【Android】AsyncTask实现异步处理

AsyncTask提供了简单易用的方式，执行后台操作并更新UI。

AsyncTask的3个泛型

Param 传入数据类型
Progress 更新UI数据类型
Result 处理结果类型

AsyncTask的4个步骤

onPreExecute 执行前的操作
doInBackGround 后台执行的操作
onProgressUpdate 更新UI操作
onPostExecute 执行后的操作

示例代码：

下载远端资源，需要INTERNET权限
将资源写入到SD，需要WRITE_EXTERNAL_STORAGE权限
在AndroidManifest.xml中进行如下配置

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

功能：

异步下载，并根据下载进步更新ProgresBar，下载完成后显示图像

package lizhen.download;import java.io.File;import java.io.FileOutputStream;import java.io.IOException;import java.io.InputStream;import org.apache.http.HttpEntity;import org.apache.http.HttpResponse;import org.apache.http.HttpStatus;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.os.AsyncTask;public class DownloadAsyncTask extends AsyncTask<String, Integer, Boolean>{protected int size;protected String errorMessage;@Overrideprotected Boolean doInBackground(String... params) {String url = params[0]; //資源地址String path = params[1]; //目標路徑try {InputStream source = requestInputStream(url); //請求源文件InputStream/* * 創建文件寫入數據，并更新進度 * */fileWrite(source, path);} catch (ClientProtocolException e) {errorMessage = e.getMessage();cancel(true);return false;} catch (IOException e) {errorMessage = e.getMessage();cancel(true);return false;}return true;}/** * 文件寫入 * @param in 數據源輸入流 * @param path 文件路徑 * @param listener 下載進度監聽器 * */private void fileWrite(InputStream in, String path) throws IOException {File file = createFile(path);FileOutputStream fileOutputStream = new FileOutputStream(file);byte buffer[] = new byte[1024];int progress = 0;        int readBytes = 0;        while ((readBytes = in.read(buffer)) != -1) {        progress += readBytes;             fileOutputStream.write(buffer, 0, readBytes);            publishProgress(progress);        }        in.close();        fileOutputStream.close();}/** * 根據資源URL地址取得資源輸入流 * @param url URL地址 * @return 資源輸入流 * @throws ClientProtocolException * @throws IOException * */private InputStream requestInputStream(String url) throws ClientProtocolException, IOException {InputStream result = null;HttpGet httpGet = new HttpGet(url);HttpClient httpClient = new DefaultHttpClient();HttpResponse httpResponse = httpClient.execute(httpGet);int httpStatusCode = httpResponse.getStatusLine().getStatusCode();if(httpStatusCode == HttpStatus.SC_OK) {HttpEntity httpEntity = httpResponse.getEntity();size = (int) httpEntity.getContentLength();result = httpEntity.getContent();}return result;}/** * 根據文件路徑創建文件 * @param path 文件路徑 * @return 文件File實例 * @return IOException * */private File createFile(String path) throws IOException {File file = new File(path);file.createNewFile();return file;}/** * 返回錯誤信息 * @return 錯誤信息 * */public String getErrorString() {return this.errorMessage;}}

package lizhen.download;import android.app.Activity;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.Bundle;import android.os.Environment;import android.view.View;import android.widget.Button;import android.widget.ImageView;import android.widget.ProgressBar;import android.widget.TextView;import android.widget.Toast;public class Download extends Activity {     private Button startButton;private ProgressBar downloadProgressBar;private TextView progressTextView;private ImageView downloadImageView;private final String source = "http://192.168.211.86/comic/test.jpg"; //源文件地址private final String path = Environment.getExternalStorageDirectory().toString()+"/test.jpg"; //目標文件地址    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.download);                startButton = (Button) findViewById(R.id.download_StartButton);        downloadProgressBar = (ProgressBar) findViewById(R.id.download_ProgressBar);        progressTextView = (TextView) findViewById(R.id.download_ProgressTextView);        downloadImageView = (ImageView) findViewById(R.id.download_ImageView);                startButton.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View v) {new DownloadTask().execute(source, path); }});    }        private class DownloadTask extends DownloadAsyncTask {@Overrideprotected void onProgressUpdate(Integer... values) {super.onProgressUpdate(values);int progress = values[0];/* * 更新進度條和下載百分率 * */downloadProgressBar.setMax(size);downloadProgressBar.setProgress(progress);int percentage = progress*100/size;progressTextView.setText("已完成"+percentage+"%");}@Overrideprotected void onPostExecute(Boolean result) {super.onPostExecute(result);if(result) {Bitmap bitmap = BitmapFactory.decodeFile(path);downloadImageView.setImageBitmap(bitmap);} else {Toast.makeText(Download.this, "Error: "+errorMessage, 1000).show();}}        }}

运行结果：

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