Question
Answer and Explanation
The OnSceneLoaded function in Unity can sometimes run twice due to a few common reasons. Understanding these reasons can help you debug the issue effectively.
Common Causes:
1. Multiple Scene Loading Calls: If your code has multiple places where you’re calling SceneManager.LoadScene() or other related scene loading methods, each call will trigger the OnSceneLoaded event once the new scene has loaded. Double-check your scripts to ensure you are not accidentally loading the same scene twice or that some part of the code is calling a scene loading mechanism unnecessarily.
2. Additive Scene Loading: When scenes are loaded additively (using LoadSceneMode.Additive), the OnSceneLoaded callback will be invoked for each loaded scene separately. If you're combining multiple scenes additively, each one will trigger this event, even if they are loaded all at once.
3. Asynchronous Scene Loading with Event Subscription: If you are using asynchronous scene loading with SceneManager.LoadSceneAsync(), and have not correctly managed the event registration (subscribing multiple times to the same event without unsubscribing first), the event may be triggered multiple times when a single scene has finished loading.
4. Scene Reloading within the Editor: The Unity Editor itself can sometimes cause a scene to load twice during certain workflows, for instance, during a script recompile if your script is also attempting to load a scene via SceneManager method. This behaviour usually doesn't occur in built versions of your game.
5. Unintentional Function Registration: It's also possible that your OnSceneLoaded function is registered multiple times as an event listener. Inspect how you attach the function to the event to ensure it’s only done once.
Debugging Steps:
1. Place Breakpoints or Debug.Log: Start by adding Debug.Log statements or using breakpoints in the OnSceneLoaded function. This will help you confirm how many times it’s actually being called and from which locations in your code.
2. Check Scene Loading Calls: Review all of your scripts for calls to SceneManager.LoadScene or SceneManager.LoadSceneAsync. Identify any redundant or accidental scene-loading calls.
3. Examine the Load Mode: If you are using additive loading, ensure you are not double-handling the same scenes or expecting the callback to only fire once.
4. Event Unsubscription: For Async scene loading ensure that you correctly unsubscribe from the events in OnDisable or any other appropriate time, to avoid duplicate function calls.
5. Editor vs Build: Test your game in a built version, not just within the Unity Editor, to rule out editor-specific issues.
By systematically looking for these common causes, you can pinpoint why your OnSceneLoaded is running twice and prevent that behavior.