ViewHolder pattern correctly implemented in custom CursorAdapter?

Question

Here is my custom CursorAdapter:

public class TasksAdapter extends CursorAdapter implements Filterable {

    private final Context context;

    public TasksAdapter(Context context, Cursor c) {
        super(context, c);
        this.context = context;
    }

    /**
     * @see android.widget.CursorAdapter#newView(android.content.Context, android.database.Cursor, android.view.ViewGroup)
     */
    @Override
    public View newView(Context context, Cursor cursor, ViewGroup parent) {
        LayoutInflater inflater = LayoutInflater.from(context);
        View v = inflater.inflate(android.R.layout.simple_list_item_checked, parent, false);        

        ViewHolder holder = new ViewHolder();
        holder.textview = (CheckedTextView)v.findViewById(android.R.id.text1);
        v.setTag(holder);

        return v;
    }

    /**  
     * @see android.widget.CursorAdapter#bindView(android.view.View, android.content.Context, android.database.Cursor)
     */
    @Override
    public void bindView(View view, Context context, Cursor cursor) {

        ViewHolder holder = (ViewHolder)view.getTag();
        int titleCol = cursor.getColumnIndexOrThrow(Tasks.TITLE);
        int completedCol = cursor.getColumnIndexOrThrow(Tasks.COMPLETED);

        String title = cursor.getString(titleCol);
        boolean completed = Util.intToBool(cursor.getInt(completedCol));

        holder.textview.setText(title);
        holder.textview.setChecked(completed);
    }

    /**
     * @see android.widget.CursorAdapter#runQueryOnBackgroundThread(java.lang.CharSequence)
     */
    @Override
    public Cursor runQueryOnBackgroundThread(CharSequence constraint) {

        StringBuffer buffer = null;
        String[] args = null;

        if (constraint != null) {
            buffer = new StringBuffer();
            buffer.append("UPPER (");
            buffer.append(Tasks.TITLE);
            buffer.append(") GLOB ?");
            args = new String[] { "*" + constraint.toString().toUpperCase() + "*" };
        }

        Cursor c = context.getContentResolver().query(Tasks.CONTENT_URI,
            null, (buffer == null ? null : buffer.toString()), args,
            Tasks.DEFAULT_SORT_ORDER);

        c.moveToFirst();
        return c;
    }

    /**
     * @see android.widget.CursorAdapter#convertToString(android.database.Cursor)
     */
    @Override
    public CharSequence convertToString(Cursor cursor) {
        final int titleCol = cursor.getColumnIndexOrThrow(Tasks.TITLE);
        String title = cursor.getString(titleCol);
        return title;
    }

    static class ViewHolder {
        CheckedTextView textview;
    }

}

Does this fall into the constraints of the ViewHolder pattern? I wasn't sure because this was a CursorAdapter, where there was no getView. If there are any problems or suggestions, could you please point them out.

Orourke

Answer 1

CursorAdapter won't call the newView each time it needs a new row; if it already has a View, it will call the bindView, so the created view is actually reused.

That said, as pointed out by Joseph in the comments, you can still use ViewHolder in order to avoid calling findViewById repeatedly.

If you are still concerned about efficiency then take a look at the SimpleCursorAdapter implementation, which uses a WeakHashMap (a map of WeakReferences):

WeakHashMap<View, View[]> mHolders = new WeakHashMap<View, View[]>();

Unwashed

So I can make expensive calls like View.findViewById(int) and my application won't lag? – Orourke

findViewById(int) is not as expensive as you think. It will only return a reference (if any exists). ViewHolder technique exists to solve other kind of problems: don't create more Views than what you really need (thus, it avoids excessive view inflations, which IS expensive). – Unwashed

@Cristian, thank you. I used newView and bindView to make a class extends SimpleCursorAdapter. See the code below. – Primateship

@Unwashed I think your comment is wrong. View Holder actually exists to avoid multiple findViewById calls, as said here: youtu.be/wDBM6wVEO70?t=10m50s Moreover, to solve the problem you talk about (don't create more Views than what you really need) another technique is used, based on the convertView. – Devitrify

@Joseph82 well... calling findViewById multiple times is the least of your concerns. You actually use ViewHolder in order to avoid inflating the view a lot of times, which is too expensive. The video actually is correct, but your interpretation of it is not. If you need me to elaborate more on this let me know. – Unwashed

