Hi all, this topic is more advanced but I can’t fi...

# getting-started

[cpe]

Hi all, this topic is more advanced but I can’t find a better channel to place it. I need to execute some code on startup without knowing of its existence, think of an expendable registry where classes can register themselves (somehow like observer). The challenge is that - as I can’t instantiate these classes (actively) - the code is currently within a companion object. So I decided to provide an interface (or an annotation) and find the functions to call on startup as follows using (using classgraph as for scanning - I’m not bound to that framework):

interface OnStartup {
  fun onStartup()
}

// a class that wil instantiated a lot of times during runtime
data class SomeClass(val name: String) {
  companion object : OnStartup {
    override fun onStartup() {
      // notify someone at startup about our existence
    }
  }
}

fun main() {
  ClassGraph().enableAllInfo().acceptPackages("com.test").scan()
    .getClassesImplementing(OnStartup::class.java)
    .filter { it.isStatic }
    .flatMap { findStartupMethods(it) }
    .forEach {
      it.call()
    }
}

private fun findStartupMethods(classInfo: ClassInfo): List<KFunction<*>> {
  return classInfo.methodInfo.filter { function ->
    function.name == "onStartup"
  }.mapNotNull { method ->
    method.loadClassAndGetMethod().kotlinFunction
  }
}

This will not work as call() expects a parameter: Callable expects 1 arguments, but 0 were provided. which should be the instance of the class (from my understanding), so I guess it should be the companionObjectReference. The code works well if I define the onStartup function on top level (but that has negative impacts for my idea). Is there any way to retrieve the companionObjectReference? Or to achieve this behaviour? Maybe there is another annotation scanner that fits better for Kotlin? Any help is appreciated.

[Youssef Shoaib [MOD]]

Since you have classInfo, can you get a Class out of it? Because then, you can do klass.kotlin.objectInstance!!

[cpe]

Ah, didn’t saw the .kotlin 🙂 thanks for your help!

Solution now looks like:

val companion = classInfo.loadClass().kotlin
if (companion.isCompanion) {
  companion.memberFunctions.filter { it.name == "onStartup" }.forEach { it.call(companion.objectInstance) }
}