Develop Reference

ListView recycles images

I have a custom ArrayAdapter that gets images from the web. I understand that the views get recycled. My code seems to work but there is a problem with the images that are loaded from the web. Occassionally, the wrong image might show for another row. For example, Mickey Mouse might be the image on Row 0 and when I scroll down Mickey Mouse might appear briefly for Row 9 (example) before changing to Donald Duck. And when I scroll back up to the top, Donald Duck might appear for Row 0 before changing back to Mickey Mouse.
Here is my code:
class OffersCustomAdapter extends ArrayAdapter<Merchant>{
Context context;
ArrayList<User> userName;
private LayoutInflater inflater;
private ImageLoadingListener animateFirstDisplayListener;
private ImageLoader imageLoader;
public OffersCustomAdapter(Context c, ArrayList<User> users) {
super(c, R.layout.single_row, users);
this.context=c;
this.userName=users;
imageLoader = ImageLoader.getInstance();
imageLoader.init(ImageLoaderConfiguration.createDefault(context));
}
static class ViewHolder{
TextView title;
TextView cat;
TextView type;
TextView desc;
ImageView pic;
}
#Override
public int getViewTypeCount() {
return 1;
}
#Override
public int getItemViewType(int position) {
return position;
}
#Override //parent is listview
public View getView(int position, View convertView, ViewGroup parent) {
View row=convertView;
ViewHolder viewHolder;
if(convertView == null){
inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
//row contains our relative layout
row =inflater.inflate(R.layout.single_row, parent, false);
viewHolder = new ViewHolder();
viewHolder.title =
(TextView) row.findViewById(R.id.textView1);
viewHolder.pic = (ImageView) row.findViewById(R.id.imageView1);
row.setTag(viewHolder);
}else{
viewHolder = (ViewHolder) convertView.getTag();
}
viewHolder = (ViewHolder) row.getTag();
User u = userName.get(position);
String titleSt = userName.get(position).getName();
viewHolder.title.setText(titleSt);
imageLoader.displayImage(imageUrl+userName.get(position).getImg(), viewHolder.pic, animateFirstDisplayListener);
return row;
}
I've looked at other examples of SO but no luck.

It's because The view is being recycled. When you scroll down, the next view that comes up will use the same view that just scrolled out of view (i.e. Mickey Mouse). You can fix this by displaying a loading image while your imageLoader fetches the new image.
If you don't have a loading image, you can do something like this at the beginning of your getView(...) method:
viewHolder.pic.setImageDrawable(null);
Edit: fixed based on comment.

It's likely because the previous image loading operation isn't canceled if still underway when the view is recycled, so this introduces a race condition on setting the view's bitmap. This issue is discussed briefly in this post, which might be worth a read through: Multithreading for Performance
The author explains it in a bit more detail:
However, a ListView-specific behavior reveals a problem with our
current implementation. Indeed, for memory efficiency reasons,
ListView recycles the views that are displayed when the user scrolls.
If one flings the list, a given ImageView object will be used many
times. Each time it is displayed the ImageView correctly triggers an
image download task, which will eventually change its image. So where
is the problem? As with most parallel applications, the key issue is
in the ordering. In our case, there's no guarantee that the download
tasks will finish in the order in which they were started. The result
is that the image finally displayed in the list may come from a
previous item, which simply happened to have taken longer to download.
This is not an issue if the images you download are bound once and for
all to given ImageViews, but let's fix it for the common case where
they are used in a list.
and provides a workaround example that may be of help.

Try picasso
Once you have the jar in your workspace, you just need one line of code.
Replace imageLoader.displayImage(imageUrl+userName.get(position).getImg(), viewHolder.pic, animateFirstDisplayListener);
with
Picasso.with(context).load(your_image_Url).into(viewholder.pic);
I believe your_image_url in your case is imageUrl+userName.get(position).getImg();

There is a 5 year old Android blog post that describes how to solve this by hand, Multithreading for Performance.
Nowadays, you should use Picasso or Volley for image loading. These network APIs will easily solve your problem and give you additional benefits, such as caching. Volley is the API that Google uses inside their own apps. I use it in my apps and am a fan.
See a thorough introduction to both frameworks.