@Unwashed You are wrong again! From the Commonsware book here you can find an implementation of getView() without ViewHolder, but that uses the convertView. As you can see, when convertView is not null, no inflating is required: the view is recycled. So, in order to avoid inflating you have to use the convertView. Instead ViewHolder is actually used for avoiding many findViewById calls, as said in the same commonsware book here – Devitrify

@Unwashed also, have a look here where it says "A way around repeated use of findViewById() is to use the "view holder" design pattern." – Devitrify

I see it now. Avoiding the inflation is provided by the convertView instance, while ViewHolder is only used as a way to avoid findViewById calls. Thanks for making this clear to me! – Unwashed

Answer 2

If you are overriding newView() and bindView(), you don't need to do anything extra in getView(). CursorAdapter has an implementation of getView() that delegates to newView() and bindView() to enforce the row recycling.

findViewById() maybe called 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. To avoid this, ViewHolder pattern is useful.

Here's an example of ViewHolder pattern implemented for a weather app:

public class ForecastAdapter extends CursorAdapter {

    public ForecastAdapter(Context context, Cursor cursor, int flags) {
        super(context, cursor, flags);
    }

    @Override
    public View newView(Context context, Cursor cursor, ViewGroup parent) {
        View view = LayoutInflater.from(context).inflate(
                R.layout.list_item_forecast, parent, false);
        ViewHolder viewHolder = new ViewHolder(view);
        view.setTag(viewHolder);
        return view;
    }

    @Override
    public void bindView(View view, Context context, Cursor cursor) {
        ViewHolder viewHolder = (ViewHolder) view.getTag();

        long date = cursor.getLong(ForecastFragment.COL_WEATHER_DATE);
        viewHolder.dateView.setText("Today");

        String weatherForecast =
                cursor.getString(ForecastFragment.COL_WEATHER_DESC);
        viewHolder.descriptionView.setText(weatherForecast);

        double high = cursor.getFloat(ForecastFragment.COL_WEATHER_MAX_TEMP);
        viewHolder.highTempView.setText("30");

        double low = cursor.getFloat(ForecastFragment.COL_WEATHER_MIN_TEMP);
        viewHolder.lowTempView.setText("24");

        int weatherConditionId =
                cursor.getInt(ForecastFragment.COL_WEATHER_CONDITION_ID);
        viewHolder.iconView.setImageResource(R.drawable.ic_snow);
    }

    /** Cache of the children views for a list item. */
    public static class ViewHolder {
        public final ImageView iconView;
        public final TextView dateView;
        public final TextView descriptionView;
        public final TextView highTempView;
        public final TextView lowTempView;

        public ViewHolder(View view) {
            iconView =
                    (ImageView) view.findViewById(R.id.item_icon);
            dateView =
                    (TextView) view.findViewById(R.id.item_date_textview);
            descriptionView =
                    (TextView) view.findViewById(R.id.item_forecast_textview);
            highTempView =
                    (TextView) view.findViewById(R.id.item_high_textview);
            lowTempView =
                    (TextView) view.findViewById(R.id.item_low_textview);
        }
    }
}

Berkshire

Answer 3

My implementation of a class extends SimpleCursorAdapter with newView and bindView but without the ViewHolder pattern

    private class CountriesAdapter extends SimpleCursorAdapter {

            private LayoutInflater mInflater;

            public CountriesAdapter(Context context, int layout, Cursor cursor, String[] from,
                    int[] to, LayoutInflater inflater) {
                super(getActivity(), layout, cursor, from, to, CURSOR_ADAPTER_FLAGS);
                mInflater = inflater;
            }

            @Override
            public View newView(Context context, Cursor cursor, ViewGroup parent) {
                return mInflater.inflate(R.layout.countries_list_row, parent, false);
            }

            @Override
            public void bindView(View rowView, Context context, Cursor cursor) {

                TextView tvCountry = (TextView) rowView.findViewById(R.id.countriesList_tv_countryName);
                TextView tvOrgs = (TextView) rowView.findViewById(R.id.countriesList_tv_orgNames);
                ImageView ivContinent =
                        (ImageView) rowView.findViewById(R.id.countriesList_iv_continentName);

                // TODO: set texts of TextViews and an icon here
                }

            }
    }

Primateship