getOrAddFragment

FragmentExt.kt

fun <T : Fragment> FragmentActivity.getOrAddFragment(tag: String, commitNow: Boolean = true, initFun: () -> T): T =
    supportFragmentManager.findFragmentByTag(tag) as? T ?: run {
        initFun().also {
            addFragment(supportFragmentManager, tag, commitNow, it)
        }
    }

fun <T : Fragment> Fragment.getOrAddFragment(tag: String, commitNow: Boolean = true, initFun: () -> T): T =
    childFragmentManager.findFragmentByTag(tag) as? T ?: run {
        initFun().also {
            addFragment(childFragmentManager, tag, commitNow, it)
        }
    }

private fun addFragment(fragmentManager: FragmentManager, tag: String, commitNow: Boolean, fragment: Fragment) {
    fragmentManager.transaction(commitNow) {
        add(fragment, tag)
    }
}

fun FragmentManager.transaction(commitNow: Boolean = true, body: FragmentTransaction.() -> Unit) {
    val transaction = beginTransaction()
    body(transaction)
    if (commitNow) transaction.commitNow() else transaction.commit()
}

MyActivity.kt

class MyActivity : AppCompatActivity() {

     override onCreate(state: Bundle) {
         val fragment = getOrAddFragment(tag = MY_FRAGMENT) { MyFragment() }
     }
     
     companion object {
         private const val MY_FRAGMENT = "my-fragment"
     }
 }