ViewHolder Pattern

Using the ViewHolder Pattern

why view holder?

in bind view we always need to find the view by id ,than put the data in it.it is slow.

Because in the adapter,the view will be reuse.

"Instead, as the user scroll through the list, items that leave the screen are kept in memory for later use and then every new row that enters the screen reuses an older row kept around in memory. "

so, we can save the reference of all the iteam, than reuse them later,no need  to always call findviewbyid.

we use View Holder to save . A viewHolder responisble to save all the iteam of a specific view.

example of ViewHolder:

/**
	* Cache of the children views for a forecast 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.list_item_icon);
	dateView = (TextView) view.findViewById(R.id.list_item_date_textview);
	descriptionView = (TextView) view.findViewById(R.id.list_item_forecast_textview);
	highTempView = (TextView) view.findViewById(R.id.list_item_high_textview);
	lowTempView = (TextView) view.findViewById(R.id.list_item_low_textview);
	}
	}

So, we use get tag and settag to match a view and viewholder !!!

First.at new View,associte a ViewHolder with a view

@Override
public View newView(Context context, Cursor cursor, ViewGroup parent) {
   // Choose the layout type
   int viewType = getItemViewType(cursor.getPosition());
   int layoutId = -1;
   // TODO: Determine layoutId from viewType
   if(viewType==VIEW_TYPE_FUTUREDAY){
       layoutId=R.layout.list_item_forecast;
   }else{
       layoutId=R.layout.list_item_forecast_today;
   }
   View view=LayoutInflater.from(context).inflate(layoutId, parent, false);
   view.setTag(new ViewHolder(view));
   return view;
}

do by setTag

Sencond.get the viewholder at bindView

public void bindView(View view, Context context, Cursor cursor) {
   // our view is pretty simple here --- just a text view
   // we'll keep the UI functional with a simple (and slow!) binding.

    // Read weather icon ID from cursor
   int weatherId = cursor.getInt(ForecastFragment.COL_WEATHER_ID);
   // Use placeholder image for now
   ViewHolder vh=(ViewHolder)view.getTag();
   ImageView iconView = vh.iconView;
   iconView.setImageResource(R.drawable.ic_launcher);

    //Read date from cursor
   Long date=cursor.getLong(ForecastFragment.COL_WEATHER_DATE);

    TextView dateview= vh.dateView;

    dateview.setText(Utility.getFriendlyDayString(context,date));
   // Read weather forecast from cursor
   String w_forecast=cursor.getString(ForecastFragment.COL_WEATHER_DESC);
   TextView forecastView=vh.descriptionView;
   forecastView.setText(w_forecast);

    // Read user preference for metric or imperial temperature units
   boolean isMetric = Utility.isMetric(context);

    // Read high temperature from cursor
   double high = cursor.getDouble(ForecastFragment.COL_WEATHER_MAX_TEMP);
   TextView highView = vh.highTempView;
   highView.setText(Utility.formatTemperature(high, isMetric));

    //  Read low temperature from cursor
   double low = cursor.getDouble(ForecastFragment.COL_WEATHER_MIN_TEMP);
   TextView lowView =vh.highTempView
   lowView.setText(Utility.formatTemperature(low, isMetric));
}

done by ViewHolder vh=(ViewHolder)view.getTag();