Life cycle

This page is an effort to thoroughly document the different life cycles in Unity... contribution needed!

I'm using it for myself right now because I keep making the same tests to figure out the exact life-cycles, and figured this would be a good place to finally centralize the info.

Static Constructor
This is called by .net as your assemblies are being loaded. The C# Reference makes very few guarantees about exactly when it will be called, but it does state that before any part of a class can be referenced its static constructor will be called.

If working from the editor, when you save a script (and thus force a recompilation), your static constructor is likely to be called immediately as Unity loads the built DLL. It will not likely be run again.

On a deployed game, this constructor will be called early in Unity's loading process.

Constructor
Unity will call the default constructor of any object but at seemingly random times. While editing a game, the constructor is likely to be called immediately after saving a script (forcing a recompilation).

Do not use a constructor to assign values to fields. Unity's Awake is designed specifically for that purpose.

0 : Awake
This is the one and only event that will ALWAYS fire (unless the object is inactive, see above). It will still fire if the object is active but the script is disabled.

0 : OnEnable
Not called if the behaviour is disabled.

[0 : OnLevelWasLoaded]
Not called on the first level.

Not called if the object is created once the level is loaded.

0 : Main
Not called if the behaviour is disabled.

Undocumented. In a Javascript behaviour, this is where your inline instructions go after compilation, but it's actually usable in C# as well.

Only think of it as a pre-Start, not another round of events. All Awakes are called, then all Main & Start are called, NOT all Awake, all Main, then all Start. Consider components Script1 & Script2, Script1 being first in the component list, the call order will be:
 * Awake Script1
 * Awake Script2
 * Main Script1
 * Start Script1
 * Main Script2
 * Start Script2

0 : Start
Not called if the behaviour is disabled.

0 : Update (first call)
Not called if the behaviour is disabled.

0 : LateUpdate (first call)
Not called if the behaviour is disabled.

0 : OnDrawGizmos (first call)
Editor-only. Called even if the behaviour is disabled.

0 : OnDrawGizmosSelected (first call)
Editor-only, called only if the object is selected in the hierarchy view. Called even if the behaviour is disabled.

[+1 : OnNetworkLoadedLevel]
Only called if the level was network-loaded.

This event isn't actually built-in, but fired by the NetworkLevelLoad script that lots of people are using. The +1 delay is also introduced by this script, but I'm not sure it would be wise to try and remove it.

Example
Don't take my word for it, try it for yourself!


 * You'll notice that if you just drop this script wherever and press play, everything starts at frame 1. I don't know if frame 0 even exists (it's called frameCount after all), or if it's used for internal initialization...
 * If you load a scene from another one, you'll notice there is a one-frame delay (eg if this script is in a scene that was loaded as soon as the game starts, everything starts at frame 2).

Collisions / Physics
TODO... explain FixedUpdate here, relation with OnCollisionXXX

GUI
TODO... explain EventType

Network life cycle
TODO... explain MasterServer connection, peer connection / disconnection, etc...

OnDisconnectedFromServer
If a method call Network.Disconnect, OnDisconnectedFromServer will be called instantly (whether in the same script or any other script that subscribes to this callback).

Example: will lead to the following output:

This behaviour is opposed to delayed callbacks, that occur one frame later than the event that triggered them (for example OnTriggerEnter is called the frame after the collision visually occurs). Also note that Debug.Break will not interrupt the current call stack, merely pause the editor at the end of the current frame. TODO: reference instant/delayed callbacks.