AsyncTask详解

摘要：注意每次需一个实例新建的任务只能执行一次否则会出现异常取消一个正在执行的任务方法将会被调用方法用于在执行后台任务前做一些操作方法内部执行后台任务不可在此方法内修改调用公布进度最后方法将被执行为了演示进度休眠毫秒方法用于更新进度信息方法用

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.baidu.com");  
  
                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  
                mTask.cancel(true);  
            }  
        });  
        progressBar = (ProgressBar) findViewById(R.id.progress_bar);  
        textView = (TextView) findViewById(R.id.text_view);  
  
    }  
  
    private class MyTask extends AsyncTask {  
        //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];  
                    int count = 0;  
                    int length = -1;  
                    while ((length = is.read(buf)) != -1) {  
                        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500毫秒  
                        Thread.sleep(500);  
                    }  
                    return new String(baos.toByteArray(), "gb2312");  
                }  
            } catch (Exception e) {  
                Log.e(TAG, e.getMessage());  
            }  
            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  
        @Override  
        protected void onCancelled() {  
            Log.i(TAG, "onCancelled() called");  
            textView.setText("cancelled");  
            progressBar.setProgress(0);  
  
            execute.setEnabled(true);  
            cancel.setEnabled(false);  
        }  
    }  
}