Creating Grid of images or custom buttons that are clickable

So I have been trying to use a canvas with bitmaps to create a grid of clickable images for a game. It is made to be a 19 x 19 board and this would be made of clickable images so that when you click a image it changes to a new image. I have tried doing this and I get the grid of images but I can not figure out a way to make them clickable. I would show code but there is nothing to show really. its just a basic custom view. Maybe I am doing this all wrong but I have seen a way that is somewhat similar that works but it does not use custom images. I can add a onTouchListener and then I get a response back but it still does not fulfill what I am trying to accomplish. I guess I need to create buttons inside of my custom view but I need the buttons to be customized by images and I cant figure out how I would go about doing that. That is where the issue lies, If there is a way to create a custom View of a grid of customizable buttons. How would I go about doing that? Sorry if this question seems.... messy but I have been working at this for awhile now and am getting pretty confused and lost. Any help is appreciated at this point.

Yep, I think so. Not sure but if you use a grid view with image views on them you can do what you want to accomplish. For more in formation, check here:
http://developer.android.com/guide/topics/ui/layout/gridview.html
http://developer.android.com/reference/android/widget/ImageView.html

You could just use a GridFragment and just implement the onGridItemClick method to handle the clicks. It is pretty straight forward.

If you want to have a gridview with buttons with custom background images I would go about it something like this.
I would store all my drawables in an int array
int[] imageResource = new int[] {R.drawable.icon, R.drawable.icon2, R.drawable.icon3};
Set your gridView with the adapter shown below and pass your drawables stored in an array
yourGridView.setAdapter(new GridAdapter(imageResource));
Now for the actual inner class Adapter
//create your own custom adapter for your gridView
public class GridAdapter extends BaseAdapter {
int[] gridImages;
public GridAdapter(int images[]){
gridImages = images;
}
#Override
public View getView(final int position, View convertView,
ViewGroup parent) {
LayoutInflater li = context.getLayoutInflater();
if (convertView == null) {
//here you inflate your xml containing a button.
convertView = li.inflate(R.layout.grid_item, null);
}
//you reference the button contained in the inflated xml
Button imageButton = (Button) convertView.findViewById(R.id.grid_button);
//now you set the button with a drawable from your int array
Drawable image = getResources().getDrawable(gridImages[position);
// Now simply add your onClickListener and whatever else you need to achieve
//return your view
return convertView;
}
public int getCount() {
return gridImages.length;
}
public Object getItem(int position) {
return gridImages[position];
}
public long getItemId(int position) {
return position;
}
}
If done correctly, this should cycle through all your drawables referenced in the int array and place them on to buttons.

