Android: when / why should I use FrameLayout instead of Fragment?

Question

I am building a layout for large screens, that is supposed to consist of 2 different parts, a left one and a right one. For doing that I thought using 2 Fragments is the right choice.

Then I had a look on the example of the navigation with the Master/Detail-Flow. It has a 2-pane layout, where on the right is the navigation, and on the left is the detail view.

But in that example, different from what I expected to see, for the detail view there is a FrameLayout that then holds a Fragment, instead of a Fragment directly.

The layout XML looks like this (an example):

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_marginLeft="16dp"
    android:layout_marginRight="16dp"
    android:baselineAligned="false"
    android:divider="?android:attr/dividerHorizontal"
    android:orientation="horizontal"
    android:showDividers="middle"
    tools:context=".WorkStationListActivity" >

    <fragment
        android:id="@+id/workstation_list"
        android:name="de.tuhh.ipmt.ialp.history.WorkStationListFragment"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1"
        tools:layout="@android:layout/list_content" />

    <FrameLayout
        android:id="@+id/workstation_detail_container"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="3" />

</LinearLayout>

My question now is: why is a FrameLayout used instead of the Fragment itself for the detail view? What is the reason or the advantage? Should I use it too?

Answer 1

The detail container is a FrameLayout because the Fragment that is displayed will be replaced using FragmentTransaction's replace() method.

The first argument to replace() is the ID of container whose Fragments will be replaced. If the FrameLayout in this example were replaced with a Fragment, then both the WorkStationListFragment and whatever detail Fragment is currently shown would be replaced by the new Fragment. By encapsulating the Fragment within a FrameLayout, you can replace just the details.