The merge tag

The merge tag allows you to avoid an unnecessary use of nested layouts when you use the include tag.
Consider the following example that consists of 3 files .xml as layout:

• activity_main.xml

  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"
      android:orientation="vertical" >
  
      <TextView
          android:id="@+id/textView1"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="row 1" />
  
      <include layout="@layout/merge_1_level"/>
      
  </LinearLayout>
  

  this file .xml creates a vertical LinearLayout with a TextView inside and the contents of the file merge_1_level.xml using the include tag

• merge_1_level.xml

  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"
      android:orientation="vertical" >
  
      <TextView
          android:id="@+id/textView2"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="row 2" />
  
      <include layout="@layout/merge_2_level" />
  
  </LinearLayout>
  

  This file adds another TextView and the contents of the file merge_2_level.xm using the include tag

• merge_2_level.xml

  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"
      android:orientation="vertical" >
  
      <TextView
          android:id="@+id/textView3"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="row 3" />
  
  </LinearLayout>
  

  this file adds the last TextView

The final result in the emulator is as follows:

The resulting layout is equivalent to:

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

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="row 1" />

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

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="row 2" />

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

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="row 3" />

</LinearLayout>

</LinearLayout>
    
</LinearLayout>

As you can see the nested LinearLayouts are useless and redundant, to optimize the code you can use the merge tag in the included files: merge_1_level.xml and merge_2_level.xml

• merge_1_level.xml

  <merge xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="match_parent"
      android:layout_height="match_parent" >
  
      <TextView
          android:id="@+id/textView2"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="row 2" />
  
      <include layout="@layout/merge_2_level" />
  
  </merge>
  

• merge_2_level.xml

  <merge xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="match_parent"
      android:layout_height="match_parent" >
  
      <TextView
          android:id="@+id/textView3"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="row 3" />
  
  </merge>
  

In this way the resulting layout is equivalent to:

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

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="row 1" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="row 2" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="row 3" />

</LinearLayout>