Unity Foundations

Core Concepts

GameObjects

GameObjects are the fundamental building blocks in Unity. Every object in a scene -- characters, props, scenery, cameras, lights -- is a GameObject. GameObjects are containers: they cannot function alone and require Components to gain functionality. Every GameObject automatically includes a Transform component that cannot be removed.

Components

Components are the functional pieces of every GameObject. Unity uses a composition-over-inheritance architecture: you build behavior by attaching multiple components to a GameObject rather than inheriting from deep class hierarchies. Each GameObject must have exactly one Transform component. Additional components (Rigidbody, Collider, MeshRenderer, custom MonoBehaviours) define what the object does.

Constraints from the docs:

Components must reside in the same project as their target GameObject
Components cannot be sourced from separate projects, unattached scripts, or uninstalled packages

Transforms

The Transform component stores position, rotation, and scale -- each relative to the parent (local coordinates) or to the world origin (world coordinates). Key points from the docs:

Child Transforms display values relative to their parent
Root GameObjects (no parent) show world coordinates
The physics engine assumes 1 unit = 1 meter
Set parent location to (0,0,0) before adding children so local coords match global coords
Avoid adjusting Transform Scale at runtime; model assets at real-life scale instead. Non-uniform scaling causes issues with Colliders, Lights, and Audio Sources

Scenes

Scenes are assets that contain all or part of a game or application. A default new scene includes a Camera and a directional Light. Projects can use a single scene or multiple scenes (e.g., one per level). Scene Templates serve as blueprints for creating new scenes.

Prefabs

Prefabs are reusable asset templates that store a complete GameObject configuration (all components, property values, and child GameObjects). Key features:

Nested Prefabs: Include prefab instances within other prefabs
Prefab Variants: Create predefined variations that maintain a base prefab relationship
Overrides: Modify components/data on specific instances without affecting the template
Unpacking: Convert prefab instances back to standalone GameObjects

ScriptableObjects

ScriptableObject is a serializable Unity type derived from UnityEngine.Object that serves as a data container independent of GameObjects. Unlike MonoBehaviours, ScriptableObjects exist as project-level .asset files. Primary use cases:

Shared data containers (reduces memory by referencing one asset instead of duplicating data across prefabs)
Editor tool foundations (EditorTool, EditorWindow derive from ScriptableObject)
Runtime configuration storage

Critical: Unity does not automatically save changes to a ScriptableObject made via script in Edit mode. You must call EditorUtility.SetDirty() after modifications.

Layers

Layers separate GameObjects for selective processing including camera rendering, lighting, physics collisions, and custom code logic. Unity supports up to 32 layers. LayerMasks define which layers an API call interacts with.

Common Patterns

Creating and Accessing Components

using UnityEngine;

public class ComponentAccess : MonoBehaviour
{
    void Start()
    {
        // Get a component on this GameObject
        Rigidbody rb = GetComponent<Rigidbody>();

        // Get a component on a child GameObject
        Collider childCollider = GetComponentInChildren<Collider>();

        // Get all components of a type on this and children
        MeshRenderer[] renderers = GetComponentsInChildren<MeshRenderer>();

        // Add a component at runtime
        BoxCollider box = gameObject.AddComponent<BoxCollider>();

        // Remove a component (destroys it)
        Destroy(box);
    }
}

Finding GameObjects

using UnityEngine;

public class FindingObjects : MonoBehaviour
{
    void Start()
    {
        // Find by tag (returns first match)
        GameObject player = GameObject.FindWithTag("Player");

        // Find all with tag
        GameObject[] enemies = GameObject.FindGameObjectsWithTag("Enemy");

        // Find by name (slow -- avoid in Update)
        GameObject manager = GameObject.Find("GameManager");

        // Compare tags efficiently (no GC allocation)
        if (gameObject.CompareTag("Player"))
        {
            Debug.Log("This is the player");
        }
    }
}

