How do I wait for a function to be done executing before returning a value?

I have an issue with this code where the return packageSize statement is triggered before the onGetStatsCompleted function and it returns 0 instead of the right value. Is there a way I can force onGetStatsCompleted to finish before returning packageSize? I know it's a logic issue because if I remove the comment at //Thread.sleep it works fine.

How do I fix this without using Thread.sleep or any other kind of time out in the application?

private var packageSize: Long = 0
private var dataSize: Long = 0
private var cacheSize: Long = 0
private var apkSize: Long = 0

/**
Get the size of the app for API < 26
*/
@Throws(InterruptedException::class)
fun getPackageSize(): Long {

    val pm = context.packageManager
    try {
        val getPackageSizeInfo = pm.javaClass.getMethod(
                "getPackageSizeInfo", String::class.java, IPackageStatsObserver::class.java)
        getPackageSizeInfo.invoke(pm, context.packageName,
                object : CachePackState() {//Call inner class
                })
    } catch (e: Exception) {
        e.printStackTrace()
    }
    //Thread.sleep(1000)
    return packageSize
}

/**
  Inner class which will get the data size for the application
 */
open inner class CachePackState : IPackageStatsObserver.Stub() {

    override fun onGetStatsCompleted(pStats: PackageStats, succeeded: Boolean) {
        //here the pStats has all the details of the package
        dataSize = pStats.dataSize
        cacheSize = pStats.cacheSize
        apkSize = pStats.codeSize
        packageSize = cacheSize + apkSize

    }
}

Feel free to ask any questions, any help is appreciated.

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