Lab project update 3: asyncTask, db file export

This is a minor update, to add the function of exporting local db file onto SD card. The reason why we do export instead of directly dragging the db file using some file explorer app it because, when testing on device, unless you root your phone, you will not be able to access your db files using apps.

Looking through android doc (btw, newest official site looks absolutely sexy) and stack overflow, I decide to use AsyncTask class to do this background job. It looks like Service, but is easier to use to communicate with the main UI thread. Unlike Service class, in which you have to care about when to create, start, handle message, stop the thread, this class provides exact functions that wrap up those details, including one before you start the task (onPreExecute), one to do background job (doInBackground), one to update main thread if you want to (onProgressUpdate) and one to return some results after the task is finished (onPostExecute). Details could be found in the doc.

Normally, it requires to override at least the doInBackground method, also in most of the time the onPostExecute method. My second method is nothing new, just making a Toast to indicate whether file has been correctly exported. My first method:

Let's go through each step. Line 3 is to locate the db file you want to export. I use the Environment class to obtain path info. Note in android, the db file of an app is created in the path "/data/data/your.package.name/databases/your_db_name.db".  The method getDataDirectory() will return the first "/data" therefore for LOCTABLE_PATH you only need to add the path after it. Line 5 gets the external dic state, which I use in line 6 to detect if the SD card is writable, defined as MEDIA_MOUNTED. If SD card is not available, I will just return a Toast and finish the task.

Line 7 calls the getExternalStorageDirectory() to obtain the SD card dir, which should be "/mnt/sdcard", you could define the dir you would like to save your db file as EXPORT_PATH. The following if statement is to check if the path you want already exists, otherwise create it. Line 11 is to create a file object at your given path. Note currently you haven't created an actual file, you just create an object and make it ready to generate a file. Also, in order to write to the external disc, you have to add following permission in your AndroidManifest.xml:

Lastly the try block is to create the file and copy your db file to it. The copyfile method should be defined by you according to what kind of file you want. I recommend just copying raw content into a .db file and then open/read it in a db browser like this. Raw file copying method in java could be found here.