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kotlinlang

I'm digging into ios samples (upd: i read all the samples, kotlinconf app too), want to understand how they work

we have `AppDelegate.h` + `AppDelegate.m` files with stub implementation in `@implementation AppDelegate`

we also have AppDelegate.kt:
```
class AppDelegate : UIResponder(), UIApplicationDelegateProtocol
override fun init() = initBy(AppDelegate())
```

Please, correct my understandings in thread &gt;

1. `initBy(AppDelegate())` - AppDelegate here is the stub declared in `AppDelegate.m`

2. what's the `initBy` fun? guess it's a `kotlinx.cinterop.initBy` but I don't understand where the receiver is.
```
@konan.internal.Intrinsic
external fun &lt;T : ObjCObjectBase&gt; T.initBy(constructorCall: T): T
```

3. Anyway, then you find `initBy` marker with
`val objCObjectInitBy = packageScope.getContributedFunctions("initBy").single()`
and use it in LLVM IR.

4. The ObjC AppDelegate declaration is needed because no straight Kotlin -&gt; C interop is developed yet. Thus iOS can't easily call `@protocol UIApplicationDelegate` implemented in Kotlin. So you feed the ios with ObjC stub, but placing the kotlin implementation on LLVM level.

Last I looked the iOS sample was a bit hackish. I think it improved in the latest release but the sample may not be up to date. The best example to look at is the Kotlinconf app. The source is available on GitHub.  

well, this concrete part is common among all the samples, including kotlinconf app.

&gt; 4. The ObjC AppDelegate declaration is needed because no straight Kotlin -&gt; C interop is developed yet. Thus iOS can’t easily call `@protocol UIApplicationDelegate` implemented in Kotlin. So you feed the ios with ObjC stub, but placing the kotlin implementation on LLVM level.

No, straight Kotlin -&gt; Objective-C interop is available, and iOS can call the protocol implemented in Kotlin. Objective-C stubs are required only to interact with Xcode.

So:

&gt; 1. `initBy(AppDelegate())` - AppDelegate here is the stub declared in `AppDelegate.m`

`AppDelegate` here is the class this code is located in.

&gt; what’s the `initBy` fun? guess it’s a `kotlinx.cinterop.initBy` but I don’t understand where the receiver is.

The receiver is the same as the receiver of the `init` method being implemented.

`initBy` is actually just an intrinsic to call the Kotlin constructor on allocated object instance. It is required because in Objective-C initializers are methods, which can be called virtually. So in some cases, e.g. when iOS allocates and initializes the user-provided class by itself, you need to override some `init*` methods. And `initBy` intrinsic allows to implement these methods by calling the corresponding Kotlin constructors.

You are welcome! The documentation will come later :slightly_smiling_face: