What is the purpose of Android s merge tag in XML layouts

Question

I ve read Romain Guy s post on the  lt merge   gt  tag  but I still don t understand how it s useful  Is it a sort-of replacement of the  lt Frame   gt  tag  or is it used like so    lt merge xmlns android        gt   lt LinearLayout     gt                     lt  LinearLayout gt   lt  merge gt    then  lt include   gt  the code in another file

User · Answer

blazeroni already made it pretty clear  I just want to add few points     lt merge gt  is used for optimizing layouts It is used for reducing unnecessary nesting   when a layout containing  lt merge gt  tag is added into another layout the  lt merge gt  node is removed and its child view is added directly to the new parent

User · Answer

lt merge  gt  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  lt include  gt  a layout from another file without using merge  the two files might look something like this   layout1 xml    lt FrameLayout gt      lt include layout   layout layout2   gt   lt  FrameLayout gt    layout2 xml    lt FrameLayout gt      lt TextView   gt      lt TextView   gt   lt  FrameLayout gt    which is functionally equivalent to this single layout    lt FrameLayout gt      lt FrameLayout gt         lt TextView   gt         lt TextView   gt      lt  FrameLayout gt   lt  FrameLayout gt    That FrameLayout in layout2 xml may not be useful    lt merge  gt  helps get rid of it   Here s what it looks like using merge  layout1 xml doesn t change    layout2 xml    lt merge gt      lt TextView   gt      lt TextView   gt   lt  merge gt    This is functionally equivalent to this layout    lt FrameLayout gt      lt TextView   gt      lt TextView   gt   lt  FrameLayout gt    but since you are using  lt include  gt  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

User · Answer

To have a more in-depth knowledge of what s happening  I created the following example  Have a look at the activity main xml and content profile xml files   activity main xml   lt LinearLayout xmlns android  http   schemas android com apk res android      android layout width  match parent      android layout height  match parent      android orientation  vertical  gt        lt include layout   layout content profile    gt    lt  LinearLayout gt    content profile xml   lt LinearLayout xmlns android  http   schemas android com apk res android      android layout width  match parent      android layout height  match parent      android orientation  vertical  gt        lt TextView         android layout width  match parent          android layout height  wrap content          android text  Howdy    gt        lt TextView         android layout width  match parent          android layout height  wrap content          android text  Hi there    gt    lt  LinearLayout gt    In here  the entire layout file when inflated looks like this    lt LinearLayout gt       lt LinearLayout gt           lt TextView   gt           lt TextView   gt       lt  LinearLayout gt   lt  LinearLayout gt    See that there is a LinearLayout inside the parent LinearLayout which doesn t serve any purpose and is redundant  A look at the layout through Layout Inspector tool clearly explains this     content profile xml after updating the code to use merge instead of a ViewGroup like LinearLayout    lt merge xmlns android  http   schemas android com apk res android  gt        lt TextView         android layout width  match parent          android layout height  wrap content          android text  Howdy    gt        lt TextView         android layout width  match parent          android layout height  wrap content          android text  Hi there    gt    lt  merge gt    Now our layout looks like this   lt LinearLayout gt       lt TextView   gt       lt TextView   gt   lt  LinearLayout gt    Here we see that the redundant LinearLayout ViewGroup is removed  Now Layout Inspector tool gives the following layout hierarchy     So always try to use merge when your parent layout can position your child layouts  or more precisely use merge when you understand that there is going to be a redundant view group in the hierarchy

User · Answer

Another reason to use merge is when using custom viewgroups in ListViews or GridViews  Instead of using the viewHolder pattern in a list adapter  you can use a custom view  The custom view would inflate an xml whose root is a merge tag   Code for adapter    public class GridViewAdapter extends BaseAdapter               typical Adapter class methods       Override      public View getView int position  View convertView  ViewGroup parent            WallpaperView wallpaperView          if  convertView    null             wallpaperView   new WallpaperView activity           else             wallpaperView    WallpaperView  convertView           wallpaperView loadWallpaper wallpapers get position   imageWidth           return wallpaperView            here is the custom viewgroup   public class WallpaperView extends RelativeLayout        public WallpaperView Context context            super context           init context                    typical constructors      private void init Context context            View inflate context  R layout wallpaper item  this           imageLoader   AppController getInstance   getImageLoader            imagePlaceHolder    ImageView  findViewById R id imgLoader2           thumbnail    NetworkImageView  findViewById R id thumbnail2           thumbnail setScaleType ImageView ScaleType CENTER CROP              public void loadWallpaper Wallpaper wallpaper  int imageWidth                  some logic that sets the views           and here is the XML    lt merge xmlns android  http   schemas android com apk res android  gt        lt ImageView         android id    id imgLoader          android layout width  30dp          android layout height  30dp          android layout centerInParent  true          android src   drawable ico loader    gt        lt com android volley toolbox NetworkImageView         android id    id thumbnail          android layout width  wrap content          android layout height  wrap content    gt    lt  merge gt

