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      1 /*
      2  * Copyright (C) 2008 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package com.example.android.apis.view;
     18 
     19 import android.app.ListActivity;
     20 import android.content.Context;
     21 import android.os.Bundle;
     22 import android.view.LayoutInflater;
     23 import android.view.View;
     24 import android.view.ViewGroup;
     25 import android.widget.BaseAdapter;
     26 import android.widget.TextView;
     27 import android.widget.ImageView;
     28 import android.graphics.BitmapFactory;
     29 import android.graphics.Bitmap;
     30 import com.example.android.apis.R;
     31 
     32 /**
     33  * Demonstrates how to write an efficient list adapter. The adapter used in this example binds
     34  * to an ImageView and to a TextView for each row in the list.
     35  *
     36  * To work efficiently the adapter implemented here uses two techniques:
     37  * - It reuses the convertView passed to getView() to avoid inflating View when it is not necessary
     38  * - It uses the ViewHolder pattern to avoid calling findViewById() when it is not necessary
     39  *
     40  * The ViewHolder pattern consists in storing a data structure in the tag of the view returned by
     41  * getView(). This data structures contains references to the views we want to bind data to, thus
     42  * avoiding calls to findViewById() every time getView() is invoked.
     43  */
     44 public class List14 extends ListActivity {
     45 
     46     private static class EfficientAdapter extends BaseAdapter {
     47         private LayoutInflater mInflater;
     48         private Bitmap mIcon1;
     49         private Bitmap mIcon2;
     50 
     51         public EfficientAdapter(Context context) {
     52             // Cache the LayoutInflate to avoid asking for a new one each time.
     53             mInflater = LayoutInflater.from(context);
     54 
     55             // Icons bound to the rows.
     56             mIcon1 = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon48x48_1);
     57             mIcon2 = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon48x48_2);
     58         }
     59 
     60         /**
     61          * The number of items in the list is determined by the number of speeches
     62          * in our array.
     63          *
     64          * @see android.widget.ListAdapter#getCount()
     65          */
     66         public int getCount() {
     67             return DATA.length;
     68         }
     69 
     70         /**
     71          * Since the data comes from an array, just returning the index is
     72          * sufficent to get at the data. If we were using a more complex data
     73          * structure, we would return whatever object represents one row in the
     74          * list.
     75          *
     76          * @see android.widget.ListAdapter#getItem(int)
     77          */
     78         public Object getItem(int position) {
     79             return position;
     80         }
     81 
     82         /**
     83          * Use the array index as a unique id.
     84          *
     85          * @see android.widget.ListAdapter#getItemId(int)
     86          */
     87         public long getItemId(int position) {
     88             return position;
     89         }
     90 
     91         /**
     92          * Make a view to hold each row.
     93          *
     94          * @see android.widget.ListAdapter#getView(int, android.view.View,
     95          *      android.view.ViewGroup)
     96          */
     97         public View getView(int position, View convertView, ViewGroup parent) {
     98             // A ViewHolder keeps references to children views to avoid unneccessary calls
     99             // to findViewById() on each row.
    100             ViewHolder holder;
    101 
    102             // When convertView is not null, we can reuse it directly, there is no need
    103             // to reinflate it. We only inflate a new View when the convertView supplied
    104             // by ListView is null.
    105             if (convertView == null) {
    106                 convertView = mInflater.inflate(R.layout.list_item_icon_text, null);
    107 
    108                 // Creates a ViewHolder and store references to the two children views
    109                 // we want to bind data to.
    110                 holder = new ViewHolder();
    111                 holder.text = (TextView) convertView.findViewById(R.id.text);
    112                 holder.icon = (ImageView) convertView.findViewById(R.id.icon);
    113 
    114                 convertView.setTag(holder);
    115             } else {
    116                 // Get the ViewHolder back to get fast access to the TextView
    117                 // and the ImageView.
    118                 holder = (ViewHolder) convertView.getTag();
    119             }
    120 
    121             // Bind the data efficiently with the holder.
    122             holder.text.setText(DATA[position]);
    123             holder.icon.setImageBitmap((position & 1) == 1 ? mIcon1 : mIcon2);
    124 
    125             return convertView;
    126         }
    127 
    128         static class ViewHolder {
    129             TextView text;
    130             ImageView icon;
    131         }
    132     }
    133 
    134     @Override
    135     public void onCreate(Bundle savedInstanceState) {
    136         super.onCreate(savedInstanceState);
    137         setListAdapter(new EfficientAdapter(this));
    138     }
    139 
    140     private static final String[] DATA = Cheeses.sCheeseStrings;
    141 }
    142