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Understanding Automatic Memory Management
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Execution Order of Event Functions

In Unity scripting, there are a number of event functions that get executed in a predetermined order as a script executes. This execution order is described below:

First Scene Load

These functions get called when a sceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
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starts (once for each object in the scene).

  • Awake: This function is always called before any Start functions and also just after a prefabAn asset type that allows you to store a GameObject complete with components and properties. The prefab acts as a template from which you can create new object instances in the scene. More info
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    is instantiated. (If a GameObject is inactive during start up Awake is not called until it is made active.)
  • OnEnable: (only called if the Object is active): This function is called just after the object is enabled. This happens when a MonoBehaviour instance is created, such as when a level is loaded or a GameObjectThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
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    with the script component is instantiated.
  • OnLevelWasLoaded: This function is executed to inform the game that a new level has been loaded.

Note that for objects added to the scene, the Awake and OnEnable functions for all scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
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will be called before Start, Update, etc(Ericsson Texture Compression) A block-based texture format that compresses textures to significantly reduce file sizes without causing a noticable reduction in image quality. More info
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are called for any of them. Naturally, this cannot be enforced when an object is instantiated during gameplay.

Editor

  • Reset: Reset is called to initialize the script’s properties when it is first attached to the object and also when the Reset command is used.

Before the first frame update

  • Start: Start is called before the first frame update only if the script instance is enabled.

For objects added to the scene, the Start function will be called on all scripts before Update, etc are called for any of them. Naturally, this cannot be enforced when an object is instantiated during gameplay.

In between frames

  • OnApplicationPause: This is called at the end of the frame where the pause is detected, effectively between the normal frame updates. One extra frame will be issued after OnApplicationPause is called to allow the game to show graphics that indicate the paused state.

Update Order

When you’re keeping track of game logic and interactions, animations, cameraA component which creates an image of a particular viewpoint in your scene. The output is either drawn to the screen or captured as a texture. More info
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positions, etc., there are a few different events you can use. The common pattern is to perform most tasks inside the Update function, but there are also other functions you can use.

  • FixedUpdate: FixedUpdate is often called more frequently than Update. It can be called multiple times per frame, if the frame rate is low and it may not be called between frames at all if the frame rate is high. All physics calculations and updates occur immediately after FixedUpdate. When applying movement calculations inside FixedUpdate, you do not need to multiply your values by Time.deltaTime. This is because FixedUpdate is called on a reliable timer, independent of the frame rate.

  • Update: Update is called once per frame. It is the main workhorse function for frame updates.

  • LateUpdate: LateUpdate is called once per frame, after Update has finished. Any calculations that are performed in Update will have completed when LateUpdate begins. A common use for LateUpdate would be a following third-person camera. If you make your character move and turn inside Update, you can perform all camera movement and rotation calculations in LateUpdate. This will ensure that the character has moved completely before the camera tracks its position.

Rendering

  • OnPreCull: Called before the camera culls the scene. Culling determines which objects are visible to the camera. OnPreCull is called just before culling takes place.
  • OnBecameVisible/OnBecameInvisible: Called when an object becomes visible/invisible to any camera.
  • OnWillRenderObject: Called once for each camera if the object is visible.
  • OnPreRender: Called before the camera starts renderingThe process of drawing graphics to the screen (or to a render texture). By default, the main camera in Unity renders its view to the screen. More info
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    the scene.
  • OnRenderObject: Called after all regular scene rendering is done. You can use GL class or Graphics.DrawMeshNow to draw custom geometry at this point.
  • OnPostRender: Called after a camera finishes rendering the scene.
  • OnRenderImage: Called after scene rendering is complete to allow post-processing of the image, see Post-processing Effects.
  • OnGUI: Called multiple times per frame in response to GUI events. The Layout and Repaint events are processed first, followed by a Layout and keyboard/mouse event for each input event.
  • OnDrawGizmos Used for drawing GizmosA graphic overlay associated with a GameObject in a Scene, and displayed in the Scene View. Built-in scene tools such as the move tool are Gizmos, and you can create custom Gizmos using textures or scripting. Some Gizmos are only drawn when the GameObject is selected, while other Gizmos are drawn by the Editor regardless of which GameObjects are selected. More info
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    in the scene viewAn interactive view into the world you are creating. You use the Scene View to select and position scenery, characters, cameras, lights, and all other types of Game Object. More info
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    for visualisation purposes.

Coroutines

Normal coroutine updates are run after the Update function returns. A coroutine is a function that can suspend its execution (yield) until the given YieldInstruction finishes. Different uses of Coroutines:

  • yield The coroutine will continue after all Update functions have been called on the next frame.
  • yield WaitForSeconds Continue after a specified time delay, after all Update functions have been called for the frame
  • yield WaitForFixedUpdate Continue after all FixedUpdate has been called on all scripts
  • yield WWW Continue after a WWW download has completed.
  • yield StartCoroutine Chains the coroutine, and will wait for the MyFunc coroutine to complete first.

When the Object is Destroyed

  • OnDestroy: This function is called after all frame updates for the last frame of the object’s existence (the object might be destroyed in response to Object.Destroy or at the closure of a scene).

When Quitting

These functions get called on all the active objects in your scene:

  • OnApplicationQuit: This function is called on all game objects before the application is quit. In the editor it is called when the user stops playmode.
  • OnDisable: This function is called when the behaviour becomes disabled or inactive.

Script Lifecycle Flowchart

The following diagram summarises the ordering and repetition of event functions during a script’s lifetime.

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Attributes
Understanding Automatic Memory Management