Utility class for handling permissions on Android

BaseActivity.kt

class BaseActivity : Activity {
    
    val permissionHandler: PermissionRequestHandler
    
    override fun onRequestPermissionsResult(requestCode: Int, permissions: Array<out String>, grantResults: IntArray) {
        val handled = permissionHandler.onRequestPermissionsResult(requestCode, permissions, grantResults)
        if(!handled) {
            super.onRequestPermissionsResult(requestCode, permissions, grantResults)
        }
    }
}

PermissionRequestHandler.kt

class PermissionRequestHandler(val activity: Activity) {
    val permissionCallbacks = mutableMapOf<Int, PermissionCallbacks>()
    val requestCodes = generateSequence(0) { it + 1 }.iterator()

    data class PermissionCallbacks(val onGranted: () -> Unit,
                                   val onDenied: (() -> Unit)? = null)

    fun needPermission(
        permission: String,
        beforeRequest: (() -> Unit)? = null,
        onDenied: (() -> Unit)? = null,
        onGranted: () -> Unit
    ) {
        if(hasPermission(permission)) {
            onGranted()
        } else {
            beforeRequest?.invoke()
            val requestCode = requestCodes.next()
            permissionCallbacks[requestCode] = PermissionCallbacks(onGranted, onDenied)
            ActivityCompat.requestPermissions(activity, arrayOf(permission), requestCode)
        }
    }

    fun onRequestPermissionsResult(requestCode: Int, permissions: Array<out String>,
                                   grantResults: IntArray): Boolean {
        val (onGranted, onDenied) = permissionCallbacks[requestCode]
            ?: return false
        if(grantResults.isNotEmpty() && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            onGranted()
        } else {
            onDenied?.invoke()
        }
        return true
    }

    fun hasPermission(perm: String) = activity.hasPermission(perm)
}

fun Context.hasPermission(perm: String): Boolean =
    ContextCompat.checkSelfPermission(this, perm) == PackageManager.PERMISSION_GRANTED