Hello guys, I have a question: I want to compile m...

# multiplatform

[Piasy]

Hello guys, I have a question: I want to compile my commonMain and iosMain kotlin code into a static library, I achieve this target with gradle plugin, but my problem is, I don't know how to implement an abstract kotlin class in Objective-C and pass it to kotlin, because when I compile kotlin code to static library, all stuff are C struct.

Here is my kotlin abstract class:

abstract class ConfEvents {
  open fun onPeerJoined(uid: String) {}
}

Here is the generated C header:

struct {
  libAvConf_KType* (*_type)(void);
  libAvConf_kref_com_piasy_avconf_ConfEvents (*ConfEvents)();
  void (*onPeerJoined)(libAvConf_kref_com_piasy_avconf_ConfEvents thiz, const char* uid);     
} ConfEvents;

If I compile kotlin code into framework, then ConfEvents would have a Objective-C interface, then I can extend it and pass it to kotlin.

[Cristián Arenas]

What I’m doing right now is building a wrapper around the C API

[Cristián Arenas]

so it woud have a private property of type libAvConf_kref_com_piasy_avconf_ConfEvents, which it would get by calling privateInstance = ConfEvents.ConfEvents() on the wrapper’s constructor, and then I would reimplement every method, like -(void)onPeerJoined(const char* uid) and it would call the correct one inside ConfEvents.onPeerJoined(privateInstance, uid)

[Cristián Arenas]

horrible, but I don’t know a better way

[Cristián Arenas]

I don’t know why it exports it as C instead of Objective-C

[Piasy]

@Cristián Arenas Thanks for your reply, but I may not explain myself clearly, what I want to do is create a sub class of the kotlin ConfEvents class in Objective-C, and its method will be called at kotlin code, and I need pass that Objective-C object to a kotlin method which receive a ConfEvents object.

That kotlin method is:

fun create(
      uid: String,
      events: ConfEvents
    ): AvConf

And the generated C API is:

libAvConf_kref_com_piasy_avconf_AvConf (*create)(libAvConf_kref_com_piasy_avconf_AvConf_Companion thiz, const char* uid,  libAvConf_kref_com_piasy_avconf_ConfEvents events);

In your description, it seems to wrap a kotlin object with an Objective-C object, and use it in Objective-C, am I right?

[Cristián Arenas]

In your description, it seems to wrap a kotlin object with an Objective-C object, and use it in Objective-C, am I right?

Yes, that’s the idea.
I do not yet know how to create an Objective-C header when creating a static library, so wrapping the C API in an Objective-C class would solve some of your problems. After wrapping all the methods, you can subclass that class in Objective-C, the only difference would be that if any function requires a pointer to the original ConfEvents class, you would have to pass it the inner private instance (of type libAvConf_kref_com_piasy_avconf_ConfEvents ).

The better solution would be to ask how to generate the static library with an Objective-C API, I have no idea how to do that

[Piasy]

Sorry I still have doubt about your suggestion: if the onPeerJoined function of ConfEvents is called in kotlin code, how could the Objective-C code be called?

[Cristián Arenas]

I’m not sure what you mean

[Piasy]

Okay, let me explain it.

so wrapping the C API in an Objective-C class

You mean I can create an Objective-C class, e.g. ObjCConfEvents, it will own a pointer of libAvConf_kref_com_piasy_avconf_ConfEvents , and its onPeerJoined method will call void (*onPeerJoined)(libAvConf_kref_com_piasy_avconf_ConfEvents thiz, const char* uid); , then I can create subclass of ObjCConfEvents , e.g. SpecialConfEvents, and doing the real logic in its onPeerJoined method, right?

the only difference would be that if any function requires a pointer to the original ConfEvents class, you would have to pass it the inner private instance (of type libAvConf_kref_com_piasy_avconf_ConfEvents )

You mean I need pass the libAvConf_kref_com_piasy_avconf_ConfEvents of ObjCConfEvents to kotlin code (the create function), right?

