Hello, I have a Java base library and a same objec...

# multiplatform

[xiaobailong24]

Hello, I have a Java base library and a same objective-C base library. And there is a

abstract class JavaAbsClass

in java library, and a

class ObjcAbsClass

in objc library.

So

except abstract class CommonAbsClass

is defined in kmm commonMain module, and

actual typealias CommonAbsClass = JavaAbsClass

that's not a problem in kmm jvmMain module.

But in kmm iosMain module, not only

actual typealias CommonAbsClass = ObjcAbsClass`

is unsupported, but also

actual typealias CommonAbsClass = ObjcAbsClassWrapper
abstract class ObjcAbsClassWrapper:ObjcAbsClass()

will get a compiler error with Non-final Kotlin subclasses of Objective-C classes are not yet supported.

So how should I do to interop abstract class with both jvm and objc library? Thanks very much!

[Big Chungus]

Does your CommonClass has any properties, constructors or methods declared?

[Big Chungus]

Easiest workaround would be this

actual abstract class CommonAbsClass: ObjCAbsClass()

[xiaobailong24]

Yes, there are some extension properties and extension function to interop jvm and objc

[Big Chungus]

When you use actual typealias, target signature must match EXACTLY to the expect declaration

[Big Chungus]

So common pattern is to moddel your expect after one of the platform declarations and typealias to it on that platform while using actual wrapper on other platforms.

e.g.:

// commonMain
expect class File {
  fun getPath(): String
}

// jvmMain
actual typealias File = java.io.File

// iosMain
actual class File {
  actual fun getPath(): String {
    TODO("Delegate to some IOS or posix APIs for this")
  }
}

[xiaobailong24]

I have try it. But another problem is that if other class refer this interop subclass, the original class type is missed

[Big Chungus]

Also note that extension functions are not part of class signature so to expand on the above example you could just as well do this without breaking typealias on jvm

// commonMain
...
expect fun File.veryCustomMethod()

// jvmMain
...
actual fun File.veryCustomMethod() = println("works")

// iosMain
...
actual fun File.veryCustomMethod() = println("works")

[Big Chungus]

You can extend native classes and interfaces on actual declaration if they're not final.

...
// iosMain
actual class File: SomeObjCFileClass() {
  actual fun getPath(): String {
    TODO("Delegate to some IOS or posix APIs for this")
  }
}

[xiaobailong24]

extend classes got another problem.

//commonMain
except class CommonAnotherClass
except var CommonAnotherClass.file : File

//iosMain
actual typealias CommonAnotherClass = ObjCAnotherClass
actual var CommonAnotherClass.file : File
    get() = this.getFile() //type mismatch. Return SomeObjCFileClass type defined in objc library
    set(value){
        this.setFile(value)
    }

[Big Chungus]

What exactly this.getFile() return? It must return your File from commonMain, not native alternative

[xiaobailong24]

this is a ObjCAnotherClass object.

[Big Chungus]

No, this is CommonAnotherClass

[Big Chungus]

You need to understand that with this approach, on jvm java.io.File == File (since it's typealias), but on ios `ObjCAnotherClass != File`(since File just extends CommonAnotherClass)

[Big Chungus]

Can you drop relevant code you have for all three - common, jvm and ios? Might be easier to explain what's wrong using specific examples

[xiaobailong24]

Sorry. Yes, this is NOT ObjCAnotherClass.
But this.getFile() returned SomeObjCFileClass defined in objc library

[Big Chungus]

Right, so THAT is what's wrong, since expected return type is File. Remember, if you have class A: B(), then B() is A() but A() !is B()

[xiaobailong24]

How to solve that?

[Big Chungus]

You need to wrap your ObjCAnotherClass in File. If it's an interface this can work

actual class CommonAnotherClass(delegate: ObjCAnotherClass): ObjCAnotherClass by delegate

actual var CommonAnotherClass.file : File
    get() = CommonAnotherClass(this.getFile()) 
    set(value){
        this.setFile(value)
    }

[xiaobailong24]

actual var CommonAnotherClass.file : File
How to interop with objc SomeObjCFileClass.getFile()

[Big Chungus]

Again, I need to see the actual code on all three sourceSets to assist you properly

[xiaobailong24]

Okay, I'll show my code at tomorrow. I am in China and on the subway after get off work at this time 🤣

[xiaobailong24]

Now using android slack app to ask for advice.👀

[xiaobailong24]

actual class CommonAnotherClass(delegate: ObjCAnotherClass): ObjCAnotherClass by delegate
It seems a workaround for me. I'll try tomorrow. Thanks!

[xiaobailong24]

@Big Chungus Here is a demo repo: https://github.com/xiaobailong24/KMMAbsClazzDemo

[xiaobailong24]

[xiaobailong24]

@Big Chungus Any idea with this demo?

[xiaobailong24]

Find an imperfect solution: