Develop Reference

Does ListView automatically recycle views?

I'm new to android programming, and I've been reading a lot about it lately. One of the features of ListView, if I understood it right, is that it recycle views and just replaces it with new data when an item is off the screen.
And just a few minutes ago, I was reading up about endless scrolling, and RecyclerView has been one of the popular choices to implement such a feature. So I looked up RecyclerView, and in this video, it is mentioned that RecyclerView recycles a view automatically to reuse it for new data (as a way to contrast its difference with ListView).
Did I misunderstand ListView about its recycling mechanism? Or if it does recycle, how do you actually implement (or how do you know you are implementing) it?

RecyclerView does recycling automatically. In order to make ListView recycle items you will need to do this modification inside of adapter class.
#Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder;
if (convertView == null) {
//brand new
convertView = LayoutInflater.from(mContext).inflate(R.layout.days_list_item, null);
holder = new ViewHolder();
// below is variables that will be different in your case
holder.numberOfDays = (TextView) convertView.findViewById(R.id.eventDays);
holder.sinceOrUntil = (TextView) convertView.findViewById(R.id.eventType);
holder.eventTitle = (TextView) convertView.findViewById(R.id.eventTitle);
holder.daysText = (TextView) convertView.findViewById(R.id.DaysText);
convertView.setTag(holder);
}
else {
//reusing item
holder = (ViewHolder) convertView.getTag();
}
// rest of the code
}
For more details refer to this link.

Large listview issues

I have a huge listview , with more then 100,000 items. What I did now..
PostPaid accountItem = (PostPaid)arr.get(position);
ViewHolder holder;
if (convertView == null) {
LayoutInflater mInflater = (LayoutInflater)
context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
convertView = mInflater.inflate(R.layout.list_support_item20, null);
holder = new ViewHolder();
holder.text = (TextView) convertView.findViewById(R.id.accountType);
convertView.setTag(holder);
}
else {
holder = (ViewHolder) convertView.getTag();
}
holder.text.setText(accountItem.getTitle());
What I did originally
LayoutInflater mInflater = (LayoutInflater) context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
convertView = mInflater.inflate(R.layout.list_support_item, null);
TextView accountType = (TextView) convertView.findViewById(R.id.accountType);
accountType.setText(accountItem.getTitle());
I want to ask about the differences between two, and which on preferable? And how can I improve performance even more. I am testing on kitat nexus and works perfectly in both cases, but am trying to figure out gingerbread issues

I want to ask about the differences between two, and which on preferable?
with a large of data to represent is always preferable to use the ViewHolder pattern. As you have surely realised, if you do not use this pattern, you have to call findViewById for every view that belongs to the layout you inflated. With this pattern you look for those only once.

Performance Tips for Android’s ListView
use the ViewHolder pattern.
Understand how listview recycling works
How ListView's recycling mechanism works
Quoting docs
Your code might call findViewById() frequently during the scrolling of ListView, which can slow down performance. Even when the Adapter returns an inflated view for recycling, you still need to look up the elements and update them. A way around repeated use of findViewById() is to use the "view holder" design pattern.
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder;
if (convertView == null) { // if convertView is null
convertView = mInflater.inflate(R.layout.mylayout,
parent, false);
holder = new ViewHolder();
// initialize views
convertView.setTag(holder); // set tag on view
} else {
holder = (ViewHolder) convertView.getTag();
// if not null get tag
// no need to initialize
}
//update views here
return convertView;
}
So using ViewHolder improves performance as you avoid initializing views everytime. You initialize only when view is null (if(convertView==null) {. Helps in smooth scrolling.
You missed the important part convertView.setTag(holder) and holder = (ViewHolder) ConvertView.getTag()
Android Doc of Smooth scrolling in lisview

ListView ImageView load same pictures

i have a big problem with my ListViewAdapter.
My listview shows 2 entrys at the same time. Each entry should get a different picture from my server.
The first and the second entrys working fine, but if i'm scrolling down, the next entrys will have the same pictures.
My Code looks like this:
if (viewHolder.imgPic != null) {
String strUrl = mainUrl+list.get(position).getUrl();
new ImageDownload(viewHolder.imgPic).execute(strUrl);
}
I'm checking the view and just doing it, if it's null.
Can someone help me?
Thanks

from your question I can assume that you don't know about the ListView recycling mechanisem
basically, view that that not visible anymore (after user scrolled it away from sight) it been recycled to displayed new item that need to shown. that's the convertView parameter at the getView() method...
probably you are see the same image because the recycled view stays with the same image..
also there is the issue of the asynchronous task (your ImageDownload class) that can finish it execute when the original item that started the request already been recycled.
I recommend you to "dig" as dipper as you can to understand all about ListView - this is one of the most complex and important UI component. reading the post I linked you is a good start.
also this video is very important:
http://www.youtube.com/watch?v=wDBM6wVEO70

