ChildEventListener.kt

package com.google.firebase.udacity.friendlychat

import com.google.firebase.database.ChildEventListener
import com.google.firebase.database.DataSnapshot
import com.google.firebase.database.DatabaseError

class _ChildEventListener(
        private val _onCancelled: (DatabaseError) -> Unit,
        private val _onChildMoved: (DataSnapshot, String?) -> Unit,
        private val _onChildChanged: (DataSnapshot, String?) -> Unit,
        private val _onChildAdded: (DataSnapshot, String?) -> Unit,
        private val _onChildRemoved: (DataSnapshot) -> Unit
) : ChildEventListener {

    override fun onCancelled(p0: DatabaseError) {
        _onCancelled(p0)
    }

    override fun onChildMoved(p0: DataSnapshot, p1: String?) {
        _onChildMoved(p0, p1)
    }

    override fun onChildChanged(p0: DataSnapshot, p1: String?) {
        _onChildChanged(p0, p1)
    }

    override fun onChildAdded(p0: DataSnapshot, p1: String?) {
        _onChildAdded(p0, p1)
    }

    override fun onChildRemoved(p0: DataSnapshot) {
        _onChildRemoved(p0)
    }
}

class _ChildEventListenerConfig {

    private var _onCancelled: (DatabaseError) -> Unit = {}
    private var _onChildMoved: (DataSnapshot, String?) -> Unit = { _, _ -> }
    private var _onChildChanged: (DataSnapshot, String?) -> Unit = { _, _ -> }
    private var _onChildAdded: (DataSnapshot, String?) -> Unit = { _, _ -> }
    private var _onChildRemoved: (DataSnapshot) -> Unit = {}

    fun onCancelled(f: (DatabaseError) -> Unit) {
        _onCancelled = f
    }

    fun onChildMoved(f: (DataSnapshot, String?) -> Unit) {
        _onChildMoved = f
    }

    fun onChildChanged(f: (DataSnapshot, String?) -> Unit) {
        _onChildChanged = f
    }

    fun onChildAdded(f: (DataSnapshot, String?) -> Unit) {
        _onChildAdded = f
    }

    fun onChildRemoved(f: (DataSnapshot) -> Unit) {
        _onChildRemoved = f
    }

    fun build() = _ChildEventListener(_onCancelled, _onChildMoved, _onChildChanged, _onChildAdded, _onChildRemoved)
}

fun childEventListener(configure: _ChildEventListenerConfig.() -> Unit): ChildEventListener {
    val config = _ChildEventListenerConfig()
    config.configure()
    return config.build()
}