KotlinでFragmentTransaction用の拡張関数

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        fragmentTransaction { ft -> ft.add(R.id.fragment_container, MainFragment()) }
    }

    fun AppCompatActivity.fragmentTransaction(transaction: (ft: FragmentTransaction) -> Unit) {
        val ft = supportFragmentManager.beginTransaction()
        transaction(ft)
        ft.commit()
    }
}