Kotlin SingleFragmentActivity

SingleFragmentActivity.kt

package com.bignerdranch.android.kotlin

import android.os.Bundle
import android.support.v4.app.Fragment
import android.support.v4.app.FragmentManager
import android.support.v7.app.AppCompatActivity

abstract class SingleFragmentActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_single_fragment)
        addFragmentIfNeeded(R.id.container, this::createFragment)
    }

    abstract fun createFragment(): Fragment
}

fun FragmentActivity.addFragmentIfNeeded(container: Int, creator: () -> Fragment) {
    supportFragmentManager.addFragmentIfNeeded(container, creator)
}

fun FragmentManager.addFragmentIfNeeded(container: Int, creator: () -> Fragment) {
    findFragmentById(container) ?: beginTransaction().add(container, creator()).commit()
}