Here is my GetView:
#Override
public View getView(final int position, View convertView, ViewGroup parent) {
View view = null;
if(rowResourceId!=R.layout.search_empty_list) {
if (convertView == null) {
LayoutInflater inflator = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
view = inflator.inflate(rowResourceId, null);
final ViewHolder viewHolder = new ViewHolder();
viewHolder.textName = (TextView) view.findViewById(R.id.textView1);
viewHolder.imgPic = (ImageView) view.findViewById(R.id.imgPic);
if (viewHolder.imgPic != null) {
String strUrl = mainUrl+list.get(position).getUrl();
new ImageDownload(viewHolder.imgPic).execute(strUrl);
}
view.setTag(viewHolder);
} else {
view = convertView;
}
ViewHolder holder = (ViewHolder) view.getTag();
holder.textName.setText(list.get(position).getName());
} else {
LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
view = inflater.inflate(rowResourceId, parent, false);
TextView textView1 = (TextView) view.findViewById(R.id.textView1);
textView1.setText(list.get(0).getName());
}
return view;
}

Recycling views in custom array adapter: how exactly is it handled?

I am having an unclear issue concerning the recycling of views in a getView method of a custom array adapter.
I understand that elements are reused, but how do I know exact what to implement in the first part of the if statement, and what in the second?
Right now I am having following code. I came to this question due to dropping the code in the second part of the statement which results in a list of the first 9 elements, which are repeated numberous times instead of all elements. I didn't really know what is causing this exactly...
#Override
public View getView(int position, View convertView, ViewGroup parent) {
View row = convertView;
if (row == null) {
LayoutInflater inflater = ((Activity) context).getLayoutInflater();
row = inflater.inflate(layoutResourceId, parent, false);
title = getItem(position).getTitle();
size = calculateFileSize(position);
txtTitle = (TextView) row.findViewById(R.id.txtTitle);
tvFileSize = (TextView) row.findViewById(R.id.tvFileSize);
txtTitle.setText(title);
tvFileSize.setText(size);
} else {
title = getItem(position).getTitle();
size = calculateFileSize(position);
txtTitle = (TextView) row.findViewById(R.id.txtTitle);
tvFileSize = (TextView) row.findViewById(R.id.tvFileSize);
txtTitle.setText(title);
tvFileSize.setText(size);
}
return row;
}

It's easy. The first time no row is created, so you have to inflate them. Afterwards, the Android os may decide to recycle the views that you already inflated and that are not visible anymore. Those are already inflated and passed into the convertView parameter, so all you have to do is to arrange it to show the new current item, for example placing the right values into the various text fields.
In short, in the first part you should perform the inflation AND fill the values, in the second if (if convertView != null) you should only overwrite the field because, given the view has been recycled, the textviews contain the values of the old item.
This post and this are good starting points

I understand that elements are reused, but how do I know exact what to implement in the first part of the if statement, and what in the second?
The organization is quite simple once you get the hang of it:
public View getView(int position, View convertView, ViewGroup parent) {
if (convertView == null) {
/* This is where you initialize new rows, by:
* - Inflating the layout,
* - Instantiating the ViewHolder,
* - And defining any characteristics that are consistent for every row */
} else {
/* Fetch data already in the row layout,
* primarily you only use this to get a copy of the ViewHolder */
}
/* Set the data that changes in each row, like `title` and `size`
* This is where you give rows there unique values. */
return convertView;
}
For detailed explanations of how ListView's RecycleBin works and why ViewHolders are important watch Turbo Charge your UI, a Google I/O presentation by Android's lead ListView programmers.

You want to create a ViewHolder class in your MainActivity. Something like
static class ViewHolder
{
TextView tv1;
TextView tv2;
}
then in your getView, the first time you get your Views from your xml in the if and reuse them after that in the else
View rowView = convertView;
if (rowView == null)
{
LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
rowView = inflater.inflate(R.layout.layout_name_to_inflate, parent, false);
holder = new ViewHolder();
holder.tv1= (TextView) rowView.findViewById(R.id.textView1);
holder.tv2 = (RadioGroup) rowView.findViewById(R.id.textView2);
rowView.setTag(holder);
}
else
{
holder = (ViewHolder) rowView.getTag();
}

I would recommend that you use the View holder and convertview pattern to create your listView as it will be more efficient.Here is a good explanation of how it works with a re-use strategy. This will answer your question on how re-cycling works. If you want to refer to a code sample, I have it on GitHub.
Hope this helps.