Forcing a BaseAdapter to update it's views when AsyncTask is done

Ok, so my approach is something like this. I am decoding locally stored encrypted and serialized Objects in an AsyncTask in an Activity. The Activity uses a BaseAdapter (HistoryAdapter) for the data to display. The AsyncTask shows a ProgressDialog until decoding is done. When onProgressUpdate() is first called, the ProgressDialog is cancelled. So far, so good. Next, in onProgressUpdate(), the HistoryAdapter is notified of the changes in the common way, triggering it's getView() method. In the HistoryAdapter's getView(), a second AsyncTask is run to modify the created convertView and set the data onto the View.
Here is where it all fails on me. I inflate the final layout in onProgressUpdate(), and set properties and data on convertView just fine here. The changes just don't show, even though all the data is set...
So, the AsyncTask in HistoryAdapter in itself in fact works perfectly, the changes are just not visible. I tried numurous suggestions mentioned on SO, like invalidating convertView, passing a reference to the ListView and using invalidateViews() (causes an eternal loop but no visible changes, which makes sense).
I really want this, because I really don't want to load the layout with image placeholders before data is available. That I got working, but looks nasty and like the easy way out. So I need the ListView to update (add the item) only when progress is done. Any ideas?
EDIT: to clarify: the data is set on the adapter in just the right time. The problem is, the adapter creates a blank View (placeholder) first (don't know any other way, otherwise you will get a NullPointerException in getView), then this View is inflated / replaced with another View in onProgressUpdate(). The second View is the one who should be visible. This works somewhat, because I can get and set properties on the newly inflated View. The changes are just not visible, and I am still seeing the blank, initially created View. I want to update the ListView on each added item, not when all items are done loading...
public View getView(int position, View convertView, ViewGroup parent) {
if (convertView == null) {
//convertView = mInflater.inflate(R.layout.history_list_item_holo_dark, null);
convertView = mInflater.inflate(R.layout.blank, parent, false); // CHEAT: LOAD BLANK/ EMPTY LAYOUT
HistoryHolder item = history.get(position);
new AsyncRequest(convertView, position).execute(item);
}
this.parent = parent;
return convertView;
}//end method
static class ViewHolder {
TextView TITLE;
TextView SUMMARY;
TextView DATE;
ImageView CONTACT_ICON;
ImageView OPTIONS_ICON;
ImageView TYPE_ICON;
}//end class
private class AsyncRequest extends AsyncTask<HistoryHolder, View, View> {
ViewHolder holder = null;
String title = "";
String summary = "";
String date = "";
long id = 0;
private View convertView = null;
private String type = "";
private int position = -1;
public AsyncRequest(View superView, int position){
this.convertView = superView;
this.position = position;
}//end constructor
#Override
protected View doInBackground(HistoryHolder... item) {
Thread.currentThread().setName(getClass().getSimpleName());
if (item[0].TYPE.equals("log")){
//massive decrypting going on here 50-100 ms
//values like title and summray set here
}
if (item[0].TYPE.equals("sms")){
//massive decrypting going on here 50-100 ms
//values like title and summray set here
}
publishProgress(convertView);
return convertView;
}// end method
#Override
protected void onProgressUpdate(View... view) {
super.onProgressUpdate(view);
}// end method
#Override
protected void onPostExecute(View result) {
super.onPostExecute(result);
if (!MathUtils.isEven(position)){
result .setBackgroundColor(context.getResources().getColor(R.color.darker)); //this works as expected, list items vary in color
} else {
result .setBackgroundColor(context.getResources().getColor(R.color.medium_dark));
} //this works as expected, list items vary in color
result = mInflater.inflate(R.layout.history_list_item_holo_dark, parent, false);
result.setTag(id);
holder = new ViewHolder();
holder.TITLE = (TextView) result .findViewById(R.id.title);
holder.SUMMARY = (TextView) result .findViewById(R.id.summary);
holder.DATE = (TextView) result .findViewById(R.id.date);
holder.CONTACT_ICON = (ImageView) result .findViewById(R.id.icon);
holder.TYPE_ICON = (ImageView) result .findViewById(R.id.type);
holder.OPTIONS_ICON = (ImageView) result .findViewById(R.id.options);
holder.OPTIONS_ICON.setFocusable(false);
holder.OPTIONS_ICON.setTag(id);
holder.TITLE.setText(title); //this change doesnt show
holder.SUMMARY.setText(summary); //and so on
result .setTag(holder);
}//end method
}//end inner class
And I know I could modify my AsynTask and that I don't need to pass reference to the View in so many places, but then again, it's code in progress. Simplified example...
EDIT
Okay, so it seems my approach was poor to begin with, resulting in the need to have a AsyncTask in my HistoryAdapter. I adressed a few issues to resolve this.
Based on #Delyan 's suggestion, I decided it was good to load/ decrypt data before it is actually needed. I am using a PropertyChangeListener for this. This implements a OnSharedPreferenceChangeListener so that it get's notified of changes to the data I need. Changes are then propagated to any interested listeners. The data is decrypted on application start and stored in a global variable, which is accesible throughout the application. See this as the 'memory cache' he referred to.
Based on the comments and on the accepted answer, decrypting now is done in the background, so there is no longer a need for AsyncTasks.
To further optimise the performance of my adapter, I am storing images needed for the ListView in a SparseArray, so they are only created and stored once. Don't use a HashMap for this! Furthermore, the images are only created for the current View if they aren't already in a HashMap (images aren't unique).
public class HistoryAdapter extends BaseAdapter{
private static Context context = ApplicationSubclass.getApplicationContext_();
private Contacts contacts = Contacts.init(context);
private SparseArray<Drawable> c_images = new SparseArray<Drawable>();
private HashMap<Long, Drawable> contact_imgs = new HashMap<Long, Drawable>();
private ArrayList<HistoryHolder> history;
private LayoutInflater mInflater;
public HistoryAdapter(Context context) {
HistoryAdapter.context = context;
mInflater = LayoutInflater.from(context);
}//end constructor
...
public View getView(int position, View convertView, ViewGroup parent) {
final HistoryHolder item = history.get(position);
Drawable d = null;
if (c_images.get(position) == null){
if (!contact_imgs.containsKey(item.CONTACT_ID)){
if (item.IN_CONTACTS && item.CONTACT_ID != 0 && item.CONTACT_ID != -1){
Bitmap photo = contacts.getContactPhotoThumbnailByContactId(item.CONTACT_ID);
if (photo != null){
d = Convert.bitmapToDrawable(context, photo, 128, 128);
} else {
d = context.getResources().getDrawable(R.drawable.ic_contact_picture);
}
} else {
d = context.getResources().getDrawable(R.drawable.ic_contact_picture);
}
contact_imgs.put(item.CONTACT_ID, d);
}
}
c_images.put(position, contact_imgs.get(item.CONTACT_ID));
ViewHolder holder;
if (convertView == null) {
convertView = mInflater.inflate(R.layout.history_list_item_holo_dark, null);
holder = new ViewHolder();
holder.POSITION = position;
holder.TITLE = (TextView) convertView.findViewById(R.id.title);
holder.SUMMARY = (TextView) convertView.findViewById(R.id.summary);
holder.DATE = (TextView) convertView.findViewById(R.id.date);
holder.CONTACT_ICON = (ImageView) convertView.findViewById(R.id.icon);
holder.CONTACT_ICON.setTag(position);
holder.OPTIONS_ICON = (ImageView) convertView.findViewById(R.id.options);
holder.OPTIONS_ICON.setFocusable(false);
holder.OPTIONS_ICON.setTag(position);
convertView.setTag(holder);
} else {
holder = (ViewHolder) convertView.getTag();
}
holder.CONTACT_ICON.setBackgroundDrawable(c_images.get(position));
holder.TITLE.setText(item.TITLE);
holder.SUMMARY.setText(item.SUMMARY);
holder.SUMMARY.setMaxLines(2);
holder.DATE.setText(item.DATE);
if (!MathUtils.isEven(position)){
convertView.setBackgroundColor(context.getResources().getColor(R.color.darker));
} else {
convertView.setBackgroundColor(context.getResources().getColor(R.color.medium_dark));
}
return convertView;
}//end method
static class ViewHolder {
TextView TITLE;
TextView SUMMARY;
TextView DATE;
ImageView CONTACT_ICON;
ImageView OPTIONS_ICON;
int POSITION;
}//end inner class
}//end class

