入门android异步任务之AsyncTask（以加载网络图片为例子）

示例最终效果图：

 

AndroidManifest.xml

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.lesson"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="7"
        android:targetSdkVersion="7" />
    
        <!-- 用于访问网络的授权 -->
    <uses-permission android:name="android.permission.INTERNET"/>
  

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        
        <activity android:name="com.lesson.MainActivity">
             <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

 基本布局文件activity_main.xml：一个ImageView和一个Button

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent" >

    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="48dp"/>
        

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:text="下载图片" />

</RelativeLayout>

 MainActivity.java

/*
 * 异步任务实现activity中加载网络图片
 */
package com.lesson;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;

import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;

import android.app.Activity;
import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.test.AndroidTestCase;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends Activity {
	
private Button button;
private ProgressDialog progerssDialog;
private ImageView imageView;//要加载的图片
private final String url="http://www.windows7en.com/desk/UploadFiles_7433/201006/2010060910062279.jpg";

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		
		super.onCreate(savedInstanceState);
		this.setContentView(R.layout.activity_main);
		button=(Button)findViewById(R.id.button1);
		imageView=(ImageView)findViewById(R.id.imageView1);
		progerssDialog=new ProgressDialog(this);
		progerssDialog.setCancelable(false);
		//设置进度条水平加载
				progerssDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
		button.setOnClickListener(new OnClickListener() {
		
			@Override
			public void onClick(View arg0) {
				//异步任务调用
				new ImageUpdate().execute(url);
				
			}
		});
	}
class ImageUpdate extends AsyncTask<String, Integer,byte[]>{

	//线程启动时，该方法被调用次序:2
	@Override
	protected byte[] doInBackground(String... arg0) {
		HttpGet httpGet=new HttpGet(arg0[0]);
		InputStream inputStream=null;
		byte[] result=null;
		byte[] data=new byte[1024];
		HttpClient client = new DefaultHttpClient();
		ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
		try {
			HttpResponse httpResponse=client.execute(httpGet);//得到响应
			if(httpResponse.getStatusLine().getStatusCode()==200){
				
				inputStream=httpResponse.getEntity().getContent();
				//文件总长度
				int filelenth=(int) httpResponse.getEntity().getContentLength();
				int readlength=0;
				int totallenth=0;
				while((readlength=inputStream.read(data))!=-1){
					totallenth+=readlength;
                                 //用于进度条刻度计算公式
					int progressValue= (int)((totallenth/(float)filelenth)*100);
                                 //将刻度值传递到第2步
					publishProgress(progressValue);
					outputStream.write(data, 0, readlength);
				}
				result=outputStream.toByteArray();
			}
			
		} catch (ClientProtocolException e) {
			
			e.printStackTrace();
		} catch (IOException e) {
			
			e.printStackTrace();
		}finally{
			client.getConnectionManager().shutdown();
		}
		return result;
	}

//线程启动时，该方法被调用次序:4

@Override protected void onPostExecute(byte[] result) { 
// TODO Auto-generated method stub 
super.onPostExecute(result);
 Bitmap bitmap=BitmapFactory.decodeByteArray(result, 0, result.length); 
imageView.setImageBitmap(bitmap); 
progerssDialog.dismiss(); 
}

//线程启动时，该方法被调用次序:1

@Override protected void onPreExecute() { 
super.onPreExecute(); 
//加载图片时的进度显示对话框
 progerssDialog.show(); 
}

//线程启动时，该方法被调用次序:3

@Override protected void onProgressUpdate(Integer... values) { 
 super.onProgressUpdate(values);
 progerssDialog.setProgress(values[0]);
 }
 } 
}