安卓AsyncTask異步下載圖片代碼

package com.example.day7_practise;

import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
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.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;

public class MainActivity extends Activity{
ImageView iv;
Button btn1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
iv=(ImageView) findViewById(R.id.imageView1);
btn1=(Button) findViewById(R.id.btn1);
btn1.setOnClickListener(new OnClickListener() {

@Override
public void onClick(View v) {
DownloadTask mTask = new DownloadTask();
String uri = "https://ss0.bdstatic.com/5aV1bjqh_Q23odCf/static/superman/img/logo/logo_white_fe6da1ec.png";
mTask.execute(uri);
}
});
}

private class DownloadTask extends AsyncTask<String,Integer , Bitmap> {
ProgressDialog pDialog;
/**
* 參數1為doInBackground參數 由onClick方法中execute設置的參數
* 參數2為onProgressUpdate參數 由publishProgress（方法用在子線程中）傳入。
* 參數3為onPostExecute參數 由doInBackground方法返傳入
*/
// (主線程)在子線程執行之前調用它
@Override
protected void onPreExecute() {
//創建對話框
pDialog = new ProgressDialog(MainActivity.this);
//創建標題欄
pDialog.setTitle("下載圖片");
//設置進度條樣式
pDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
pDialog.setMax(100);
pDialog.setMessage("正在下載");
pDialog.show();
}

// (子線程)執行子線程
@Override
protected Bitmap doInBackground(String... params) {
HttpURLConnection conn = null;
URL url;
try {
url = new URL(params[0]);
conn=(HttpURLConnection) url.openConnection();
int code=conn.getResponseCode();
if(code==200){
//取出流中圖片信息解析成圖片
Bitmap bmap=BitmapFactory.decodeStream(conn.getInputStream());
return bmap;
}else {
throw new RuntimeException("網絡訪問失敗");
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}finally {
if(conn!=null){
conn.disconnect();
conn=null;
}
}
return null;
}

// (主線程)在子線程執行時更新UI界面
@Override
protected void onProgressUpdate(Integer... values) {
//pDialog.setProgress(value);
}

// (主線程)在子線程執行之後執行，擦屁股專用
@Override
protected void onPostExecute(Bitmap bmap) {
if(bmap==null){
Toast.makeText(MainActivity.this, "下載失敗", 0).show();
}else {
iv.setImageBitmap(bmap);
}
pDialog.cancel();
}
}
}