When an asynchronous task is executed, the task goes through 4 steps:

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1. onPreExecute(), invoked on the UI thread before the task     is executed. This step is normally used to setup the task, for instance by     showing a progress bar in the user interface.

2. doInBackground(Params...), invoked on the background thread     immediately after onPreExecute() finishes executing. 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(Progress...) to publish one or more units     of progress. These values are published on the UI thread, in the      onProgressUpdate(Progress...) step.

3. onProgressUpdate(Progress...), invoked on the UI thread after a     call to publishProgress(Progress...). 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(Result), invoked on the UI thread after the background     computation finishes. The result of the background computation is passed to     this step as a parameter.

当在主线程中调用new MyTask().execute(args)时，触发

execute（）将参数传给doInBackground（）

doInBackground（）返回值传给onPostExecute()

在doInBackground()中调用的publishProgress()的参数，传递给onProgressUpdate()

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