Dmmh, what you say, you are wanting to do (So I need the ListView to update (add the item) only when progress is done) and what you are doing (AsyncTask in getView) are quite the opposite things.
AsyncTask in the end of getView is used (however in different way) for lazy image load i.e. to show large images and show text+imageplaceholder until download is complete.
You are trying to gradually fill in your adapter's datasourse in the First AsyncTask, but every time, you notify observers about changes in dataset you will have another cycle of getView calls for every item in dataset. No good.
First, never, NEVER!!! assume that getView will supply you back a convertview, previously filled for this very position. So you MUST either refill convert view with new values, or turn off performance optimization and supply new view every time you are asked for it. There's no way for ListView to turn off recycling attempts because this is the essence of ListView, the feature it is build upon.
Second (resulted from first), avoid at all means storing time-expensive (or user input) data into your newly created Views only. Views come and go, and you do not want to walk the long way to get the expensive data (or just lose user input). The only partial exclusion are simple ineffective implementations of big image lazy loading. To reduce memory usage they download only images that are currently visible by user. More effective implementations use off-ListView caching.
So, if you really want to have the items in your ListView to be added one at a time, but in full glory, you should:
0*. If you have to load user-provided icons and this takes significant time to load (I have not understand your initial post about that) make an empty ArrayList to cache loaded ones and access them by index. If all images are already available by some index, ignore this matter.
class DecryptedRecord {
String title = "";
String summary = "";
String date = "";
int contactViewIndex = -1;
int contactOptionsIndex = -1;
int contactImageIndex = -1;
int typeImageIndex = -1;
int optionsImageIndex = -1; //if option icon varies. ignore otherwise
}
Declare DecryptedRecord class, containing necessary data to fill in the view, e.g.:
In your AsyncTask: after loading every HistoryHolder, perform "heavy decrypting" and fill new DecryptedRecord with the values. If it is necessary to load cusom image(see no.0*), load it here and store its index in cache(ignore this if 0* is irrelevant). Attach filled DecryptedRecord to the HistoryHolder with setTag(). Call publishProgress(HistoryHolder)
In onProgressUpdate just add HistoryHolder to Adapter and call historyAdapter.notifyDataSetChanged();
In getView() synchronously inflate the view if convertView is null, and IN ALL CASES populate it with the data from DecryptedRecord acqired from HistoryHolder.getTag(). Inclusive, call setImageBitmap() on your ImageViews adressing necessary Bitmap in the corresponding list by index.
This should do what you want. Please, if you would have errors/problems, try to include complete code in your question, or it will be very difficult to understand the details of your situation. Hope that helps.

