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.

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;
}

ListView adapter gets confused when convertView is recycled

I have a straight forward BaseAdapter for my ListView. It downloads a JSON feed and displays the data in the rows. There is a ViewHolder which contains the views and a data object called "Story". Everything works just fine.
However, after some scrolling of longer lists, I notice two things.
1) My log shows that the adapter is reloading the feed when scrolling further down. This is strange, as I put the whole JSON array into a variable, so why does it have to reload?
2) More importantly, after some scrolling back and forth, the rows contain the wrong "Story" objects. Here are the relevant parts of the getView routine:
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder;
Story story = stories.get(position);
if (convertView == null) {
//create holder
holder = new ViewHolder();
convertView = inflator.inflate(R.layout.story_list_item, parent, false);
holder.titleView = (TextView) convertView.findViewById(R.id.story_list_title);
holder.dateView = (TextView) convertView.findViewById(R.id.story_list_date);
holder.story = story;
holder.imageView = (ImageView) convertView.findViewById(R.id.story_list_image);
convertView.setTag(holder);
} else {
holder = (ViewHolder) convertView.getTag();
}
// configure the view
holder.titleView.setText(story.title);
return convertView;
}
Simple enough. Now the strange thing is that I can fix the problem by eliminating the if statement if (convertView == null) (and, I presume, eliminating the row recycling as well).
But will I not run into memory problems this way? Why does the plain vanilla version not work?
Thanks for your help.
Regards,
S

You are aware that you're only assigning
holder.story = story
when convertView == null ? Consider moving holder.story = story to just after your convertView if-case and it should work a lot better. Btw, do you even need to store the "story" inside your view holder? Typically that pattern should only be used to store Views and view state information, not the data of the actual position.

android gallery custom adapter

I have a question, maybe a silly one, but I think it is important.
Why the parameter: convertView (View) on the
public View getView(int position, View convertView, ViewGroup parent)
is always null? android is supposed to recycle the views once they're created the first time, isn't it? or how can I do to recycle those views?
I feel like the method receives those 3 parameters, but in none of the google examples they use either of them.

Unfortunately, convertView will always be null, due to Android bug 3376. Gallery does not implement View recycling (at least as of Gingerbread/2.3.4).
A commenter in the bug suggests forking Gallery.java (from AOSP) and implementing it yourself, which may be the best option.

The convertView parameter indeed will be null a few first times, when this function will be called. Then if you scroll the list / galery Android will give you the same view, which was constructed earlier using this function, and you should use it to optimally construct the new view, based on the old one.
Also, you should store the references to child view somewhere.
To better undestand that, look at this code example (taken from Android Developers):
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.list_item_icon_text, null);
// Creates a ViewHolder and store references to the two children views
// we want to bind data to.
holder = new ViewHolder();
holder.text = (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.text.setText(DATA[position]);
holder.icon.setImageBitmap((position & 1) == 1 ? mIcon1 : mIcon2);
return convertView;
}

On getView() method normally you check if convertView is null, and if it isn't you just rewrite the fields in the View adapting it to the data you get based on the position instead of creating a new View (from inflation or whatever method you want).
Hope it helped,
JQCorreia

getView() has a second parameter as view(convertView).This convertView is the view which is returned from previous iteration.For the first iteration it will be null and adapter will create (instance) view.When it is done with creating required layout,the view will be returned to its caller.This returned value will be available as 2nd parameter from the next iteration onwards.So one can decide to reuse previously returned view instead of recreating by looking at this parameter.Thus Android achives re-usability feature while creating multiple list items.

Since convertView will always be null you should implement your own algorithm of caching and reusing items.
This is my implementation of Gallery adapter
public View getView(int position, View convertView, ViewGroup parent) {
int arrPosition = position % VIEW_CHACHE_SIZE;
ImageView imageView;
mCursor.moveToPosition(position);
if (parent.getHeight() > 0 && layoutParams.height == 0) {
layoutParams = new Gallery.LayoutParams(parent.getWidth() / VISIBLE_IMAGES_COUNT, (int) (parent.getHeight() * IMAGE_HEIGHT_COEFICIENT));
viewsList[0].setLayoutParams(layoutParams);
}
if (convertView != null) {
Log.i("GALLERY", "convert view not null");
}
// check views cache
if (viewsList[arrPosition] == null) {
imageView = new ImageView(mContext);
imageView.setPadding(3, 3, 3, 3);
viewsList[arrPosition] = imageView;
} else {
imageView = viewsList[arrPosition];
if (position == arrPosition) {
if (imageView.getDrawable().equals(imagesList.get(position))) {
return imageView;
}
}
}
// check images cache
if (imagesList.get(position) != null) {
imageView.setImageDrawable(imagesList.get(position));
} else {
byte[] photo = mCursor.getBlob(mCursor.getColumnIndex(DataProxy.PHOTO_COLUMN));
imagesList.put(position, new BitmapDrawable(BitmapFactory.decodeByteArray(photo, 0, photo.length)));
imageView.setImageDrawable(imagesList.get(position));
}
imageView.setScaleType(ImageView.ScaleType.FIT_CENTER);
imageView.setLayoutParams(layoutParams);
return imageView;
}
.........................................................
private SparseArray<Drawable> imagesList = new SparseArray<Drawable>();
private ImageView[] viewsList = new ImageView[VIEW_CHACHE_SIZE];
private Gallery.LayoutParams layoutParams = new LayoutParams(0, 0);
private static final int VIEW_CHACHE_SIZE = 4;

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).