Coordinator Layout with Toolbar in Fragments or Activity

As for me it sounds too weird to have appbar and toolbar in each fragment. So I've chosen to have single appbar with toolbar in activity.

To solve that issue with CoordinatorLayout you will have to set different behaviour of your FrameLayout (or any other Layout) that supposed to hold fragments from each fragment that you want to override default behaviour.

Lets assume, that your default behaviour is app:layout_behavior="@string/appbar_scrolling_view_behavior"

Then in your fragment_activity_layout.xml you may have something like that:

<android.support.design.widget.CoordinatorLayout
    android:id="@+id/coordinator"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.design.widget.AppBarLayout
        android:id="@+id/appbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <android.support.v7.widget.Toolbar
            android:id="@+id/dashboard_toolbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:theme="@style/AppTheme.Toolbar"
            app:layout_scrollFlags="scroll|enterAlways"/>
    </android.support.design.widget.AppBarLayout>

    <FrameLayout
        android:id="@+id/dashboard_content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"/>
</android.support.design.widget.CoordinatorLayout>

And in each fragment you wish not to implement app:layout_behavior="@string/appbar_scrolling_view_behavior" you will have to override onAttach and onDetach methods that will change behaviour of your FrameLayout:

CoordinatorLayout.Behavior behavior;

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);

    if(behavior != null)
        return;

    FrameLayout layout =(FrameLayout) getActivity().findViewById(R.id.dashboard_content);
    CoordinatorLayout.LayoutParams params = (CoordinatorLayout.LayoutParams) layout.getLayoutParams();

    behavior = params.getBehavior();
    params.setBehavior(null);

}

@Override
public void onDetach() {
    super.onDetach();
    if(behavior == null)
        return;

    FrameLayout layout =(FrameLayout) getActivity().findViewById(R.id.dashboard_content);
    CoordinatorLayout.LayoutParams params = (CoordinatorLayout.LayoutParams) layout.getLayoutParams();

    params.setBehavior(behavior);

    layout.setLayoutParams(params);

    behavior = null;
}

After that CoordinatorLayout won't collapse appbar, etc. and will allow fragment layouts to be full-height.

Here's my solution

<!-- Put your fragment inside a Framelayout and set the behavior for this FrameLayout -->
<FrameLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior">

    <!-- Your fragment -->
    <include layout="@layout/content_main" />

</FrameLayout>

<android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:layout_scrollFlags="scroll|enterAlways"
        app:popupTheme="@style/AppTheme.PopupOverlay" />

</android.support.design.widget.AppBarLayout>

This is a realy good question: should Toolbars that need to act like an ActionBar be kept in a Activity or a Fragment? Having searched around different questions and documentation, I couldn't find a solution that covers all cases. It therefore really depends on your situation wich way to go.

Case 1: Toolbar must be ActionBar replacement

If the Toolbar has to behave like a normal ActionBar (or if max 1 fragment is shown from time to time), I think the best/simplest way is to use traditional Activities with there own Toolbar and put your Fragment in there. This way you don't have to worry about when which Toolbar must be shown.

Changing the ActionBar (-behaviour) from Fragments is also possible, but I would not recommend it, since that forces you to keep track which Fragment changed the ActionBar when. I don't even know if setting the ActionBar can be done multiple times.

Case 2: Each Fragment should have its own (part of) Toolbar

You could also choose to put different stand alone Toolbars in different Fragments, with their own actions. This way you could display different Fragments next to each other - each with their own actions in their Toolbar - and suggest that it is 1 Toolbar (maybe like the Gmail-app, although I am unsure). This however means that you would have to inflate those Toolbars yourself, but it must not be very difficult.

I hope this will help making a choice.

(Sorry if I made any (language-)mistakes)

That's what the navigation documentation says:

Adding the top app bar to your activity works well when the app bar’s layout is similar for each destination in your app. If, however, your top app bar changes substantially across destinations, then consider removing the top app bar from your activity and defining it in each destination fragment, instead.

Actually it's very easy to setup a Toolbar using NavigationUI, and this solution doesn't require the Fragment to have any knowledge about its parent. For example:

<!-- my_fragment.xml -->
<androidx.constraintlayout.widget.ConstraintLayout ...>

  <com.google.android.material.appbar.MaterialToolbar
    android:id="@+id/toolbar"
    ... />

</androidx.constraintlayout.widget.ConstraintLayout>

class MyFragment : Fragment() {
  ...

  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    val navController = findNavController()
    binding.toolbar.setupWithNavController(navController)
  }
}

You can find the full GitHub example here. There's also a relative question that might be interested Is setSupportActionbar required anymore?