Android ListView延迟加载-优快云博客

前段在EOE上讨论了延迟加载,然后做了个,模拟延迟加载的List.估计以后会用到吧.毕竟List还是经常遇到的.

UI线程必须是自己更新,不能由其它线程来更新,这个问题已经很多人讨论过了.而Android使用Handler的HandleMessage来处理把控制返回给UI线程.如果直接一个Runnable是不行的.
由于没有到网络上去载图片,所以线程睡眠了500毫秒来模拟.在这里可以用一个线程去下载图片,然后Notify通知图片已经准备好了.
如果有不对的喜欢指正,讨论.把代码放上来.
有个小问题:在开始时显示的是未选中的图片,没有处理初始化加载图片,而主要关注的是滚动时加载图片.

Java代码

package com.me.list;
import java.util.ArrayList;
import java.util.HashMap;
import android.app.Activity;
import android.app.ListActivity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.util.Log;
import android.view.ContextMenu;
import android.view.MenuItem;
import android.view.View;
import android.view.ContextMenu.ContextMenuInfo;
import android.view.LayoutInflater;
import android.view.View.OnCreateContextMenuListener;
import android.view.ViewGroup;
import android.widget.AbsListView;
import android.widget.AbsListView.OnScrollListener;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;
import com.me.*;
import java.util.List;
/**
*
* @author archko
*/
public class ListViewLazy3 extends ListActivity implements OnScrollListener{
private TextView mStatus;
private boolean mBusy= false ;
//private boolean needToUpdate=false;
private static final String TAG= "List13" ;
private AbsListView absListView;
private ArrayList<HashMap<String,Object>> users= new ArrayList<HashMap<String,Object>>();
private Bitmap mIcon1, mIcon2;
private int firstPos= 0 ;
private NoLooperThread noLooerThread;
private MySimpleAdapter adapter;
@Override
public void onCreate(Bundle savedInstanceState){
super .onCreate(savedInstanceState);
setContentView(R.layout.list_13);
mStatus=(TextView)findViewById(R.id.status);
mStatus.setText("Idle" );
for ( int i= 0 ; i< 30 ; i++){
HashMap<String,Object> user=new HashMap<String,Object>();
user.put("img" ,R.drawable.checkoff);
user.put("username" , "姓名(" +i+ ")" );
user.put("age" ,( 20 +i)+ "" );
users.add(user);
}
// Use an existing ListAdapter that will map an array
// of strings to TextViews
adapter=new MySimpleAdapter( this );
setListAdapter(adapter);
getListView().setOnScrollListener(this );
}
//这个方法只要在列表中移动就会调用.
@Override
public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount){
int lastItem=firstVisibleItem+visibleItemCount- 1 ;
Log.d(TAG,"firstPos." +firstPos+ " *firstVisibleItem." +firstVisibleItem); //这个位置不太正确,在我这总是会差一个
if (firstVisibleItem!=firstPos){
firstPos=firstVisibleItem;
//needToUpdate=true;
//if(needToUpdate){ //需要更新,启动线程
this .absListView=view;
//Log.d(TAG,"noLooperThread."+noLooerThread+" isAlive."+noLooerThread.isAlive());
if (noLooerThread!= null ){
//if(noLooerThread.isAlive()){
//noLooerThread.stop();
noLooerThread.interrupt();
Log.d(TAG,"onScroll.noLooerThread.interrupt" );
noLooerThread=null ;
}
noLooerThread=new NoLooperThread();
noLooerThread.start();
//}
}
}
//为什么会对称出现呢?更新5个就是从头开始5个和从尾开始5个?
@Override
public void onScrollStateChanged(AbsListView view, int scrollState){
//没有使用这个,因为这是在鼠标拖动时才会发生,而上面的只要焦点变化都会触发.
//Log.d(TAG,"scrollState."+scrollState);
/*switch(scrollState){
case OnScrollListener.SCROLL_STATE_IDLE:
mBusy=false;
this.absListView=view;
noLooerThread=new NoLooperThread();
noLooerThread.start();
mStatus.setText("Idle");
break;
case OnScrollListener.SCROLL_STATE_TOUCH_SCROLL:
mBusy=true;
mStatus.setText("Touch scroll");
break;
case OnScrollListener.SCROLL_STATE_FLING:
mBusy=true;
mStatus.setText("Fling");
break;
}*/
}
//更新List里的项,这里只作通知数据更新了.
public void update(){
Log.d(TAG,"update." );
adapter.notifyDataSetChanged();
}
ViewHolder holder;
class MySimpleAdapter extends BaseAdapter{
private LayoutInflater mInflater;
//private Bitmap mIcon1, mIcon2;
private Context ctx;
public MySimpleAdapter(Context c){
mInflater=LayoutInflater.from(c);
ctx=c;
// Icons bound to the rows.
mIcon1=BitmapFactory.decodeResource(c.getResources(),R.drawable.checkon);
mIcon2=BitmapFactory.decodeResource(c.getResources(),R.drawable.checkoff);
}
public int getCount(){
return users.size();
}
public Object getItem( int position){
return position;
}
public long getItemId( int position){
return position;
}
//ViewHolder holder;
//而convertView只有在Activity初始化时才会是空.
@Override
// <editor-fold defaultstate="collapsed" desc="comment">
public View getView( int position,View convertView,ViewGroup parent){
if (convertView== null ){
convertView=mInflater.inflate(R.layout.list_items,null );
// Creates a ViewHolder and store references to the two children views
// we want to bind data to.
holder=new ViewHolder();
