Android之ViewHolder用法
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先声明一下ViewHolder在Android自定义的适配器中使用。目的:优化资源,节省空间,避免重复绘制view而引起的不必要的内存损耗。
学习过ViewHolder之后的写法:
看一下官方的API:
我自己以前的写法:
- public class PlateAdapter extends BaseAdapter {
- private List<Plate> list;
- private Context context;
- public PlateAdapter(List<Plate> list, Context context) {
- super();
- this.list = list;
- this.context = context;
- }
- @Override
- public int getCount() {
- // TODO Auto-generated method stub
- return list.size();
- }
- @Override
- public Object getItem(int arg0) {
- // TODO Auto-generated method stub
- return list.get(arg0);
- }
- @Override
- public long getItemId(int arg0) {
- // TODO Auto-generated method stub
- return arg0;
- }
- @Override
- public View getView(int arg0, View arg1, ViewGroup arg2) {
- // TODO Auto-generated method stub
- if(arg1 == null){
- arg1 = LayoutInflater.from(context).inflate(R.layout.select_car_type_list_item, null);
- }
- TextView text = (TextView)arg1.findViewById(R.id.text);
- text.setText(list.get(arg0).getPlateType());
- return arg1;
- }
- }
- @SuppressWarnings("unused")
- public class NoticeAdapter extends BaseAdapter{
- private Context _context;
- private List<ExamNotice> _list;
- public NoticeAdapter(Context context, List<ExamNotice> list) {
- super();
- this._context = context;
- this._list = list;
- }
- public void set_list(List<ExamNotice> _list) {
- this._list = _list;
- }
- @Override
- public int getCount() {
- return _list.size();
- }
- @Override
- public Object getItem(int arg0) {
- return _list.get(arg0);
- }
- @Override
- public long getItemId(int arg0) {
- return arg0;
- }
- @Override
- public View getView(int position, View convertView, ViewGroup parent) {
- Holder holder;
- if(null==convertView){
- convertView=View.inflate(_context, R.layout.notice_item, null);
- holder=new Holder();
- holder.studyPlanName=(TextView)convertView.findViewById(R.id.xxjh_item_name);
- holder.studyPlanDate=(TextView)convertView.findViewById(R.id.xxjh_item_date);
- convertView.setTag(holder);
- }else{
- holder=(Holder)convertView.getTag();
- }
- ExamNotice notice=(ExamNotice) getItem(position);
- holder.studyPlanName.setText(notice.getNoticeTitle());
- SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
- String noticeDate = sdf.format(notice.getNoticeDate());
- holder.studyPlanDate.setText(noticeDate);
- return convertView;
- }
- private static class Holder{
- public TextView studyPlanName,studyPlanDate;
- }
- }
看一下官方的API:
- /*
- * Copyright (C) 2008 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
- package com.example.android.apis.view;
- import android.app.ListActivity;
- import android.content.Context;
- import android.os.Bundle;
- import android.view.LayoutInflater;
- import android.view.View;
- import android.view.ViewGroup;
- import android.widget.BaseAdapter;
- import android.widget.TextView;
- import android.widget.ImageView;
- import android.graphics.BitmapFactory;
- import android.graphics.Bitmap;
- import com.example.android.apis.R;
- /**
- * Demonstrates how to write an efficient list adapter. The adapter used in this example binds
- * to an ImageView and to a TextView for each row in the list.
- *
- * To work efficiently the adapter implemented here uses two techniques:
- * - It reuses the convertView passed to getView() to avoid inflating View when it is not necessary
- * - It uses the ViewHolder pattern to avoid calling findViewById() when it is not necessary
- *
- * The ViewHolder pattern consists in storing a data structure in the tag of the view returned by
- * getView(). This data structures contains references to the views we want to bind data to, thus
- * avoiding calls to findViewById() every time getView() is invoked.
- */
- public class List14 extends ListActivity {
- private static class EfficientAdapter extends BaseAdapter {
- private LayoutInflater mInflater;
- private Bitmap mIcon1;
- private Bitmap mIcon2;
- public EfficientAdapter(Context context) {
- // Cache the LayoutInflate to avoid asking for a new one each time.
- mInflater = LayoutInflater.from(context);
- // Icons bound to the rows.
- mIcon1 = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon48x48_1);
- mIcon2 = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon48x48_2);
- }
- /**
- * The number of items in the list is determined by the number of speeches
- * in our array.
- *
- * @see android.widget.ListAdapter#getCount()
- */
- public int getCount() {
- return DATA.length;
- }
- /**
- * Since the data comes from an array, just returning the index is
- * sufficent to get at the data. If we were using a more complex data
- * structure, we would return whatever object represents one row in the
- * list.
- *
- * @see android.widget.ListAdapter#getItem(int)
- */
- public Object getItem(int position) {
- return position;
- }
- /**
- * Use the array index as a unique id.
- *
- * @see android.widget.ListAdapter#getItemId(int)
- */
- public long getItemId(int position) {
- return position;
- }
- /**
- * Make a view to hold each row.
- *
- * @see android.widget.ListAdapter#getView(int, android.view.View,
- * android.view.ViewGroup)
- */
- public View getView(int position, View convertView, ViewGroup parent) {
- // A ViewHolder keeps references to children views to avoid unneccessary calls
- // to findViewById() on each row.
- ViewHolder holder;
- // When convertView is not null, we can reuse it directly, there is no need
- // to reinflate it. We only inflate a new View when the convertView supplied
- // by ListView is null.
- if (convertView == null) {
- convertView = mInflater.inflate(R.layout.list_item_icon_text, null);
- // Creates a ViewHolder and store references to the two children views
- // we want to bind data to.
- holder = new ViewHolder();
- holder.text = (TextView) convertView.findViewById(R.id.text);
- holder.icon = (ImageView) convertView.findViewById(R.id.icon);
- convertView.setTag(holder);
- } else {
- // Get the ViewHolder back to get fast access to the TextView
- // and the ImageView.
- holder = (ViewHolder) convertView.getTag();
- }
- // Bind the data efficiently with the holder.
- holder.text.setText(DATA[position]);
- holder.icon.setImageBitmap((position & 1) == 1 ? mIcon1 : mIcon2);
- return convertView;
- }
- static class ViewHolder {
- TextView text;
- ImageView icon;
- }
- }
- @Override
- public void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- setListAdapter(new EfficientAdapter(this));
- }
- private static final String[] DATA = Cheeses.sCheeseStrings;
- }
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