You need to separate your concerns. Decrypting/decoding data has no place in the UI layer.
In general, your current approach to have AsyncTasks per item is difficult and (some would say) wrong for multiple reasons:
Honeycomb and above, there's only ever one AsyncTask running at any one point in time, unless you explicitly set the Executor.
More importantly, by holding a reference to convertView, you're leaking abstraction from the ListView - it's possible that the view you're holding a reference to is being reused for a different position. Unless you take painstaking care to cancel AsyncTasks and ensure proper result delivery, this will cause you trouble.
As mentioned above, decrypting/decoding data has no place in the UI layer (and I consider the Adapters UI layer, since they have similar constraints on execution speed).
If I were you, I'd use a memory cache of decrypted data, expanding/shrinking it as demand changes. Then, I would just fetch decrypted data in the getView() method of the adapter. In order to avoid decrypting items when scrolling, you can set up a scroll listener on the ListView, so that you only show the items when the list is not moving. There's a demo in ApiDemos that does something similar.
EDIT:
As for your obvious problem, you're reinflating a view (result) without adding it to the list item (the convertView field in the task). You can fix that by adding it to convertView (in an empty layout, for example). Again, this will not work as you expected in all cases.

You could create a content provider which has your decoded data stored in a db/temp data structure and use that to update your views. The decoding etc could happen in a background via a service/thread. The adapter could talk to the decoded data via this provider. This is related to alexei burmistrov idea mentioned above.
Not the good option - To use your current layout - a layout file that has both blank & filled layouts. When the layout is first shown, the blank view is visible. and when task is finished the visibility is set to gone.
This solution is not optimal as the asynctask would run each time a getView is called. And Android re-uses views so it could be many times based on how you scroll on the UI etc.
sample code:
public void getView(){
result = mInflater.inflate(R.layout.history_list_item_holo_dark, parent, false);
holder = new ViewHolder();
holder.TITLE = (TextView) result .findViewById(R.id.title);
holder.SUMMARY = (TextView) result .findViewById(R.id.summary);
..
holder.OPTIONS_ICON.setTag(id);
result.setTag(holder);
result.setTag(R.id.view_id ,id);
AsyncTask.execute(result);
convertView = result;
}
AsyncTask(View ...){
onPostExecute(View view){
ViewHolder holder = view.getTag();
if(holder != null){
//set visibility of views in holder
//update Text & data
}
}
}

First, when the async task runs the onPostExecute you will have to set the data to the adapter and notifydatasetchanged().
Now be a little more clean with the list view when is empty using something like creating a progressBar and set it to the listview like this:
listView.setEmptyView(progressbar);

