How to write custom ExpandableListAdapter

Question

I'm looking to write my own ExpandableListAdapter which operates similarly to ArrayAdapter. My data model is this:

public class Group {

    private String name;   

    private List<Child> children;
}

public class Child {

     private String name;
}

Pretty simple. How can I map this relationship into a ExpandableListAdapter implementation? I have a working SimpleExpandableListAdapter working right now, but I need more custom control over items (displaying icons, etc.). What should I do for this?

The main thing is that I need an add() method to be able to add groups and to invalidate the list when children are added and removed from the adapter. I'm actually surprised that there isn't an implementation in the SDK (even an abstract one) which helps one accomplish this.

Answer 1

Here's an implementation I just whipped up. I have no idea if it works or not, but seems "smart" to me :) By the way, how should one go about getting the combined child id or the combined group id, so I just kinda improvised there.

package example;

import java.util.ArrayList;
import java.util.List;
import java.util.Map.Entry;

import android.content.Context;
import android.database.DataSetObservable;
import android.database.DataSetObserver;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ExpandableListAdapter;

public abstract class AbstractExpandableListAdapter<A, B> implements ExpandableListAdapter {

    private final List<Entry<A, List<B>>> objects;

    private final DataSetObservable dataSetObservable = new DataSetObservable();

    private final Context context;

    private final Integer groupClosedView;

    private final Integer groupExpandedView;

    private final Integer childView;

    private final LayoutInflater inflater;

    public AbstractExpandableListAdapter(Context context, int groupClosedView, 
            int groupExpandedView, int childView, List<Entry<A, List<B>>> objects) {
        this.context = context;
        this.objects = objects;
        this.groupClosedView = new Integer(groupClosedView);
        this.groupExpandedView = new Integer(groupExpandedView);
        this.childView = new Integer(childView);

        this.inflater = (LayoutInflater) this.context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    public void add(Entry<A, List<B>> group) {
        this.getObjects().add(group);
        this.notifyDataSetChanged();
    }

    public void remove(A group) {
        for (Entry<A, List<B>> entry : this.getObjects()) {
            if (entry != null && entry.getKey().equals(group)) {
                this.getObjects().remove(group);
                this.notifyDataSetChanged();
                break;
            }
        }
    }

    public void remove(Entry<A, List<B>> entry) {
        remove(entry.getKey());
    }

    public void addChild(A group, B child) {
        for (Entry<A, List<B>> entry : this.getObjects()) {
            if (entry != null && entry.getKey().equals(group)) {
                if (entry.getValue() == null) 
                    entry.setValue(new ArrayList<B>());

                entry.getValue().add(child);
                this.notifyDataSetChanged();
                break;
            }
        }
    }

    public void removeChild(A group, B child) {
        for (Entry<A, List<B>> entry : this.getObjects()) {
            if (entry != null && entry.getKey().equals(group)) {
                if (entry.getValue() == null)
                    return;

                entry.getValue().remove(child);
                this.notifyDataSetChanged();
                break;
            }
        }
    }

    public void notifyDataSetChanged() {
        this.getDataSetObservable().notifyChanged();
    }

    public void notifyDataSetInvalidated() {
        this.getDataSetObservable().notifyInvalidated();
    }

    public void registerDataSetObserver(DataSetObserver observer) {
        this.getDataSetObservable().registerObserver(observer);
    }

    public void unregisterDataSetObserver(DataSetObserver observer) {
        this.getDataSetObservable().unregisterObserver(observer);
    }

    public int getGroupCount() {
        return getObjects().size();
    }

    public int getChildrenCount(int groupPosition) {
        return getObjects().get(groupPosition).getValue().size();
    }

    public Object getGroup(int groupPosition) {
        return getObjects().get(groupPosition).getKey();
    }

    public Object getChild(int groupPosition, int childPosition) {
        return getObjects().get(groupPosition).getValue().get(childPosition);
    }

    public long getGroupId(int groupPosition) {
        return ((Integer)groupPosition).longValue();
    }

    public long getChildId(int groupPosition, int childPosition) {
        return ((Integer)childPosition).longValue();
    }

    public boolean hasStableIds() {
        return true;
    }

    public View getGroupView(int groupPosition, boolean isExpanded,
            View convertView, ViewGroup parent) {

        if (convertView != null && convertView.getId() != 
                (isExpanded ? getGroupExpandedView() : getGroupClosedView())) {
//          do nothing, we're good to go, nothing has changed.
        } else {
//          something has changed, update.
            convertView = inflater.inflate(isExpanded ? getGroupExpandedView() :
                    getGroupClosedView(), parent, false);
            convertView.setTag(getObjects().get(groupPosition));
        }

        return convertView;
    }

    public View getChildView(int groupPosition, int childPosition,
            boolean isLastChild, View convertView, ViewGroup parent) {

        if (convertView != null) {
//          do nothing 
        } else {
//          create
            convertView = inflater.inflate(getChildView(), parent, false);
            convertView.setTag(getObjects().get(groupPosition).getValue().get(childPosition));
        }

        return convertView;
    }

    public boolean isChildSelectable(int groupPosition, int childPosition) {
        return true;
    }

    public boolean areAllItemsEnabled() {
        return true;
    }

    public boolean isEmpty() {
        return getObjects().size() == 0;
    }

    public void onGroupExpanded(int groupPosition) {

    }

    public void onGroupCollapsed(int groupPosition) {

    }

    public long getCombinedChildId(long groupId, long childId) {
        return groupId * 10000L + childId;
    }

    public long getCombinedGroupId(long groupId) {
        return groupId * 10000L;
    }

    protected DataSetObservable getDataSetObservable() {
        return dataSetObservable;
    }

    protected List<Entry<A, List<B>>> getObjects() {
        return objects;
    }

    protected Context getContext() {
        return context;
    }

    protected Integer getGroupClosedView() {
        return groupClosedView;
    }

    protected Integer getGroupExpandedView() {
        return groupExpandedView;
    }

    protected Integer getChildView() {
        return childView;
    }
}

Any comments or criticisms are welcome.

Answer 2

I was fairly surprised I didn't find better documentation on this either. If you find one, please post it here. The best implementation example I found was in ApiDemos. There is an ExpandableListActivity that implements a BaseExpandableListAdapter. The class is called ExpandableList1.java.

You will have to create your own add() method that adds your Group and Child classes to the adapter. I don't think it will be that difficult at first glance. In fact you might just be able to create the references to the class objects. When I implemented mine, my data set was small and didn't change so I only needed to refer to my array.xml file.