The last part of the question I really couldn't grasp without a picture of the effect but for the first part "what to implement in the first part of the if statement, and what in the second" I think I've found the this implementation very common.
You would find the view references first and store them to a static class ViewHolder which then you attach to the tag of the new inflated view. As the listview recycles the views and a convertView is passed getView you get the ViewHolder from the convertView's tag so you don't have to find the references again (which greatly improves performance) and update the view data with that of your object at the position given.
Technically you don't care what position the view was since all you care for is the references to the views you need to update which are held within it's ViewHolder.
#Override
public View getView(int position, View convertView, ViewGroup container) {
ViewHolder holder;
Store store = getItem(position);
if (convertView == null) {
convertView = mLayoutInflater.inflate(R.layout.item_store, null);
// create a holder to store references
holder = new ViewHolder();
// find references and store in holder
ViewGroup logoPhoneLayout = (ViewGroup) convertView
.findViewById(R.id.logophonelayout);
ViewGroup addressLayout = (ViewGroup) convertView
.findViewById(R.id.addresslayout);
holder.image = (ImageView) logoPhoneLayout
.findViewById(R.id.image1);
holder.phone = (TextView) logoPhoneLayout
.findViewById(R.id.textview1);
holder.address = (TextView) addressLayout
.findViewById(R.id.textview1);
// store holder in views tag
convertView.setTag(holder);
} else {
// Retrieve holder from view
holder = (ViewHolder) convertView.getTag();
}
// fill in view with our store (at this position)
holder.phone.setText(store.phone);
holder.address.setText(store.getFullAddress());
UrlImageViewHelper.setUrlDrawable(holder.image, store.storeLogoURL,
R.drawable.no_image);
return convertView;
}
private static class ViewHolder {
ImageView image;
TextView phone;
TextView address;
}

How to implement a view holder?

i am using a viewholder to display from a dynamic arrayadapter.it works but the data displayed changes irregularly when i scroll the List.i want my List View to be populated only once ,Not all the time when i scroll my list. Any suggestion?
Here is my Code
public View getView(int position, View convertView, ViewGroup parent) {
// A ViewHolder keeps references to children views to avoid unneccessary calls
// to findViewById() on each row.
ViewHolder holder;
// When convertView is not null, we can reuse it directly, there is no need
// to reinflate it. We only inflate a new View when the convertView supplied
// by ListView is null.
if (convertView == null) {
convertView = mInflater.inflate(R.layout.sample, null);
// Creates a ViewHolder and store references to the two children views
// we want to bind data to.
holder = new ViewHolder();
holder.name = (TextView) convertView.findViewById(R.id.text);
holder.icon = (ImageView) convertView.findViewById(R.id.icon);
convertView.setTag(holder);
} else {
// Get the ViewHolder back to get fast access to the TextView
// and the ImageView.
holder = (ViewHolder) convertView.getTag();
}
// Bind the data efficiently with the holder.
if(_first==true)
{
if(id<myElements.size())
{
holder.name.setText(myElements.get(id));
holder.icon.setImageBitmap( mIcon1 );
id++;
}
else
{
_first=false;
}
}
//holder.icon.setImageBitmap(mIcon2);
/*try{
if(id<myElements.size())
id++;
else
{
id--;
}
}
catch(Exception e)
{
android.util.Log.i("callRestService",e.getMessage());
}*/
return convertView;
}
static class ViewHolder {
TextView name;
ImageView icon;
}
when the list is loaded it looks like this : http://i.stack.imgur.com/NrGhR.png after scrolling some data http://i.stack.imgur.com/sMbAD.png it looks like this, and again if i scroll to the beginning it looks http://i.stack.imgur.com/0KjMa.png
P.S : my list have to be in alphabetic order

Have you tried this?
public View getView(int position, View convertView, ViewGroup parent) {
// A ViewHolder keeps references to children views to avoid unneccessary calls
// to findViewById() on each row.
ViewHolder holder;
// When convertView is not null, we can reuse it directly, there is no need
// to reinflate it. We only inflate a new View when the convertView supplied
// by ListView is null.
if (convertView == null) {
convertView = mInflater.inflate(R.layout.sample, null);
// Creates a ViewHolder and store references to the two children views
// we want to bind data to.
holder = new ViewHolder();
holder.name = (TextView) convertView.findViewById(R.id.text);
holder.icon = (ImageView) convertView.findViewById(R.id.icon);
convertView.setTag(holder);
} else {
// Get the ViewHolder back to get fast access to the TextView
// and the ImageView.
holder = (ViewHolder) convertView.getTag();
}
// Bind the data efficiently with the holder.
holder.name.setText(myElements.get(id));
holder.icon.setImageBitmap( mIcon1 );
return convertView;
}
static class ViewHolder {
TextView name;
ImageView icon;
}
If yes, what's wrong with it?
I don't think loading all the rows at once is a good idea. You will end up having plenty of useless Views in memory that are going to slow the application down for nothing.
Views and operations on views (like inflate, findViewById, getChild..) are expensive, you should try to reuse them as much as possible. That's why we use ViewHolders.

You would need to write you own version of ListView to do that (which is bad). If the ListView doesn't work properly, it probably means that you are doing something wrong.
Where does the id element come from? You are getting the position in your getView() method, so you don't need to worry about exceeding list bounds. The position is linked to the element position in your list, so you can get the correct element like this:
myElements.get(position);
When the data in your list changes, you can call this:
yourAdapter.notifyDataSetChanged()
That will rebuild your list with new data (while keeping your scrolling and stuff).