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====== SceneManager ======

**When does scene loading happen?** Scene loading and unloading happens at the start of the Update loop, within the ''EarlyUpdate.UpdatePreloading'' phase of the [[PlayerLoop]], and once during the Initialization phase of the player, before the first Update loop.

Player Loop Phases:
  * Initialization (Scene Loading happens here) 
  * Game Loop
    * Early Update
      * `UpdatePreloading` (Scene Loading happens here)
    * Update

===== The Scene Loading Queue =====

The scene loading queue is a pipeline, and only one AsyncOperation can progress at a time. Calling ''SceneManager.LoadSceneAsync'' or ''SceneManager.UnloadSceneAsync'' adds a new AsyncOperation onto the end of the queue.

During the ''UpdatePreloading'' phase, Unity will execute the next operation in the scene loading queue.

The scene loading pipeline only seems to execute a single operation, every other frame. That is to say if you have 3 small scenes in the pipeline (Scene1, Scene2, Scene3), loading additively, the engine will load them at the following times:

* Frame N+0: Scene1  
  * Frame N+1: ----  
  * Frame N+2: Scene 2  
  * Frame N+3: ----  
  * Frame N+4: Scene 3

==== Initialization ====

The Engine simply "flushes" the scene loading queue during initialization.

The Unity scene flushing pipeline works differently in the initialization phase of the player. **All** AsyncOperations that are queued during initialization are fully loaded during initialization. For example, if ''SceneManager.LoadSceneAsync'' is called in the Awake of the very first scene((ie. called before the first Update)), then the engine will fully load the new scene before starting the first Update.

(The scene load pipeline is fully flushed) I don't yet know how it handles unloads if they are in the pipeline then, so if we need this behaviour we should debug and see how it works. If scene unloads get flushed too, we'll need to add a bit of logic for it, in the same way LoadSceneAsync does.

===== Build Pipeline =====

Scenes are built during standalone in the following way: '

- The active scenes are closed.  
  - Each Scene in the BuildSettings is opened. At this point prefabs are still prefab instances.  
  - EditorSceneManager.sceneOpened callback is called.  
  - Prefabs in the scene are flattened (turned into normal game objects).  
  - [[IProcessScene]] and PostProcessScene callbacks are called.  
  - The final scene file is saved into the standalone build file.  
  - Repeat for next scene

===== Sample Scene Load (Editor) =====

Here is an example of a scene loading additively **in the middle** of a game, **in the editor**. Note that:

* Prefabs.MergePrefabs (turning prefabs into gameobjects) happens before any other callback. (editor only)
  * Lightmapping is applied / loaded after Awake/OnEnable.
  * SceneManager.sceneLoaded happens after PostProcessScene;

===== Scattered Notes =====
  * During a scene load (ie. Awake) the loading scene has ''scene.isLoaded=false''. 
  * During a scene unload (ie. Destroy) the unloading scene has ''scene.isLoaded=false''. ((You might expect during the unload phase that isLoaded=true, but this is not the case))

----

===== API Notes: =====
=== SceneManager.sceneLoaded ===
  * Called *after* the loaded Scene Awakes.

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