Wie kann ich meinen ArrayAdapter dem ViewHolder-Muster folgen lassen?

Question

Hier ist mein ArrayAdapter. Ich mag diese effizienter gestalten, indem nach dem ViewHolder Muster:

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

aber bin nicht sicher, wie dies zu erreichen.

UPDATE: ViewHolder Muster

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

Die ViewHolder ist im Grunde eine statische Klasse Instanz, die Sie mit Blick assoziieren, wenn es erstellt wird, das Kind das Caching sieht Sie suchen zur Laufzeit auf. Wenn die Ansicht bereits vorhanden ist, rufen Sie die Halterinstanz ab und verwenden Sie ihre Felder, anstatt findViewById aufzurufen.

In Ihrem Fall:

@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; 
} 

Vorbehalt: Ich versuche nicht, dies zu kompilieren, so mit einem Körnchen Salz.