Navigation Architecture Component - Dialog Fragments

May 2019 Update:

DialogFragment are now fully supported starting from Navigation 2.1.0, you can read more here and here

Old Answer for Navigation <= 2.1.0-alpha02:

I proceeded in this way:

1) Update Navigation library at least to version 2.1.0-alpha01 and copy both files of this modified gist in your project.

2) Then in your navigation host fragment, change the name parameter to your custom NavHostFragment

<fragment
    android:id="@+id/nav_host_fragment"
    android:name="com.example.app.navigation.MyNavHostFragment"
    app:defaultNavHost="true"
    app:navGraph="@navigation/nav_graph"
    android:layout_width="0dp"
    android:layout_height="0dp"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@id/toolbar" />

3) Create your DialogFragment subclasses and add them to your nav_graph.xml with:

<dialog
    android:id="@+id/my_dialog"
    android:name="com.example.ui.MyDialogFragment"
    tools:layout="@layout/my_dialog" />

4) Now launch them from fragments or activity with

findNavController().navigate(R.id.my_dialog)

or similar methods.

No, as of the 1.0.0-alpha01 build, there is no support for dialogs as part of your Navigation Graph. You should just continue to use show() to show a DialogFragment.

Yes it is possible, You can access view of parent fragment from dialog fragment by calling getParentFragment().getView(). And use the view for navigation.

Here is the example

Navigation.findNavController(getParentFragment().getView()).navigate(R.id.nextfragment);

Yes. The framework is made in such a way that you can create a class extending the Navigator abstract class for the views that does not come out-of-the box and add it to your NavController with the method getNavigatorProvider().addNavigator(Navigator navigator)

If you are using the NavHostFragment, you will also need to extend it to add the custom Navigator or just create your own MyFragment implementing NavHost interface. It's so flexible that you can create your own xml parameters with custom attrs defined in values, like you do creating custom views. Something like this (not tested):

@Navigator.Name("dialog-fragment")
class DialogFragmentNavigator(
        val context: Context,
        private val fragmentManager: FragmentManager
) : Navigator<DialogFragmentNavigator.Destination>() {

    override fun navigate(destination: Destination, args: Bundle?,
                          navOptions: NavOptions?, navigatorExtras: Extras?
    ): NavDestination {
        val fragment = Class.forName(destination.name).newInstance() as DialogFragment
        fragment.show(fragmentManager, destination.id.toString())
        return destination
    }

    override fun createDestination(): Destination = Destination(this)

    override fun popBackStack() = fragmentManager.popBackStackImmediate()

    class Destination(navigator: DialogFragmentNavigator) : NavDestination(navigator) {

        // The value of <dialog-fragment app:name="com.example.MyFragmentDialog"/>
        lateinit var name: String

        override fun onInflate(context: Context, attrs: AttributeSet) {
            super.onInflate(context, attrs)
            val a = context.resources.obtainAttributes(
                    attrs, R.styleable.FragmentNavigator
            )
            name = a.getString(R.styleable.FragmentNavigator_android_name)
                    ?: throw RuntimeException("Error while inflating XML. " +
                            "`name` attribute is required")
            a.recycle()
        }
    }
}

Usage

my_navigation.xml

<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/navigation"
    app:startDestination="@id/navigation_home">

    <fragment
        android:id="@+id/navigation_assistant"
        android:name="com.example.ui.HomeFragment"
        tools:layout="@layout/home">
        <action
            android:id="@+id/action_nav_to_dialog"
            app:destination="@id/navigation_dialog" />
    </fragment>

    <dialog-fragment
        android:id="@+id/navigation_dialog"
        android:name="com.example.ui.MyDialogFragment"
        tools:layout="@layout/my_dialog" />

</navigation>    

The fragment that will navigate.

class HomeFragment : Fragment(), NavHost {

    private val navControllerInternal: NavController by lazy(LazyThreadSafetyMode.NONE){
        NavController(context!!)
    }

    override fun getNavController(): NavController = navControllerInternal

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // Built-in navigator for `fragment` XML tag
        navControllerInternal.navigatorProvider.addNavigator(
            FragmentNavigator(context!!, childFragmentManager, this.id)
        )
        // Your custom navigator for `dialog-fragment` XML tag
        navControllerInternal.navigatorProvider.addNavigator(
            DialogFragmentNavigator(context!!, childFragmentManager)
        )
        navControllerInternal.setGraph(R.navigation.my_navigation)
    }

    override fun onCreateView(inflater: LayoutInflater, 
                              container: ViewGroup?, savedInstanceState: Bundle?): View? {
        super.onCreateView(inflater, container, savedInstanceState)
        val view = inflater.inflate(R.layout.home)
        view.id = this.id

        view.button.setOnClickListener{
            getNavController().navigate(R.id.action_nav_to_dialog)
        }

        return view
    }
}