Android札记（十三）AsyncTask（二）

通过onProgressUpdate()来反馈当前任务的执行进度

一、建立布局，通过点击按钮来执行异步任务。

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent"    android:orientation="vertical" >    <Button        android:id="@+id/button"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:text="下载图片" />    <ImageView        android:id="@+id/image"        android:layout_width="match_parent"        android:layout_height="wrap_content"        android:src="@drawable/ic_launcher" /></LinearLayout>

二、MainAcitivity
在编写代码之前，需要给任务一个权限来实现网络下载。
在注册文件中加入权限
uses-permission android:name=”android.permission.INTERNET”

public class MainActivity extends ActionBarActivity {    private Button button;    private ImageView image;    private String image_path = "http://img1.3lian.com/img2008/06/019/13.jpg";    private ProgressDialog dialog;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        button = (Button) findViewById(R.id.button);        image = (ImageView) findViewById(R.id.image);        dialog = new ProgressDialog(this);        dialog.setMessage("正在下载，请稍候...");        dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);        dialog.setCancelable(false);        button.setOnClickListener(new OnClickListener() {            @Override            public void onClick(View v) {                // TODO Auto-generated method stub                new Mytask().execute(image_path);            }        });    }    public class Mytask extends AsyncTask<String, Integer, Bitmap> {        @Override        protected Bitmap doInBackground(String... params) {            // TODO Auto-generated method stub            HttpClient httpClient = new DefaultHttpClient();            HttpGet httpGet = new HttpGet(params[0]);            Bitmap bitmap = null;            ByteArrayOutputStream out = new ByteArrayOutputStream();            InputStream input = null;            try {                HttpResponse httpResponse = httpClient.execute(httpGet);                if (httpResponse.getStatusLine().getStatusCode() == 200) {                    input = httpResponse.getEntity().getContent();                    long file_length = httpResponse.getEntity()                            .getContentLength();                    int len = 0;                    byte[] data = new byte[1024];                    int total_length = 0;//已下载长度                    while ((len = input.read(data)) != -1) {//读取data.length个字节存入data中                        total_length += len;                        int value = (int) ((total_length / file_length) * 100);                        publishProgress(value);//发布进度                        out.write(data, 0, len);//把data中的从0到len个字节写入out输出流，因为不一定是读取了data.length个字节，实际运行往往有偏差                    }                    byte[] result = out.toByteArray();                    bitmap = BitmapFactory.decodeByteArray(result, 0,                            result.length);                }            } catch (IOException e) {                // TODO Auto-generated catch block                e.printStackTrace();            } finally {                if (input != null)                    try {                        input.close();                    } catch (IOException e) {                        // TODO Auto-generated catch block                        e.printStackTrace();                    }            }            return bitmap;        }        @Override        protected void onPreExecute() {            // TODO Auto-generated method stub            super.onPreExecute();            dialog.show();        }        @Override        protected void onProgressUpdate(Integer... values) {            // TODO Auto-generated method stub            super.onProgressUpdate(values);            dialog.setProgress(values[0]);        }        @Override        protected void onPostExecute(Bitmap result) {            // TODO Auto-generated method stub            super.onPostExecute(result);            dialog.dismiss();            image.setImageBitmap(result);        }    }    }

三、效果如图所示