简介
上一篇文章介绍了Handler、Message、MessageQueue等Android线程交互方面的内容，Android之理解Looper、Handler、Message、MessageQueue。下面开始实践，学习如何去使用以及应用到程序里面。
实例
在这里使用ListView作为异步下载图片的环境。
1、Handle+Runnable
实现思路是：
1：在UI线程中启动一个线程,让这个线程去下载图片。
2：图片完成下载后发送一个消息去通知UI线程
3：UI线程获取到消息后，更新UI。
package com.example.handlerloadiage;
import java.io.IOException;
import java.net.URL;
import android.app.Activity;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.os.Handler;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.ListView;
public class Handler_Runnable_Mode extends Activity {
private ListView listview;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handlerimageloader);
listview = (ListView) findViewById(R.id.listview);
listview.setAdapter(new MyAdapter());
}
private class MyAdapter extends BaseAdapter {
public MyAdapter() {
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return 100;
}
@Override
public Object getItem(int position) {
// TODO Auto-generated method stub
return null;
}
@Override
public long getItemId(int position) {
// TODO Auto-generated method stub
return 0;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if (convertView == null) {
convertView = LayoutInflater.from(getApplicationContext())
.inflate(R.layout.list_item, null);
}
final ImageView image = (ImageView) convertView
.findViewById(R.id.imageview);
final String imageURL = "http://avatar.csdn.net/D/1/4/3_wangjinyu501.jpg";
Handler handler = new Handler();
handler.post(new Runnable() {
public void run() {
Drawable drawable = null;
try {
drawable = Drawable.createFromStream(
new URL(imageURL).openStream(), "image.jpg");
} catch (IOException e) {
Log.d("test", e.getMessage());
}
if (drawable == null) {
Log.d("test", "null drawable");
} else {
Log.d("test", "not null drawable");
}
if (drawable == null) {
image.setImageResource(R.drawable.ic_launcher);
} else {
image.setImageDrawable(drawable);
}
}
});
return convertView;
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}
效果如下：
快速滑动的过程中，还是出现了ANR的现象。
这是因为handler.post(new Runnable()这个方法，并没有开启一个新的线程，他还是在UI主线程中，所以导致出现ANR现象。
2、Handle+Runnable+Message
实现思路:
实现代码：
package com.example.handlerloadiage;
import java.io.IOException;
import java.net.URL;
import android.app.Activity;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.ListView;
public class Handler_Runnable_Mode extends Activity {
private ListView listview;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_handlerimageloader);
listview = (ListView) findViewById(R.id.listview);
listview.setAdapter(new MyAdapter());
}
private class MyAdapter extends BaseAdapter {
public MyAdapter() {
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return 100;
}
@Override
public Object getItem(int position) {
// TODO Auto-generated method stub
return null;
}
@Override
public long getItemId(int position) {
// TODO Auto-generated method stub
return 0;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
if (convertView == null) {
convertView = LayoutInflater.from(getApplicationContext())
.inflate(R.layout.list_item, null);
}
final ImageView image = (ImageView) convertView
.findViewById(R.id.imageview);
final String imageURL = "http://avatar.csdn.net/D/1/4/3_wangjinyu501.jpg";
final Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Drawable d=(Drawable) msg.obj;
if (d == null) {
image.setImageResource(R.drawable.ic_launcher);
} else {
image.setImageDrawable(d);
}
}
};
handler.post(new Runnable() {
public void run() {
Drawable drawable = null;
try {
drawable = Drawable.createFromStream(
new URL(imageURL).openStream(), "image.jpg");
Message message=handler.obtainMessage();
message.obj=drawable;
handler.sendMessage(message);
} catch (IOException e) {
Log.d("test", e.getMessage());
}
if (drawable == null) {
Log.d("test", "null drawable");
} else {
Log.d("test", "not null drawable");
}
}
});
return convertView;
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}
也是会出现ANR的现象，原因和之前一样。