holder.itemTitle=(TextView)convertView.findViewById(R.id.ItemTitle);
holder.itemText=(TextView)convertView.findViewById(R.id.ItemText);
holder.itemIcon=(ImageView)convertView.findViewById(R.id.ItemImage);
//holder.itemIcon.setImageBitmap(mIcon2);
convertView.setTag(holder);
}else {
// Get the ViewHolder back to get fast access to the TextView and the ImageView.
holder=(ViewHolder)convertView.getTag();
}
holder.itemTitle.setText(users.get(position).get("username" ).toString());
holder.itemText.setText(users.get(position).get("age" ).toString());
int i=Integer.valueOf(users.get(position).get( "img" ).toString());
holder.itemIcon.setImageBitmap(BitmapFactory.decodeResource(ctx.getResources(),i));
//Log.d(TAG,"getView.setIcon");
return convertView;
}// </editor-fold>
}
static class ViewHolder{
TextView itemTitle, itemText;
ImageView itemIcon;
}
class EventHandler extends Handler{
public EventHandler(Looper looper){
super (looper);
}
public EventHandler(){
super ();
}
public void handleMessage(Message msg){
//可以根据msg.what执行不同的处理，这里没有这么做
switch (msg.what){
case 1 :
Log.d(TAG,"1.EventHandler.handleMessage.before" );
update();
Log.d(TAG,"1.EventHandler.handleMessage.after" );
break ;
case 2 :
Log.d(TAG,"2.EventHandler.handleMessage.before" );
update();
Log.d(TAG,"2.EventHandler.handleMessage.after" );
mBusy=true ;
//needToUpdate=false;
break ;
case 3 :
//不能在非主线程的线程里面更新UI，所以这里通过Log打印收到的消息
Log.e(TAG,(String)msg.obj);
//ownLooperThread.stop();
break ;
default :
//不能在非主线程的线程里面更新UI，所以这里通过Log打印收到的消息
Log.e(TAG,(String)msg.obj);
break ;
}
}
}
//这个Thread可以不用这么复杂的.因为当时还没明白Handler,Message所以也是借用别人的代码.简单点就是new Runnable(){run(){
XXX..........
Message msg=new Message();
msg.what=XYZ..............;
mHandler.sendMessage(msg);
}}
Android为每个线程产生一个Looper这个在程序异常时看Logcat就会明白的.
class NoLooperThread extends Thread{
private EventHandler mNoLooperThreadHandler;
private boolean loadIngImg= true ; //正在加载图片
private boolean needToUpdate= false ; //是否需要更新图片列表
private Object lock= new Object();
// <editor-fold defaultstate="collapsed" desc="comment">
class InnerThread extends Thread{
public void run(){
//Log.d(TAG,"NoLooper.run.loadImg.run");
int count=absListView.getChildCount();
int first=absListView.getFirstVisiblePosition();
boolean flag= false ;
for ( int i= 0 ; i<count ; i++){
try {
Thread.sleep(500 );
}catch (InterruptedException ex){
Thread.interrupted();//虽然这里捕获到异常了,不过,它还是会继续执行的.线程不会停止,应该设置共享变量来停止非阻塞的线程,暂时没有使用.
}
if (Integer.valueOf(users.get(first+i).get( "img" ).toString())==R.drawable.checkoff){
users.get(first+i).put("img" ,R.drawable.checkon);
Log.d(TAG,"thead." +Thread.currentThread()+ " update ." +(first+i));
flag=true ;
}
}
synchronized (lock){
lock.notify();
loadIngImg=false ; //加载完毕
if (flag){
needToUpdate=true ; //加载后需要更新
}
Log.d(TAG,"NoLooper.run.loadImg.notify" );
}
}
}// </editor-fold>
public void run(){
InnerThread it=new InnerThread();
synchronized (lock){
it.start();
while (loadIngImg){
try {
lock.wait();
}catch (InterruptedException e){
loadIngImg=false ;
it.interrupt();
}
Log.d(TAG,"needToUpdate." +needToUpdate);
if (needToUpdate){ //需要更新列表,启动线程.
Looper myLooper, mainLooper;
myLooper=Looper.myLooper();
mainLooper=Looper.getMainLooper(); //这是一个static函数
String obj;
if (myLooper== null ){
mNoLooperThreadHandler=new EventHandler(mainLooper);
obj="NoLooperThread has no looper and handleMessage function executed in main thread!" ;
}else {
mNoLooperThreadHandler=new EventHandler(myLooper);
obj="This is from NoLooperThread self and handleMessage function executed in NoLooperThread!" ;
}
Log.d(TAG,"NoLooperThread.run.mNoLooperThreadHandler" );
mNoLooperThreadHandler.removeMessages(0 );
//send message to main thread:
//what=2 when=0 arg1=1 arg2=1 obj=NoLooperThread has no looper and handleMessage
//function executed in main thread!
Message m=mNoLooperThreadHandler.obtainMessage(2 , 1 , 1 ,obj);
mNoLooperThreadHandler.sendMessage(m);
}
loadIngImg=true ;
}
}
}
}
}
list_13这个只是一个ListView和一个文本:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent" >
<ListView android:id="@android:id/list"
android:layout_width="fill_parent"
android:layout_height="0dip"
android:layout_weight="1"
android:drawSelectorOnTop="false" />
<TextView android:id="@+id/status"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:paddingLeft="8dip"
android:paddingRight="8dip" />
</LinearLayout>
行布局只是一个ImageView和两个TextView,没有特别的.
使用Asynctask 是Android专门为线程处理而写的类,封装了一些方法,更易于使用的.

list 异步加载图片