What is the purpose of Android's <merge> tag in XML layouts?

Question

I've read Romain Guy's post on the <merge /> tag, but I still don't understand how it's useful. Is it a sort-of replacement of the <Frame /> tag, or is it used like so:

<merge xmlns:android="....">
<LinearLayout ...>
    .
    .
    .
</LinearLayout>
</merge>

then <include /> the code in another file?

Answer 1

<merge/> is useful because it can get rid of unneeded ViewGroups, i.e. layouts that are simply used to wrap other views and serve no purpose themselves.

For example, if you were to <include/> a layout from another file without using merge, the two files might look something like this:

layout1.xml:

<FrameLayout>
   <include layout="@layout/layout2"/>
</FrameLayout>

layout2.xml:

<FrameLayout>
   <TextView />
   <TextView />
</FrameLayout>

which is functionally equivalent to this single layout:

<FrameLayout>
   <FrameLayout>
      <TextView />
      <TextView />
   </FrameLayout>
</FrameLayout>

That FrameLayout in layout2.xml may not be useful. <merge/> helps get rid of it. Here's what it looks like using merge (layout1.xml doesn't change):

layout2.xml:

<merge>
   <TextView />
   <TextView />
</merge>

This is functionally equivalent to this layout:

<FrameLayout>
   <TextView />
   <TextView />
</FrameLayout>

but since you are using <include/> you can reuse the layout elsewhere. It doesn't have to be used to replace only FrameLayouts - you can use it to replace any layout that isn't adding something useful to the way your view looks/behaves.