Using native plugin with callbacks in editor (OS X)

Questions & Answers
1

I have a native plugin that has callbacks back to the C# code. This is how the callbacks are setup:

C# Side (Unity):

public delegate void Callback(int value);

[DllImport("NativePlugin")]
public static extern void setup_callback(Callback callback);

[MonoPInvokeCallback(typeof(Callback))]
private static void OnCallback(int value)
{
	Debug.Log("Callback called with value: " + value);
}

void Start()
{
	setup_callback(OnCallback);
}

The Objective-C side (with C wrapper so you’d be able to call it from C#):

typedef void(*Callback)(int);

Callback _callback = NULL;

void setup_callback(Callback callback)
{
	_callback = callback;
}

void on_something_happened(int value)
{
	if(_callback)
		_callback(value);
}

Now, most of the time the above code works ok, and I receive the correct values in Unity. But sometimes when I make changes to any one of my C# scripts, Unity crashes with this:

Exception Type: EXC_BAD_ACCESS (SIGABRT)
Exception Codes: KERN_INVALID_ADDRESS at 0x00000000ffffffa9
Exception Note: EXC_CORPSE_NOTIFY

Looking at Unity’s Editor logs I found out that when I make changes to a C# script it recompiles Assembly-CSharp.dll and reloads it togheter with some other C# assemblies. So that made me think that this might cause the pointer saved in the native plugin to now point to an invalid address (because Assembly-CSharp.dll is now probably in a different address space). Note that it does not happen every time the dll gets reloaded, is it possible that it gets reloaded to the exact same address space sometimes?

Can I get an event from Unity when the editor reloads the DLL and then re-register the callbacks (assuming that this is the real problem)? Is this even the correct way to setup a callback from native code?

2

I ended up using LockReloadAssemblies Unity - Scripting API: EditorApplication.LockReloadAssemblies

It let’s you prevent Unity from reloading until you call UnlockReloadAssemblies.
I just created a new script:

    void Awake()
    {
        //Prevent editor from reloading dlls files during the gameplay
        #if UNITY_EDITOR
        UnityEditor.EditorApplication.LockReloadAssemblies();
        #endif
    }

    void OnApplicationQuit()
    {
        //Prevent editor from reloading dlls files during the gameplay
         #if UNITY_EDITOR
        UnityEditor.EditorApplication.UnlockReloadAssemblies();
        #endif
    }

And put it on some object in the scene.

3

For anyone else with a similar issue, another solution that might work for you is documented here:

In the code below, Unity will call OnDisable immediately before unloading the assembly and performing a recompile.

 [InitializeOnLoad]
 internal sealed class MyScriptableFriend : ScriptableObject {
     static MyScriptableFriend() { s_Instance = CreateInstance<MyScriptableFriend>(); }
     private static MyScriptableFriend s_Instance;

     private void OnDisable() {
         // Incoming recompile... save yourself!
     }
 }