Issue when fast scroll a listview

I am working on a small project where I create a listview bound to an ArrayAdapter. In the getView() function of ArrayAdapter, I do a loading of images from web urls on thread, and set them to list items (based on position of course, so url[0] image is set to list_item[0] etc). It all seems to work well.
However when I was testing the app, I noticed that if I wait my listview to fully display, then perform a fast scroll back and forth, I see sometimes the image on one list item is misplaced on other (like being in an intermediate state). However it's not going away until I scroll the wrongly-displayed-item out of screen and then back.
I do not know if it relates to my loading web url using thread, or maybe loading image from local resource folder can have the same issue.
This actually leads to a question I have about getView() function. I think the logic is correct in my getView() because it's as simple as a binding of url to view based on position. And whenever getView() get a chance to be called, like when I scroll an item out of screen then back, it will make the list item display correctly.
The thing I do not understand is how to explain the issue that happened (like an intermediate state), and how to avoid it when writing code?
I paste my adapter code piece below, but I think the question maybe a general one:
#Override
public View getView(int position, View v, ViewGroup parent) {
ViewHolder viewHolder = null;
if (v == null) {
LayoutInflater inflater = (LayoutInflater) getContext()
.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
v = inflater.inflate(layoutResourceId, parent, false);
viewHolder = new ViewHolder();
viewHolder.title = (TextView) v.findViewById(R.id.title);
viewHolder.description = (TextView) v.findViewById(R.id.description);
viewHolder.image = (ImageView) v.findViewById(R.id.image);
v.setTag(viewHolder);
} else {
viewHolder = (ViewHolder) v.getTag();
}
listItem item = items[position]; //items is some global array
//passed in to ArrayAdapter constructor
if (item != null) {
viewHolder.title.setText(item.title);
viewHolder.description.setText(item.description);
if (!(item.imageHref).equalsIgnoreCase("null")) {
mDrawableManager.fetchDrawableOnThread(item.imageHref, viewHolder.image);
} else {
viewHolder.image.setImageDrawable(null);
}
}
return v;
}
}
static class ViewHolder {
TextView title;
TextView description;
ImageView image;
}

I have same issue when scroll quickly it alternate the vales of some item to others, just like background color of some items if changes randomly. I solved this issue by searching a lot and find exact solution is just adding these two methods in your adapter if you are using ViewHolder in your adapter
#Override
public int getViewTypeCount() {
return getCount();
}
#Override
public int getItemViewType(int position) {
return position;
}

Assuming that you are not caching the downloaded image.. lets see the following code:
if (!(item.imageHref).equalsIgnoreCase("null")) {
mDrawableManager.fetchDrawableOnThread(item.imageHref, viewHolder.image);
} else {
viewHolder.image.setImageDrawable(null);
}
Now if the image view is getting reused then it would already have the old image for the assigned list item. So until the thread download the image from the network it would display the old image and when the thread download the image for the current item it would be replaced with the new image. Try to change it to:
if (!(item.imageHref).equalsIgnoreCase("null")) {
viewHolder.image.setImageDrawable(SOME_DEFULAT_IMAGE);
mDrawableManager.fetchDrawableOnThread(item.imageHref, viewHolder.image);
} else {
viewHolder.image.setImageDrawable(null);
}
Or you can use something link smart image view that supports HTTP URI and also caches the images. Check out following link for smart image view:
https://github.com/loopj/android-smart-image-view
http://loopj.com/android-smart-image-view/

Add ImageLoader class from below link in your project.
link
just call DisplayImage() methode of Image loader class as below in getView()
ImageLoader imageLoader = new ImageLoader();
yourImageView.setTag(URL_FOR_Your_Image);
imageLoader.DisplayImage(URL_FOR_Your_Image,ACTIVITY.this, yourImageView);
Your images will load in background as you want without wait.

I think you should declare your downloader method fetchDrawableOnThread() as "synchronized" . Because a lot of threads are working simultaneously and any thread which started later, can end earlier. So there are chances of images being misplaced.
It happened to me for a long time. Finally "synchronized" helped me do it cleanly. I hope it helps you too.