Instantiating Prefabs

From the Unity docs example:

using UnityEngine;

public class InstantiationExample : MonoBehaviour
{
    // Reference to the prefab. Drag a prefab into this field in the Inspector.
    public GameObject myPrefab;

    void Start()
    {
        // Instantiate at position (0, 0, 0) and zero rotation.
        Instantiate(myPrefab, new Vector3(0, 0, 0), Quaternion.identity);
    }
}

Instantiating with Parent Transform

using UnityEngine;

public class SpawnWithParent : MonoBehaviour
{
    public GameObject prefab;
    public Transform parentTransform;

    void SpawnChild()
    {
        // Instantiate as child of a parent transform
        GameObject instance = Instantiate(prefab, parentTransform);

        // Instantiate at specific world position under a parent
        GameObject positioned = Instantiate(
            prefab,
            new Vector3(5, 0, 0),
            Quaternion.identity,
            parentTransform
        );
    }
}

ScriptableObject Data Container

From the Unity docs:

using UnityEngine;

[CreateAssetMenu(fileName = "Data", menuName = "ScriptableObjects/SpawnData", order = 1)]
public class SpawnDataScriptableObject : ScriptableObject
{
    public GameObject prefab;
    public int count;
    public Vector3[] positions;
}

using UnityEngine;

public class SpawnManager : MonoBehaviour
{
    public SpawnDataScriptableObject spawnData;

    void Start()
    {
        for (int i = 0; i < spawnData.count; i++)
        {
            Instantiate(spawnData.prefab, spawnData.positions[i], Quaternion.identity);
        }
    }
}

Scene Management

using UnityEngine;
using UnityEngine.SceneManagement;

public class SceneLoader : MonoBehaviour
{
    void Start()
    {
        // Subscribe to scene loaded event
        SceneManager.sceneLoaded += OnSceneLoaded;
    }

    void OnSceneLoaded(Scene scene, LoadSceneMode mode)
    {
        Debug.Log("Loaded: " + scene.name);
    }

    public void LoadLevel(string sceneName)
    {
        // Load scene by name (replaces current)
        SceneManager.LoadScene(sceneName);
    }

    public void LoadAdditive(string sceneName)
    {
        // Load scene additively (keeps current scenes)
        SceneManager.LoadSceneAsync(sceneName, LoadSceneMode.Additive);
    }

    public void UnloadLevel(string sceneName)
    {
        SceneManager.UnloadSceneAsync(sceneName);
    }

    public void GetSceneInfo()
    {
        Scene active = SceneManager.GetActiveScene();
        Debug.Log("Active scene: " + active.name);
        Debug.Log("Loaded scene count: " + SceneManager.loadedSceneCount);
    }
}

Tag-Based Spawning

From the Unity docs example:

using UnityEngine;

public class RespawnSystem : MonoBehaviour
{
    public GameObject respawnPrefab;
    private GameObject respawn;

    void Update()
    {
        if (respawn == null)
            respawn = GameObject.FindWithTag("Respawn");

        if (respawn != null)
        {
            Instantiate(respawnPrefab, respawn.transform.position,
                respawn.transform.rotation);
        }
    }
}

Activating and Deactivating GameObjects

using UnityEngine;

public class ToggleVisibility : MonoBehaviour
{
    public GameObject target;

    public void Toggle()
    {
        // SetActive controls whether the GameObject is active
        target.SetActive(!target.activeSelf);

        // activeSelf: this object's own active state
        // activeInHierarchy: effective state (considers parent chain)
        Debug.Log("Self: " + target.activeSelf);
        Debug.Log("InHierarchy: " + target.activeInHierarchy);
    }
}

Layer Masks for Raycasting

using UnityEngine;

public class LayerRaycast : MonoBehaviour
{
    void Update()
    {
        // Create a layer mask for layer named "Ground"
        int groundLayer = LayerMask.NameToLayer("Ground");
        int layerMask = 1 << groundLayer;

        // Raycast only against the Ground layer
        if (Physics.Raycast(transform.position, Vector3.down, out RaycastHit hit, 100f, layerMask))
        {
            Debug.Log("Hit ground at: " + hit.point);
        }

