How does the getView method work when creating your own custom adapter

Question

My questions are    What is exactly the function of the LayoutInflater  Why do all the articles that I ve read check if convertview is null or not first  What does it mean when it is null and what does it mean when it isn t  What is the parent parameter that this method accepts

User · Answer

What is exactly the function of the LayoutInflater?

When you design using XML, all your UI elements are just tags and parameters. Before you can use these UI elements, (eg a TextView or LinearLayout), you need to create the actual objects corresponding to these xml elements. That is what the inflater is for. The inflater, uses these tags and their corresponding parameters to create the actual objects and set all the parameters. After this you can get a reference to the UI element using findViewById().

Why do all the articles that I've read check if convertview is null or not first? What does it mean when it is null and what does it mean when it isn't?

This is an interesting one. You see, getView() is called everytime an item in the list is drawn. Now, before the item can be drawn, it has to be created. Now convertView basically is the last used view to draw an item. In getView() you inflate the xml first and then use findByViewID() to get the various UI elements of the listitem. When we check for (convertView == null) what we do is check that if a view is null(for the first item) then create it, else, if it already exists, reuse it, no need to go through the inflate process again. Makes it a lot more efficient.

You must also have come across a concept of ViewHolder in getView(). This makes the list more efficient. What we do is create a viewholder and store the reference to all the UI elements that we got after inflating. This way, we can avoid calling the numerous findByViewId() and save on a lot of time. This ViewHolder is created in the (convertView == null) condition and is stored in the convertView using setTag(). In the else loop we just obtain it back using getView() and reuse it.

What is the parent parameter that this method accepts?

The parent is a ViewGroup to which your view created by getView() is finally attached. Now in your case this would be the ListView.

Hope this helps :)

User · Answer

getView   method in Adapter is for generating item s view of a ListView  Gallery       LayoutInflater is used to get the View object which you define in a layout xml  the root object  normally a LinearLayout  FrameLayout  or RelativeLayout  convertView is for recycling  Let s say you have a listview which can only display 10 items at a time  and currently it is displaying item 1 -  item 10  When you scroll down one item  the item 1 will be out of screen  and item 11 will be displayed  To generate View for item 11  the getView   method will be called  and convertView here is the view of item 1  which is not neccessary anymore   So instead create a new View object for item 11  which is costly   why not re-use convertView     we just check convertView is null or not  if null create new view  else re-use convertView  parentView is the ListView or Gallery    which contains the item s view which getView   generates     Note  you don t call this method directly  just need to implement it to tell the parent view how to generate the item s view

User · Answer

LayoutInflater is used to generate dynamic views of the XML for the ListView item or in onCreateView of the fragment    ConvertView is basically used to recycle the views which are not in the view currently  Say you have a scrollable ListView  On scrolling down or up  the convertView gives the view which was scrolled  This reusage saves memory   The parent parameter of the getView   method gives a reference to the parent layout which has the listView  Say you want to get the Id of any item in the parent XML you can use   ViewParent nv   parent getParent     while  nv    null         if  View class isInstance nv             final View button     View  nv  findViewById R id remove           if  button    null                   FOUND IT                 do something  then break              button setOnClickListener new OnClickListener                       Override                 public void onClick View v                           TODO Auto-generated method stub                     Log d  Remove    Remove clicked                           Button  button  setText  Hi                                                        break

User · Answer

You could have a look at this video about the list view  Its from last years Google IO and still the best walk-through on list views in my mind   http   www youtube com watch v wDBM6wVEO70   It inflates layouts  the xml files on your res layout  folder  into java Objects such as LinearLayout and other views  Look at the video  will get you up to date with whats the use of the convert view  basically its a recycled view waiting to be reused by you  to avoid creating a new object and slowing down the scrolling of your list  Allows you to reference you list-view from the adapter

User · Answer

Layout inflator inflates adds external XML to your current view  getView   is called numerous times including when scrolled  So if it already has view inflated we don t wanna do it again since inflating is a costly process   thats why we check if its null and then inflate it  The parent view is single cell of your List

User · Answer

getView   method create new View or ViewGroup for each row of Listview or Spinner   You can define this View or ViewGroup in a Layout XML file in res layout folder and can give the reference it to Adapter class Object   if you have 4 item in a Array passed to Adapter  getView   method will create 4 View for 4 rows of Adaper    LayoutInflater class has a Method inflate   whic create View Object from XML resource layout

User · Answer

1  The LayoutInflater takes your layout XML-files and creates different View-objects from its contents     2  The adapters are built to reuse Views  when a View is scrolled so that is no longer visible  it can be used for one of the new Views appearing  This reused View is the convertView  If this is null it means that there is no recycled View and we have to create a new one  otherwise we should use it to avoid creating a new   3  The parent is provided so you can inflate your view into that for proper layout parameters   All these together can be used to effectively create the view that will appear in your list  or other view that takes an adapter    public View getView int position   Nullable View convertView  ViewGroup parent       if  convertView    null              We must create a View          convertView   inflater inflate R layout my list item  parent  false               Here we can do changes to the convertView  such as set a text on a TextView        or an image on an ImageView      return convertView      Notice the use of the LayoutInflater  that parent can be used as an argument for it  and how convertView is reused

User · Answer

You can also find useful information about getView at the Adapter interface in Adapter java file  It says          Get a View that displays the data at the specified position in the data set  You can either    create a View manually or inflate it from an XML layout file  When the View is inflated  the    parent View  GridView  ListView     will apply default layout parameters unless you use      link android view LayoutInflater inflate int  android view ViewGroup  boolean      to specify a root view and to prevent attachment to the root          param position The position of the item within the adapter s data set of the item whose view           we want      param convertView The old view to reuse  if possible  Note  You should check that this view           is non-null and of an appropriate type before using  If it is not possible to convert           this view to display the correct data  this method can create a new view            Heterogeneous lists can specify their number of view types  so that this View is           always of the right type  see   link  getViewTypeCount    and             link  getItemViewType int         param parent The parent that this view will eventually be attached to     return A View corresponding to the data at the specified position      View getView int position  View convertView  ViewGroup parent

[android] How does the getView() method work when creating your own custom adapter?

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