android 异步任务 AsyncTask

android 3,0之后就不允许在主线程进行网络操作，所以在开发中，在联网过程中必须使用异步线程，有俩种方式

1.AsyncTask

2.handler

接下来使用AsyncTask的基本使用示例：从网络下载图片，进度条更新下载进度。

package com.zx.progress;

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

import org.apache.http.HttpResponse;
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.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends Activity {

//图片地址
private String imageUrl="http://f.hiphotos.baidu.com/image/pic/item/a2cc7cd98d1001e93cbe5587ba0e7bec55e797b3.jpg";
private Button btn;
private ImageView iv;
private ProgressDialog dialog;//进度条
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
dialog=new ProgressDialog(this);
dialog.setMessage("正在下载。。。");
dialog.setCancelable(false);
dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
btn=(Button) findViewById(R.id.button1);
iv=(ImageView) findViewById(R.id.imageView1);
btn.setOnClickListener(new OnClickListener() {

@Override
public void onClick(View v) {

new MyTask().execute(imageUrl);
}
});
}

class MyTask extends AsyncTask{

@Override
protected void onPreExecute() {
// TODO Auto-generated method stub
super.onPreExecute();
dialog.show();
}

//耗时任务在该方法中执行
@Override
protected Bitmap doInBackground(String... params) {

Bitmap bitmap=null;
HttpClient client=new DefaultHttpClient();
HttpGet httpGet=new HttpGet(params[0]);
InputStream inputStream=null;
ByteArrayOutputStream outputStream=new ByteArrayOutputStream();
try {

HttpResponse httpResponse=client.execute(httpGet);
if(httpResponse.getStatusLine().getStatusCode()==200){

inputStream=httpResponse.getEntity().getContent();
long file_length=httpResponse.getEntity().getContentLength();//文件总长度
int progress=0;
int len=0;
int value=0;
byte[] data=new byte[1024];//每个读取1024个字节
while((len=inputStream.read(data))!=-1){
progress+=len;
value=(int)(progress/(float)file_length*100);
publishProgress(value);
outputStream.write(data, 0, len);
}
byte[] result=outputStream.toByteArray();//转换为数组
bitmap= BitmapFactory.decodeByteArray(result, 0, result.length);
}
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
}
finally{

}
return bitmap;
}

@Override
protected void onProgressUpdate(Integer... values) {
// TODO Auto-generated method stub
super.onProgressUpdate(values);
dialog.setProgress(values[0]);//更新进度
}

@Override
protected void onPostExecute(Bitmap result) {
// TODO Auto-generated method stub
super.onPostExecute(result);
dialog.dismiss();
iv.setImageBitmap(result);
}

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}

}

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