Android AsyncTask实例

下面是AsyncTask实例，AsyncTask使用说明详见注释。

package com.example.asynctask;import java.io.ByteArrayOutputStream;  import java.io.InputStream;  import org.apache.http.HttpEntity;  import org.apache.http.HttpResponse;  import org.apache.http.HttpStatus;  import org.apache.http.client.HttpClient;  import org.apache.http.client.methods.HttpGet;  import org.apache.http.impl.client.DefaultHttpClient;  import com.example.androidtestproject.R;import android.app.Activity;  import android.os.AsyncTask;  import android.os.Bundle;  import android.util.Log;  import android.view.View;  import android.widget.Button;  import android.widget.ProgressBar;  import android.widget.TextView;    public class MainActivity extends Activity {            private static final String TAG = "ASYNC_TASK";            private Button execute;      private Button cancel;      private ProgressBar progressBar;      private TextView textView;      private MyTask mTask;            @Override      public void onCreate(Bundle savedInstanceState) {          super.onCreate(savedInstanceState);          setContentView(R.layout.main);                    execute = (Button) findViewById(R.id.execute);          execute.setOnClickListener(new View.OnClickListener() {              @Override              public void onClick(View v) {                  //注意每次需new一个实例,新建的任务只能执行一次,否则会出现异常                  mTask = new MyTask();                  mTask.execute("http://www.csdn.net");                                    execute.setEnabled(false);                  cancel.setEnabled(true);              }          });                  cancel = (Button) findViewById(R.id.cancel);          cancel.setOnClickListener(new View.OnClickListener() {              @Override              public void onClick(View v) {                  //取消一个正在执行的任务,onCancelled方法将会被调用            if ((mTask != null)) {            mTask.cancel(true);              }            }          });                  progressBar = (ProgressBar) findViewById(R.id.progress_bar);          textView = (TextView) findViewById(R.id.text_view);      }          private class MyTask extends AsyncTask<String, Integer, String> {              //onPreExecute方法用于在执行后台任务前做一些UI操作          @Override          protected void onPreExecute() {              Log.i(TAG, "onPreExecute() called");              textView.setText("loading...");          }                    //doInBackground方法内部执行后台任务,不可在此方法内修改UI          @Override          protected String doInBackground(String... params) {              Log.i(TAG, "doInBackground(Params... params) called");              try {                  HttpClient client = new DefaultHttpClient();                  HttpGet get = new HttpGet(params[0]);                  HttpResponse response = client.execute(get);                  if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {                      HttpEntity entity = response.getEntity();                      InputStream is = entity.getContent();                      long total = entity.getContentLength();                      ByteArrayOutputStream baos = new ByteArrayOutputStream();                      byte[] buf = new byte[1024];                      long count = 0;                      int length = -1;                    Log.i(TAG, "total=" + total);                    while ((length = is.read(buf)) != -1) {                          if (isCancelled()) {                        Log.i(TAG, "isCancelled=true");                        return null; // don't forget to terminate this method                        }                                                baos.write(buf, 0, length);                          count += length;                          //调用publishProgress公布进度,最后onProgressUpdate方法将被执行                          publishProgress((int) ((count / (float) total) * 100));                          //为了演示进度,休眠50毫秒                          Thread.sleep(50);                    }                                          return new String(baos.toByteArray(), "utf-8");                  }              } catch (Exception e) {                  Log.e(TAG, "exception");              }                          return null;          }                    //onProgressUpdate方法用于更新进度信息          @Override          protected void onProgressUpdate(Integer... progresses) {              Log.i(TAG, "onProgressUpdate(Progress... progresses) called");              progressBar.setProgress(progresses[0]);              textView.setText("loading..." + progresses[0] + "%");          }                    //onPostExecute方法用于在执行完后台任务后更新UI,显示结果          @Override          protected void onPostExecute(String result) {              Log.i(TAG, "onPostExecute(Result result) called");              textView.setText(result);                            execute.setEnabled(true);              cancel.setEnabled(false);        }        //onCancelled方法用于在取消执行中的任务时更改UI@Overrideprotected void onCancelled() {// TODO Auto-generated method stubLog.i(TAG, "onCancelled called");textView.append(", Cancelled");execute.setEnabled(true);cancel.setEnabled(false);}      }@Overrideprotected void onDestroy() {// TODO Auto-generated method stubsuper.onDestroy();Log.i(TAG, "onDestroy called");// 当Activity Destory时判断任务是否还在运行，还在运行的话则取消任务运行，否则Activity Destory掉后mTask还在后台运行，直到结束。if ((mTask != null) && (mTask.getStatus() == AsyncTask.Status.RUNNING)) {Log.i(TAG, "mTask.cancel");mTask.cancel(true);}}          }

Android AsyncTask实例_第1张图片

说明，该实例中

long total = entity.getContentLength();

total可能返回-1，一旦返回-1则进度显示是错误的。只是测试实例，暂且不处理返回-1的情况。

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