an engine and editor. More is coming over the next few weeks, so consider subscribing for updates.

The Unity Engine runtime is primarily written in C++, and much of the Engine’s primitives (such as the game objects and their components) live in C++ land. You’ll also know that the Unity Engine API is in C#. The API gives you access to all of Unity’s native objects in a way thatsave for a few pitfalls we’ll discuss todayfeels like intuitive, idiomatic C#.

A GameObject, MonoBehaviour, and ScriptableObject all inherit from UnityEngine Object.

The C# class hierarchy of UnityEngine.Object, GameObject, ScriptableObject, Component, and their children. This should map to the conceptual hierarchy of how to think about these objects, though sometimes the runtime implementation will look different.

UnityEngine.Object

At the top of the Unity Object hierarchy sits UnityEngine.Object. For the most part provides a name string, an int GetInstanceID() method, and a bunch of equality comparers.

The class also provides a static void Destroy(Object obj) method (and some overloads) that destroys a UnityEngine.Object and any of its subclasses. When an Object is destroyed, the native part of the object is freed from memory, and the smaller managed part will be garbage collected at some point after there are no more references to it.

Because your valid reference to a UnityEngine.Object can point to a destroyed native object, UnityEngine.Object overrides C#‘s operator== and operator!= to make a destroyed Object appear null. Simply accessing methods on a destroyed object will return NullReferenceException, albeit with a friendlier error message that tells you which object you were trying to access.

GameObject

A GameObject derives from Object and represents anything in your scene.

Let’s start at a high-level: A GameObject inherits a name and instance ID from its parent. Otherwise, conceptually, a GameObject

  • has a list of Components on it,
  • has a tag string for organizational purposes, and
  • belongs to a layer.

A GameObject’s state

  • is the product of all of its Components’ state, and
  • whether an object is active or not.

Let’s dig a bit deeper. When starting, most of the interesting stuff in a GameObject is in its Components. A GameObject has at least one Component: its Transform. A Transform describes the position and rotation of the GameObject. A Transform includes helper properties that show an object’s absolute world position and rotation, as well as the position and rotation relative to its parent. In the Editor, the Transform position and rotation are set from the parent relative variants.

Since every GameObject has a Transform (and also, given that a Transform is frequently needed/accessed), the GameObject directly exposes a Transform transform public property.

You can access individual components from T GetComponent<T>(), or lists of components from T[] GetComponents<T>(), etc. These methods search through all components on a GameObject and return ones with a compatible type (or null, if none exist in the singular case). Since these methods search through components and check type compatibility, it is often recommended to cache this lookup.

If you are building/extending a GameObject by hand, you can always use T AddComponent<T>(). In most cases, however, you’re better off using the Editor.

Individual Objects (a Component or ScriptableObject) might refer to other GameObjects in a few ways:

  • By reference. By exposing a GameObject serialized field that you then set from the inspector.

    We have discussed serialization extensively throughout the series: as a fundamental concept and in our tour of the Editor, when describing the Inspector, and the practice of using the Inspector as an injection framework.

  • Using tags. Every Game Object can have a tag string. You can find objects in the scene using that tag through the static functions GameObject.FindGameObjectsWithTag and GameObject.FindGameObjectWithTag. A GameObject also exposes a public bool CompareTag(string tag) method.

    This is a quick-and-dirty way to get the job done, but is still a popular way. A common use of this in the wild is to have a "Player" tag to find the Player. Ideally, these methods should not be called every frame, so if you have to use them, consider caching the result.

  • Using layers. A layer is an int between 0 and 31. Every Game Object is in exactly one layer.

    While you can’t directly look up all objects in a layer, if you already have a reference to a GameObject (e.g., in a collision event), you can check a GameObject against a LayerMask. A LayerMask is typically used in functions like Physics.Raycast(). This allows you to find objects with colliders intersecting with a given ray. Passing a LayerMask to Physics.Raycast() will only return objects within the specified set of layers.

    Inside the Unity Engine, Cameras make heavy use of layers. E.g., you can have one camera that renders “everything but UI”, and overlay another camera for an in-game HUD, etc.

  • Using indirect references. There are many reasons why the methods above might be insufficient: you might not want to use tags to avoid depending on copy-pasted strings, and layers might not fit your use case. If referencing a fellow object in-scene is not an option (e.g., you’re dealing with a dynamic set of objects or don’t have access to the current scene objects in the context you need this reference, etc.), then you might want to look further.

    For this, an increasingly popular concept is runtime sets Scriptable Objects. You can read more about this in Unity’s how-to article on architecting your game with ScriptableObjects, based on the talk by Ryan Hipple. If you have an hour to spare, you might want to watch the whole thing.

A GameObject also exposes a BroadcastMessage and SendMessage functions that propagate messages (described in the Component section) to all components in or under it.

Photo of a Building Structure

Photo by Ramin Khatibi via Unsplash.

Component

Every behavior on a GameObject is driven through its Components. User-implemented Components will usually extend the MonoBehaviour subclass (more on that later).

A Component inherits a name and instance ID from its parent. Otherwise, conceptually, a Component

  • always belongs to a single GameObject, exposed as a public GameObject gameObject property, and
  • can receive messages, driving much of its behavior.

The state of a component on an active GameObject lies entirely in its implementation.

In addition to its GameObject, a component exposes shorthand properties and methods such as Transform transform, T GetComponent<T>(), etc. These are simply convenience shorthands for accessing those same methods on the corresponding gameObject.

The most important functionality of a Component is driven through Unity Messages (also sometimes called Unity Event Functions when referring to built-in messages). These are effectively callbacks functions triggered by the Engine in certain situations. Every Component will receive a Awake(), Start(), Update() and other messages, for example. The Unity Docs on the Order of Execution of these messages is a convenient resource.

To have your component receive a particular message, simply add a private void method with the appropriate message name. The runtime will use reflection to call these messages, when applicable. This is why you don’t see an override directive on these messages. Messages like Update, LateUpdate, and FixedUpdate are inspected once per type, so don’t worry about reflection being used in every frame. See more details in the ”10000 Update() calls” Unity blog post for more information.

A Behaviour is a type of component that can be enabled or disabled. When a Behaviour is disabled, Start, Update, FixedUpdate, LateUpdate, OnEnable, and OnDisable messages are not called.

A MonoBehaviour is a Behaviour that also enables using Coroutines.

A Note on Inactive Objects and Disabled Components

A GameObject in a loaded scene will exist in memory until the object is Destroyed explicitly or the scene is unloaded. A GameObject can be set to inactive, which will cause it to stop receiving Update (and related) events.

When an object is created, the messages called on a component depend on if: (1) the GameObject is active, and (2) the component is enabled:

GameObject is activeGameObject is inactive
Component is EnabledAwake, OnEnable, StartComponent implicitly disabled
Component is DisabledAwakeAwake

When an object is set to active or a Behaviour is set to enabled:

  • OnEnable will be called.
  • If Start has never been called on this Behaviour, it will be called exactly once.

Takeaways

Some takeaways of all this:

  1. A Unity object might appear to become null when destroyed. == null checking does more than you think.
  2. As a result, null-coalescing operators (??, ??=) and null-conditional operators (?., ?[]) don’t work as expected.
  3. Yes, your Unity Messages can be private!
  4. Don’t create abstract classes that unnecessarily declare Update or other messages to make overriding easier; that’ll result in the engine always calling these events.
  5. Disabling an Object or Component is a great way to limit its game logic or save on CPU-bound effort, but these objects still have a memory overhead.