I give it a try with synchronization again. Either synchronize the fetchDrawableOnThread(), or synchronize the global hashmap data within fetchDrawableOnThread(). First i thought the issue is gone, but when i tried more times later, i found the issue is still there.
Then i thought about the synchronization. fetchDrawableOnThread() is called from getView(), and getview() itself does not have a concurrency issue. Even if as Yogesh said, what happened INSIDE getView() is thread-based, and return early or late, it can not affect the correctness of getView(), i.e. the list item's display, only the sooner or later.
What i did(synchronization) inside fetchDrawableOnThread() i think it's still correct, 'cause i used a hashmap to cache images downloaded from remote url, and this hashmap is read/write upon in a multi-thread situation, so it should be locked. But i do not think it's the rootcause of the UI misplace, if hashmap is messed up, the image misplacement will be permanent.
Then i looked further on convertView reuse mechanism based on Praful's explanation. He explained clearly what happened when image always comes from remote and no cache locally, but my situation is i waited my list to display fully, i.e. all images download complete and cached complete, before i do the fast scroll. So in my experiment, the images are read from cache.
Then when inspecting my code, i saw one minor difference in the use of convertView as in getView() method, a lot of the example usages are like this:
public View getView(int position, View convertView, ViewGroup parent) { // case 1
View v = convertView;
.... // modify v
return v;
}
However the example i happened to copy from use:
public View getView(int position, View convertView, ViewGroup parent) { // case 2
.... // modify convertView
return convertView;
}
I thought it makes no difference at first, 'cause according to what android says, 'ListView sends the Adapter an old view that it's not used any more in the convertView param.', so why not use 'convertView' para directly?
But i guess i was wrong. I changed my getView() code to case 1. Boom! everything works. No funny business ever no matter how fast i scroll the list.
Quite strange, is convertView only old, or is it old & in-use? If the later, we should only get a copy and then modify..... ??

Android Adapter multiple getView

i have read about the issue of getView called multiple times and all the answers. However, i don't find a solution for my problem.
I have a list where rows have two states: read or not. Well, i want to the items seen for first time have a different color and when i scroll the list, they change their color to "read state".
In order to do this, in the method getView of my adapter i set a field isRead when the row for that item is painted. But the problem is the following: since the method getView is called multiple times the field is marked as read and when the list is shown in the screen it appears as if it had already been read.
Any idea to fix this problem?
Thanks

I assume you mean the issue of getView requesting the same view several times.
ListView does this because it needs to get measurements for the views for different reasons (scrollbar size, layout, etc)
This issue can usually be avoided by not using the "wrap_content" property on your listview.
Other than that, using getView to determine if a view has been displayed is simply a bad idea. ListView has many optimizations that mess with the order getView is called on for each row, so there is no way to know what will happen and your app will start showing odd behavior.
Try to avoid any relationship between the view and the data other than the concept of view as a display of that data.
Instead, have some worker thread or event listener in your listactivity watch the list for which items in the list have been displayed to the user, update the data, and call dataSetChanged on your adaptor.

I had the same problem and I had no reference at all to "wrap_content" in the layouts attirbute. Although this is an old thread I couldn't figured it out how to solve the issue. Thus, I mitigated it by adding a List in the Adapter that holds the positions already drawn, as shown in the code below. I think that it is not the right away of doing that, but it worked for me.
public class ImageAdapter extends BaseAdapter {
private Context mContext;
private List<Integer> usedPositions = new ArrayList<Integer>();
public ImageAdapter(Context c, List<String> imageUrls) {
mContext = c;
...
}
...
// create a new ImageView for each item referenced by the Adapter
public View getView(int position, View convertView, ViewGroup parent) {
ImageView imageView;
if (convertView == null) {
imageView = new ImageView(mContext);
imageView.setLayoutParams(new GridView.LayoutParams(85, 85));
imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
imageView.setPadding(8, 8, 8, 8);
} else {
imageView = (ImageView) convertView;
}
if (!usedPositions.contains(position)) {
// Your code to fill the imageView object content
usedPositions.add(position); // holds the used position
}
return imageView;
}
}

Not the way you want it to work. The reason that getView() is called multiple times is to allow the OS to measure the rows so it knows how to lay them out. You would need to have it marked as read when they click it or check a box or something.

Here is a very simple way of avoiding the double call when you know nothing below this block of code will distort the layout.
private static List<String> usedPositions = new ArrayList<String>();
...
#Override
public View getView(int position, View rowView, ViewGroup parent) {
rowView = inflater.inflate(R.layout.download_listview_item, null);
Log.d(LOG_TAG, "--> Position: " + position);
if (!usedPositions.contains(String.valueOf(position))){
usedPositions.add(String.valueOf(position));
return rowView;
}else{
usedPositions.remove(String.valueOf(position));
}
...