java - tipos - recyclerview viewholder example

¿Cómo puedo hacer que mi ArrayAdapter siga el patrón ViewHolder?

Question

Aquí está mi ArrayAdapter. Me gustaría hacerlo más eficiente siguiendo el patrón ViewHolder:

http://developer.android.com/resources/samples/ApiDemos/src/com/example/android/apis/view/List14.html

pero no estoy seguro de cómo lograr esto.

ACTUALIZACIÓN: patrón ViewHolder

private class QuoteAdapter extends ArrayAdapter<Quote> {

    private ArrayList<Quote> items;
    // used to keep selected position in ListView
    private int selectedPos = -1; // init value for not-selected

    public QuoteAdapter(Context context, int textViewResourceId, ArrayList<Quote> items) {
        super(context, textViewResourceId, items);
        this.items = items;
    }

    public void setSelectedPosition(int pos) {
        selectedPos = pos;
        // inform the view of this change
        notifyDataSetChanged();
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View v = convertView;
        ViewHolder holder; // to reference the child views for later actions

        if (v == null) {
            LayoutInflater vi = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            v = vi.inflate(R.layout.mainrow, null);

            // cache view fields into the holder
            holder = new ViewHolder();
            holder.nameText = (TextView) v.findViewById(R.id.nameText);
            holder.priceText = (TextView) v.findViewById(R.id.priceText);
            holder.changeText = (TextView) v.findViewById(R.id.changeText);

            // associate the holder with the view for later lookup
            v.setTag(holder);
        }
        else {
            // view already exists, get the holder instance from the view
            holder = (ViewHolder)v.getTag();
        }

        // change the row color based on selected state
        if (selectedPos == position) {
            v.setBackgroundResource(R.drawable.stocks_selected_gradient);
            holder.nameText.setTextColor(Color.WHITE);
            holder.priceText.setTextColor(Color.WHITE);
            holder.changeText.setTextColor(Color.WHITE);
        } else {
            v.setBackgroundResource(R.drawable.stocks_gradient);
            holder.nameText.setTextAppearance(getApplicationContext(), R.style.BlueText);
            holder.priceText.setTextAppearance(getApplicationContext(), R.style.BlueText);
            holder.changeText.setTextAppearance(getApplicationContext(), R.style.BlueText);
        }

        Quote q = items.get(position);
        if (q != null) {
            if (holder.nameText != null) {
                holder.nameText.setText(q.getSymbol());
            }
            if (holder.priceText != null) {
                holder.priceText.setText(q.getLastTradePriceOnly());
            }
            if (holder.changeText != null) {
                try {
                    float floatedChange = Float.valueOf(q.getChange());
                    if (floatedChange < 0) {
                        if (selectedPos != position)
                            holder.changeText.setTextAppearance(getApplicationContext(), R.style.RedText); // red
                    } else {
                        if (selectedPos != position)
                            holder.changeText.setTextAppearance(getApplicationContext(), R.style.GreenText); // green
                    }
                } catch (NumberFormatException e) {
                    System.out.println("not a number");
                } catch (NullPointerException e) {
                    System.out.println("null number");
                }
                holder.changeText.setText(q.getChange() + " (" + q.getPercentChange() + ")");
            }
        }
        return v;
    }
}

Answer 1

El ViewHolder es básicamente una instancia de clase estática que asocia con una vista cuando se crea, almacenando en caché las vistas secundarias que está buscando en el tiempo de ejecución. Si la vista ya existe, recupere la instancia del titular y use sus campos en lugar de llamar a findViewById .

En tu caso:

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    View v = convertView;
    ViewHolder holder; // to reference the child views for later actions

    if (v == null) {
        LayoutInflater vi = 
            (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        v = vi.inflate(R.layout.mainrow, null);
        // cache view fields into the holder
        holder = new ViewHolder();
        holder.nameText = (TextView) v.findViewById(R.id.nameText);
        holder.priceText = (TextView) v.findViewById(R.id.priceText);
        holder.changeText = (TextView) v.findViewById(R.id.changeText);
        // associate the holder with the view for later lookup
        v.setTag(holder);
    }
    else {
        // view already exists, get the holder instance from the view
        holder = (ViewHolder) v.getTag();
    }
    // no local variables with findViewById here

    // use holder.nameText where you were 
    // using the local variable nameText before

    return v;
}

// somewhere else in your class definition
static class ViewHolder {
    TextView nameText;
    TextView priceText;
    TextView changeText;
}

advertencia: no intenté compilar esto, así que tómelo con un grano de sal.