Android初学-AsyncTask下载网络图片

AsyncTask 异步处理：

mainfest:
注意添加的：

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.INTERNET" />

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name=".Main2Activity"></activity>
</application>

---

代码 ：MainActivity.java

package com.example.wgscd.myapplication;

import android.graphics.BitmapFactory;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.graphics.Bitmap;
import android.os.AsyncTask;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

import static android.R.attr.bitmap;

public class MainActivity extends AppCompatActivity {
Button btn1=null;
TextView txtView1=null;
ProgressBar progressBar;
int j=1;
String msg="";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btn1=(Button) findViewById(R.id.btn1);
txtView1=(TextView)findViewById(R.id.textView1);
progressBar = (ProgressBar)findViewById(R.id.progressBar);
progressBar.setVisibility(View.INVISIBLE);
}

protected void btn1_Click(View view){
  //  txtView1.setText(  j +"");
 //   j++;
    downLoadPictue();

}


void downLoadPictue(){
    DownImageTask asyncTask = new DownImageTask();
    asyncTask.execute("http://www.xitongzijia.net/meinv/e/data/tmp/titlepic/pic43847d162a1452bfeb482be70d24bff9e5c99_200.jpg");
}

class DownImageTask  extends AsyncTask<String, Integer, Bitmap> {
    // 执行预处理
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        // 显示进度条
        progressBar.setVisibility(View.VISIBLE);
        progressBar.setMax(100);

    }
    // 后台进程的执行
    @Override
    protected Bitmap doInBackground(String... params) {

        Bitmap   bitmap=null;
        try {
            URL url = new URL(params[0]);
            HttpURLConnection conn = (HttpURLConnection) url
             .openConnection();
            // 进度条的更新，我这边只是用一个循环来示范，在实际应用中要使用已下载文件的大小和文件总大小的比例来更新
            for (int i = 1; i <= 10; i++) {
               publishProgress(i * 10);
               Thread.sleep(200);
            }
           InputStream inputStream = conn.getInputStream();
            bitmap = BitmapFactory.decodeStream(inputStream);
            inputStream.close();


        } catch (Exception e) {
            e.printStackTrace();
            msg= e.getMessage();
        }
        return bitmap;
    }
    // 运行于UI线程，对后台任务的结果做出处理，doInBackground方法执行的结果作为此方法的参数
    @Override
    protected void onPostExecute(Bitmap result) {
        super.onPostExecute(result);
       ImageView imageView = (ImageView) findViewById(R.id.imageView);
       imageView.setImageBitmap(result);
        progressBar.setVisibility(View.GONE);

    }
    // 运行于UI线程，如果在doInBackground(Params...)中使用了publishProgress(Progress...)，就会触发此方法
    @Override
    protected void onProgressUpdate(Integer... values) {
        super.onProgressUpdate(values);
        txtView1.setText( ""+ values[0] +"%" + msg);
        progressBar.setProgress(values[0]);

    }
}

}