Initialization paths for native iOS plug-ins

Learn about the different initialization paths available for native iOS plug-ins.

Unity and Apple platforms provide multiple mechanisms for initializing a native plug-in. The best approach depends on whether you need to initialize the plug-in early (at application startup) or lazily (on its first use). You can perform initialization from managed C# code, native code, or within Unity’s generated Xcode project code.

Managed C# initialization paths

• RuntimeInitializeOnLoadMethodAttribute: Automatically runs a method at different points in the application lifecycle, such as BeforeSplashScreen or AfterSceneLoad. Use RuntimeInitializeOnLoadMethodAttribute to initialize your plug-in’s C# API and make the first call into native code. For more information, refer to RuntimeInitializeOnLoadMethodAttribute.
• SceneManager Callbacks: Use SceneManager.sceneLoaded and SceneManager.sceneUnloaded for lazy initialization tied to specific scene content.
• MonoBehaviour Lifecycle: Use standard MonoBehaviour methods like Awake, OnEnable, and Start for initialization tied to a GameObject’s lifecycle. Refer to Event function execution order for more information.

Native initialization paths

The native initialization paths and their timing differ depending on your plug-in’s language. Swift follows a different path than C++ or Objective-C. The following sections outline the various initialization paths available for native code.

C++ and Objective-C++ initialization paths

You can use a global C++ object’s constructor to run code before main() and its destructor to run code after main() completes. A global C++ object constructor is invoked automatically when the application binary loads and its destructor is invoked when the binary unloads.

Characteristics:

• Timing: Before main().
• Execution order: Non-deterministic.
• Unity state: Unity isn’t initialized.
• Execution: Triggered automatically on binary load and unload.
• Compatibility: Works with .c, .cpp, .m, .mm, and .a files.
• Dependency: Behavior depends on UnityFramework.framework linking (static versus dynamic).

Code example

// Global object — constructor runs before main(), destructor after main()

struct GlobalFoo {
    GlobalFoo() {
        std::cout << "GlobalFoo constructor running!\n";
    }

    ~GlobalFoo() {
        std::cout << "GlobalFoo destructor running!\n";
    }
};

GlobalFoo g_foo;

Clang and GCC initialization paths

This method uses the __attribute__((constructor)) and __attribute__((destructor)) attributes to register functions that run when a library loads and unloads.

Characteristics:

• Timing: Runs before main().
• Execution order: Non-deterministic.
• Unity state: Unity isn’t initialized.
• Execution: Triggered automatically on binary load and unload.
• Compatibility: Works with .c, .cpp, .m, and .mm files.
• Dependency: Behavior depends on UnityFramework.framework linking.

Example

// runs before main
__attribute__((constructor))
static void MyInit() {
    printf("Init called\n");
}

// after program exit/shared library unload
__attribute__((destructor))
static void MyDone() {
    printf("Done called\n");
}

Objective-C

These methods are for use in .m and .mm files.

+load

The +load method is executed by the Objective-C runtime when a class loads, which occurs before main() and before C++ global constructors.

Characteristics:

• Timing: Early. Runs before main().
• Execution order: Non-deterministic.
• Unity state: Unity isn’t initialized.
• Execution: Runs automatically for every class and category that implements it, even if never referenced. Can’t be disabled.
• Compatibility: Works with .m and .mm files.
• Dependency: Behavior depends on UnityFramework.framework linking.

Example

// Runs as soon as the Obj-C runtime loads the class (before main()).
// Categories get +load before classes
@implementation MyClass
+ (void)load {
    NSLog(@"+load called");
}
@end

+initialize

The +initialize method is invoked lazily, just before the first message is sent to the class. It’s ideal for on-demand initialization.

Characteristics:

• Timing: Runs on the first message sent to the class.
• Execution order: Deterministic.
• Unity state: Unity might be initialized, depending on when the first message is sent.
• Execution: Lazy. Doesn’t run if the class is never used, thread-safe, and runs exactly once per class.
• Compatibility: Works with .m and .mm files.

Example:

// Called on first message dispatch to the class
@implementation MyClass
+ (void)initialize {
    NSLog(@"+initialize called");
}
@end

Swift

In Swift, global variables and objects are lazily initialized. This means the NativeManager constructor runs only when the manager object is accessed for the first time. The initial access occurs when calling GetValueFromManager.

Characteristics:

• Timing: On first use.
• Execution order: Deterministic.
• Execution: Lazy.

Example:

class NativeManager {
init() { rint("NativeManager initialized") }
func value() -> Int { return 42 }
}

// global object
let manager = NativeManager()

@_cdecl("GetValueFromManager")
public func GetValueFromManager() -> Int { return manager.value() }

Change App Controller Class

You can modify the Unity-provided AppController class to customize your application’s startup behavior. The IMPL_APP_CONTROLLER_SUBCLASS macro achieves this by replacing Unity’s default AppController class with your own custom Objective-C subclass.

IMPL_APP_CONTROLLER_SUBCLASS works as follows:

1. Declare an Objective-C category on your subclass.
2. Use +load (executed before main()) to override the global AppControllerClassName symbol that Unity uses to decide which UIApplicationDelegate or AppController class to instantiate.

Use IMPL_APP_CONTROLLER_SUBCLASS if you need early lifecycle hooks, such as:

• preStartUnity
• startUnity
• initUnityWithScene
• application:didFinishLaunchingWithOptions
• applicationDidEnterBackground
• applicationWillTerminate
• App-level system integrations that must run before Unity initializes.

Characteristics:

• Dependency: Only compatible with an Objective-C project type.
• Compatibility: Works with .m and .mm files.

Code example:

// MyAppController.mm
#import "UnityAppController.h"

// Custom AppController
@interface MyAppController : UnityAppController
@end

@implementation MyAppController

    (BOOL)application:(UIApplication*)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions
    {
    NSLog(@"MyAppController didFinishLaunching");
    return [super application:application didFinishLaunchingWithOptions:launchOptions];
    }
    @end

// Register this class as Unity's AppController
IMPL_APP_CONTROLLER_SUBCLASS(MyAppController)

Additional resources

• Configure a plug-in for iOS with the Inspector window
• Conditional compilation behavior for native iOS plug-ins