android 异步 执行 任务 例子 附讲解

代码说明：

Rules::The AsyncTask instance must be created in UI thread. .execute must be invoked on the UI thread.Never call  objMyTask.onPreExecute(), objMyTask.doInBackground(), objMyTask.onProgressUpdate(),  objMyTask.onPostExecute manually.The AsyncTask can be executed only once (an exception will be thrown if a second execution is attempted.)AsyncTask have Four Main Method... onPreExecute()  doInBackground() onProgressUpdate() onPostExecute()  onPreExecute-This method is called first when you start AsyncTask using objAsync.execute().And mostly this method is use for initializing dialog(ProgressDialog,CustomDialog) and showing. doInBackground-The main purpose of AsyncTask is accomplished by this method.Any non-UI thread process is running in this method.Such as Rss Feed Reader,Image and video Uploading and Downloading.You cant handle your View in this method.Because this method is non-UI thread.While any background process is running if you want to handle UI therea are  onProgressUpdate method. after completion of process this method send result to OnPostExecute. onProgressUpdate-While backgrounding task is running ,you can handle your UI using this method .Such as status of downloading or uploading task.and this method is called from  doInBackground.Using publishProgress() you can call onProgressUpdate method to update UI while process is running. onPostExecute -This method is called after the background computation finishes.The result of background process in passed in this method as parameters.And now you can dismiss progress dialog ,to indicate that background task is completed. You can cancel AsyncTask using objAsyncTask.cancel().then you just check in doInBackground, if (isCancelled()) { break; } else {        //continue... } See this Image For more Clear.