        // Use GetMask for multiple layers
        int combinedMask = LayerMask.GetMask("Ground", "Water");
        Physics.Raycast(transform.position, Vector3.forward, 50f, combinedMask);
    }
}

Anti-Patterns

1. Using GameObject.Find in Update

// BAD: Find is expensive -- runs every frame with string lookup
void Update()
{
    GameObject player = GameObject.Find("Player"); // Avoid!
}

// GOOD: Cache the reference
private GameObject player;
void Start()
{
    player = GameObject.FindWithTag("Player");
}

2. Non-Uniform Transform Scale

From the docs: "Don't adjust the Scale of your GameObject in the Transform component." Non-uniform scaling (e.g., 2, 4, 2) causes:

Colliders, Lights, Audio Sources to behave incorrectly
Rotated children to appear skewed
Performance degradation when instantiating scaled objects

Model assets at real-life scale instead.

3. Duplicating Data Across Prefabs Instead of Using ScriptableObjects

// BAD: Every prefab instance duplicates this data
public class EnemyStats : MonoBehaviour
{
    public int health = 100;
    public float speed = 5f;
    public string enemyName = "Goblin";
}

// GOOD: Use a ScriptableObject -- one asset, many references
[CreateAssetMenu(menuName = "ScriptableObjects/EnemyConfig")]
public class EnemyConfig : ScriptableObject
{
    public int health = 100;
    public float speed = 5f;
    public string enemyName = "Goblin";
}

public class Enemy : MonoBehaviour
{
    public EnemyConfig config; // All instances share the same asset
}

4. Forgetting EditorUtility.SetDirty for ScriptableObject Changes

// BAD: Changes in Edit mode won't persist
settings.value += 10;

// GOOD: Mark asset dirty so Unity saves it
settings.value += 10;
EditorUtility.SetDirty(settings);

5. String-Based Tag Comparison

// BAD: Allocates a new string for comparison (GC pressure)
if (gameObject.tag == "Player") { }

// GOOD: CompareTag avoids allocation
if (gameObject.CompareTag("Player")) { }

6. Ignoring Parent-Child Transform Relationships

Set a parent's location to (0,0,0) before adding children. Otherwise child local coordinates will not match global coordinates, causing confusion when positioning objects.

Key API Quick Reference

API	Description	Notes
`GetComponent<T>()`	Get component on same GameObject	Returns null if not found
`GetComponentInChildren<T>()`	Get component on self or children	Searches depth-first
`GetComponentsInChildren<T>()`	Get all matching components in hierarchy	Returns array
`AddComponent<T>()`	Attach new component at runtime	Returns the new component
`Destroy(obj)`	Destroy a GameObject or Component	Deferred to end of frame
`Instantiate(prefab, pos, rot)`	Clone a prefab at position/rotation	Returns the clone
`GameObject.FindWithTag(tag)`	Find first active GO with tag	Returns null if none
`GameObject.FindGameObjectsWithTag(tag)`	Find all active GOs with tag	Returns array
`GameObject.Find(name)`	Find by name (expensive)	Avoid in Update loops
`gameObject.SetActive(bool)`	Activate/deactivate	Disables all components
`gameObject.CompareTag(tag)`	Tag comparison without GC alloc	Preferred over `== tag`
`SceneManager.LoadScene(name)`	Load scene (replaces current)	Must be in Build Settings
`SceneManager.LoadSceneAsync(name, mode)`	Async scene loading	Additive or Single mode
`SceneManager.UnloadSceneAsync(name)`	Unload a loaded scene	Returns AsyncOperation
`SceneManager.GetActiveScene()`	Get current active scene	Returns Scene struct
`LayerMask.NameToLayer(name)`	Get layer index from name	Returns int
`LayerMask.GetMask(names)`	Get combined mask from layer names	Params string array
`Camera.main`	Get MainCamera-tagged camera	Cached by Unity

Related Skills

unity-scripting -- C# scripting patterns, MonoBehaviour lifecycle, coroutines, events
unity-physics -- Rigidbody, Colliders, physics layers, raycasting, triggers
unity-editor-tools -- Custom inspectors, editor windows, gizmos, build pipeline

unity-foundations