从sdk的文档中看到，当一个AsyncTask运行的过程中，经历了4个步骤:
1、onPreExecute(), 在excute调用后立即在ui线程中执行。 This step is normally used to setup the task, for instance by showing a progress bar in the user interface.
2、doInBackground, 当 onPreExecute() 完成后, 立即在后台线程中运行. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate step.
3、onProgressUpdate, 在调用publishProgress后，在ui线程中运行. The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field.
4、onPostExecute, 后台运算完成时在ui线程中调用. The result of the background computation is passed to this step as a parameter.
