Android ListView: Endless adapter

This post explains how to create an endless adapter for a ListView. An endless adapter is an adapter that loads more data when user reaches the ListView end. This kind of adapter is useful when there is a large number of items and we don’t want to show them all to avoid a long loading time. To achieve this goal we will create:

• a custom listview
• a listener
• a custom adapter

 
In this case the custom adapter is very simple and we can replace it with something more complex or use standard android adapter.

Custom Endless ListView component

This is the main component, that holds the business logic. In this component we have to find a way to check if the user scrolled all the item inside the ListView and we reached its end. The first step is creating a custom component so we can extend the standard ListView behavior.

public class EndlessListView extends ListView implements OnScrollListener {
..
    @Override
    public void onScroll(AbsListView view, int firstVisibleItem,
            int visibleItemCount, int totalItemCount) {
        ...
        }

    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {}

}

At line 1, we extends ListView and implements OnScrollListener to get notified when user scrolls the time on the ListView. At line 4, we override onScroll method, called during the scroll. When we reach the ListView end we have to show a view that informs the user to wait until the all data is loaded. We can use a “trick” to show the wait indicator, we can exploit the ListView footer. We can add and remove it as we need. To make our component configurable we can simply set a view to use a footer:

public void setLoadingView(int resId) {
    LayoutInflater inflater = (LayoutInflater) super.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    footer = (View) inflater.inflate(resId, null);
    this.addFooterView(footer);

}

Let’s focus on the onScroll method. Here we have to check if the firstVisibleElement plus itemCounts (the number of items show inside the ListView) is greater that the total number of items. If this condition is verified then we can fire the event to load more data:

@Override
public void onScroll(AbsListView view, int firstVisibleItem,
        int visibleItemCount, int totalItemCount) {

    if (getAdapter() == null)
        return ;

    if (getAdapter().getCount() == 0)
        return ;

    int l = visibleItemCount + firstVisibleItem;
    if (l >= totalItemCount && !isLoading) {
        // It is time to add new data. We call the listener
        this.addFooterView(footer);
        isLoading = true;
        listener.loadData();
    }
}

At line 12, we simply add the footer to inform the user we are loading more data, then at line 13 we set a true a boolean attribute to not fire the event again while we are still loading the data and then we call the listener.

As you can see the listener is very simple:

public static interface EndlessListener {
    public void loadData() ;
}

Just one more thing, when the loading data process is finished our custom ListView must be informed so that we can refresh the items and remove the footer:

public void addNewData(List<String> data) {
    this.removeFooterView(footer);
    adapter.addAll(data);
    adapter.notifyDataSetChanged();
    isLoading = false;
}

At line 4, we call notifyDataSetChanged to inform the adapter that the dataset is changed.

Test custom component

To test the custom component we can create a simple layout including our custom ListView:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <com.survivingwithandroid.endlessadapter.EndlessListView android:id="@+id/el"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"  />

</RelativeLayout>

and we need a main activity so that :

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    lv = (EndlessListView) findViewById(R.id.el);
    EndlessAdapter adp = new EndlessAdapter(this, createItems(mult), R.layout.row_layout);
    lv.setLoadingView(R.layout.loading_layout);
    lv.setAdapter(adp);
    lv.setListener(this);

}

At line 7, we set the footer view then at line the adapter and at the end we set our activity as a listener for our custom ListView. To emulate the internet loading data, we can simply create an AsyncTask and make our thread sleeps for some seconds. Notice that in the onPostExecute:

@Override
protected void onPostExecute(List<String> result) {            
    super.onPostExecute(result);
    lv.addNewData(result);
}

At line 4, we add the new loaded data.

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