Android之ViewHolder用法

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。

我自己以前的写法：

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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;
}
}

学习过ViewHolder之后的写法：

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@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：

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/*
*
* 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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