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 EfficientAdapter(Context context)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 */ getCount()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 */ getItem(int position)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 */ getItemId(int position)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 */ getView(int position, View convertView, ViewGroup parent)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 onCreate(Bundle savedInstanceState)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