If my understanding is correct, when kotlin code call onPeerJoined of libAvConf_kref_com_piasy_avconf_ConfEvents , how could the ObjCConfEvents get called?

Sorry if my understanding is wrong, and please correct me if so.

Thank you very much!

[Cristián Arenas]

ah

[Cristián Arenas]

I get your problem now

[Cristián Arenas]

what I describe solves the “call kotlin code from obj-c” problem

[Cristián Arenas]

but you want to “call obj-c code from kotlin”

[Piasy]

yeah I understand that

[Cristián Arenas]

I would think that that is not possible using subclasses

[Cristián Arenas]

I would personally go with closures/blocks to do that

[Cristián Arenas]

but maybe it does work if you can generate an Objective-C static library

[Cristián Arenas]

but this wrapper idea breaks communication in the other direction

[Piasy]

okay, if so, then that's bad 😞

but fortunately we can compile kotlin code as framework, although the naming is out of our control, but we can create another wrapper framework over it.

[Cristián Arenas]

I’m in the same boat as you

[Cristián Arenas]

also, I haven’t been able to make static libraries with bitcode

[Piasy]

yeah, the gradle plugin doesn't have bitcode option when compiling static lib

[Cristián Arenas]

that means I have to disable bitcode in the iOS app, which is not an option for me (used by external clients), so I can only make frameworks

[Cristián Arenas]

I’ve been trying the umbrella framework approach

[Cristián Arenas]

and it turns out that Apple explicitly tells you in the documentation that you should avoid them

[Cristián Arenas]

and Xcode does not support them, so they don’t get codesigned and the apps don’t compile

[Piasy]

I haven't use umbrella framework before, I haven't even hard about it, lol

[Cristián Arenas]

umbrella framework = framework with frameworks inside, it’s what you suggested

[Cristián Arenas]

Apple docs:
https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/CreationGuidelines.html

Don’t Create Umbrella Frameworks

While it is possible to create umbrella frameworks using Xcode, doing so is unnecessary for most developers and is not recommended. Apple uses umbrella frameworks to mask some of the interdependencies between libraries in the operating system. In nearly all cases, you should be able to include your code in a single, standard framework bundle. Alternatively, if your code was sufficiently modular, you could create multiple frameworks, but in that case, the dependencies between modules would be minimal or nonexistent and should not warrant the creation of an umbrella for them.

[Piasy]

oh, I didn't mean that, by saying wrapper framework, I mean another separate framework, which just contains classes that wrap API of the original framework's classes.

[Cristián Arenas]

so you would need to install both on the iOS app?

[Piasy]

yes, not perfect, but it works

[Piasy]

I mean, the app project need depend on both of them.

[Cristián Arenas]

well, if you go the wrapping framework route as I did:
The way to fix this codesigning problem, is to add a Run Script build phase, where you can use the codesign command line tool.

f="$CODESIGNING_FOLDER_PATH/Frameworks/YourOuterFrameworkName.framework/Frameworks/YourInnerFrameworkName.framework"

codesign --force --sign "$EXPANDED_CODE_SIGN_IDENTITY_NAME" --preserve-metadata=identifier,entitlements --timestamp=none "$f"

That way the app project only needs to depend on the outer one

[Piasy]

great! thanks for sharing

[Cristián Arenas]

just in case you ever try that

[Cristián Arenas]

I would’ve killed for someone to tell me all of this 3 weeks ago

[Piasy]

😂

[Cristián Arenas]

and if someone had said this 2 months ago, I would’ve never chosen kotlin as a way to share code

[Cristián Arenas]

it’s been probably harder than maintaining 2 codebases 😅

[Piasy]

long way to go for kotlin mpp

[Cristián Arenas]

If you figure anything else out, please share it with me 🙏

[Piasy]

Sure I will

[Artyom Degtyarev [JB]]

Just a word on static libs with bitcode - please try this approach(https://kotlinlang.slack.com/archives/C3SGXARS6/p1575971172220200?thread_ts=1575913985.211900&cid=C3SGXARS6), and tell if this is not working.