User · Answer

The include tag  The  lt include gt  tag lets you to divide your layout into multiple files  it helps dealing with complex or overlong user interface   Let s suppose you split your complex layout using two include files as follows   top level activity xml     lt LinearLayout xmlns android  http   schemas android com apk res android      android id    id layout1       android layout width  match parent      android layout height  match parent      android orientation  vertical   gt        lt  -- First include file -- gt       lt include layout   layout include1 xml    gt        lt  -- Second include file -- gt       lt include layout   layout include2 xml    gt    lt  LinearLayout gt    Then you need to write include1 xml and include2 xml   Keep in mind that the xml from the include files is simply dumped in your top level activity layout at rendering time  pretty much like the  INCLUDE macro for C    The include files are plain jane layout xml   include1 xml    lt  xml version  1 0  encoding  utf-8   gt   lt TextView xmlns android  http   schemas android com apk res android      android id    id textView1      android text  First include      android textAppearance   android attr textAppearanceMedium   gt        and include2 xml    lt  xml version  1 0  encoding  utf-8   gt   lt Button xmlns android  http   schemas android com apk res android      android id    id button1      android text  Button    gt    See  Nothing fancy   Note that you still have to declare the android namespace with xmlns android  http   schemas android com apk res android   So the rendered version of  top level activity xml is     lt LinearLayout xmlns android  http   schemas android com apk res android      android id    id layout1       android layout width  match parent      android layout height  match parent      android orientation  vertical   gt        lt  -- First include file -- gt       lt TextView         android id    id textView1          android text  First include          android textAppearance   android attr textAppearanceMedium   gt        lt  -- Second include file -- gt       lt Button         android id    id button1          android text  Button    gt     lt  LinearLayout gt    In your java code  all this is transparent  findViewById R id textView1  in your activity class returns the correct widget   even if that widget was declared in a xml file different from the activity layout    And the cherry on top  the visual editor handles the thing swimmingly  The top level layout is rendered with the xml included   The plot thickens  As an include file is a classic layout xml file  it means that it must have one top element  So in case your file needs to include more than one widget  you would have to use a layout   Let s say that include1 xml has now two TextView  a layout has to be declared  Let s choose  a LinearLayout   include1 xml     lt LinearLayout xmlns android  http   schemas android com apk res android      android id    id layout2       android layout width  match parent      android layout height  match parent      android orientation  vertical   gt        lt TextView         android id    id textView1          android text  Second include          android textAppearance   android attr textAppearanceMedium   gt        lt TextView         android id    id textView2          android text  More text          android textAppearance   android attr textAppearanceMedium   gt    lt  LinearLayout gt    The top level activity xml will be rendered as      lt LinearLayout xmlns android  http   schemas android com apk res android      android id    id layout1       android layout width  match parent      android layout height  match parent      android orientation  vertical   gt        lt  -- First include file -- gt       lt LinearLayout          android id    id layout2           android layout width  match parent          android layout height  match parent          android orientation  vertical   gt           lt TextView             android id    id textView1              android text  Second include              android textAppearance   android attr textAppearanceMedium   gt           lt TextView             android id    id textView2              android text  More text              android textAppearance   android attr textAppearanceMedium   gt       lt  LinearLayout gt         lt  -- Second include file -- gt      lt Button         android id    id button1          android text  Button    gt    lt  LinearLayout gt    But wait the two levels of LinearLayout are redundant   Indeed  the two nested LinearLayout serve no purpose as the two TextView could be included under layout1for exactly the same rendering   So what can we do    Enter the merge tag  The  lt merge gt  tag is just a dummy tag that provides a top level element to deal with this kind of redundancy issues   Now include1 xml becomes      lt merge xmlns android  http   schemas android com apk res android  gt        lt TextView         android id    id textView1          android text  Second include          android textAppearance   android attr textAppearanceMedium   gt        lt TextView         android id    id textView2          android text  More text          android textAppearance   android attr textAppearanceMedium   gt    lt  merge gt    and now top level activity xml is rendered as      lt LinearLayout xmlns android  http   schemas android com apk res android      android id    id layout1       android layout width  match parent      android layout height  match parent      android orientation  vertical   gt        lt  -- First include file -- gt        lt TextView         android id    id textView1          android text  Second include          android textAppearance   android attr textAppearanceMedium   gt        lt TextView         android id    id textView2          android text  More text          android textAppearance   android attr textAppearanceMedium   gt        lt  -- Second include file -- gt       lt Button         android id    id button1          android text  Button    gt    lt  LinearLayout gt    You saved one hierarchy level  avoid one useless view  Romain Guy sleeps better already   Aren t you happier now

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

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