Margin of a fragment's root view not registered (when used in ViewPager)

Question

So I have this simple main layout:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical">

    <include layout="@layout/top_bar" />

    <android.support.v4.view.ViewPager
        android:id="@+id/VpPager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

Then I use a FragmentPagerAdapter to fill the ViewPager with different fragments, which works as expected, except for one thing.

Let's say one of the fragments has this layout, where window is an XML-file containing a simple shape with rounded corners:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_margin="16dp"
    android:orientation="vertical"
    android:background="@drawable/window">

    ...

</LinearLayout>

Then, for some reason, the margin of 16dp is completely ignored. But if I set the layout to:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_margin="16dp"
        android:orientation="vertical"
        android:background="@drawable/window">

        ...

    </LinearLayout>

</FrameLayout>

Then it works. I really don't want to nest the whole layout in another just to make the margin work. Plus, because of the background, I can't use padding instead. Any idea what the problem is with the first version? Any alternatives where I don't have to put the layout in another?

Answer 1

The reason for this is that margins are not supported by the ViewPager.LayoutParams, as you can see from the documentation.

However, FrameLayout.LayoutParams does support ViewGroup.MarginLayoutParams, which is why wrapping your LinearLayout inside a FrameLayout actually does provide the results you're seeking.