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Unity Engine

Enter Play mode without scene reload

Understand how entering Play mode without scene reload affects your application and how you can compensate for these effects in your code.
Read time 3 minutesLast updated 12 days ago
Scene reload on entering Play mode is on by default. This means that when you enter Play mode, Unity destroys all existing scene GameObjects and reloads the scene from disk. As your project gets more complex, the time between pressing the Play button and the scene fully loading in the Editor increases.
When you turn scene reloading off, the process takes less time. Instead of reloading the scene from disk, Unity only resets the scene's modified contents. Unity still calls the same event functions (such as 
OnEnable
, 
OnDisable
and 
OnDestroy
) as if the scene were freshly loaded.

Effects of turning scene reload off when entering Play mode

When you turn scene reload off, the time it takes to start your application in the Editor is no longer representative of the startup time in the built version. If you want to debug or profile exactly what happens during your project's startup, you should enable scene reloading to more accurately represent the true loading time and processes that happen in the built version of your application.
Otherwise, turning scene reload off should have minimal effects on your project. However, because scene reloading is closely connected to domain reload, there are a few important differences:
  • Unity doesn't recreate existing objects or call constructors, which means non-serialized fields keep the values assigned to them during Play mode on returning to Edit mode. This applies for fields of all script types, including MonoBehaviours, ScriptableObjects, and your own custom C# types. For detailed information on what is and isn't serialized in different contexts, refer to Serialization rules.
    • Note
      private
      fields are not serialized as part of the regular build pipeline but are serialized as part of the Editor's hot reloading of scripts. This is why 
      private
      fields you modify in Play mode might reset to their original values on exiting Play mode even when scene and domain reload on enter Play mode are both off.
  • Unity converts null 
    private
    and 
    internal
    fields of array and 
    List
    type to an empty array or 
    List
    object during domain reload and they stay non-null for runtime (non-Editor) scripts.
  • Scripts decorated with 
    [ExecuteInEditMode]
     or 
    [ExecuteAlways]
     don't receive 
    OnDestroy
    or 
    Awake
    calls. While in Edit mode, these scripts might modify their own fields or those of other runtime scripts. To mitigate this, you can initialize any affected fields in an 
    OnEnable
    callback with code inside a condition that checks the value of 
    EditorApplication.isPlaying
    . For an example of this and more context on the importance of separating Play mode and Edit mode code, refer to the 
    [ExecuteAlways]
     API description.
For more details on the events skipped when scene reload is off, refer to Domain and scene reload execution order reference.

Additional resources