android design considerations: AsyncTask vs Service (IntentService?)

Question

I'm designing an android app which will need to do the following steps:

1. user pushes a button or otherwise indicates to "sync data".
2. sync process will use REST web services to move data to and from the server.
3. the data will be stored locally in a sqlite database.
4. the sync process should provide status updates/messages to the UI
5. the user should not be allowed to wander off to other parts of the application and do more work during the sync process.

The first time the sync process runs, it may take 10-20 minutes. After the initial sync, less data will be transferred and stored and I expect the process to take 1-2 minutes or less.

I've been doing a lot of reading about android's AsyncTask and various examples of using a Service ... But I don't fully understand the design considerations and trade-offs of choosing one design over the other. I currently have my demo project stubbed out using an AsyncTask. After watching (most of) Developing Android REST client applications: http://code.google.com/events/io/2010/sessions/developing-RESTful-android-apps.html# I'm left confused the design patterns described here feel overly complex, perhaps because I just "don't get it" yet.

I come from a java, spring, web and desktop application background. Thinking and designing in terms of a handheld device is quite new to me. (What happens when the screen layout is changed? What happens when the phone rings while I'm running a sync?) Taking 2 steps back, if the initial sync IS going to be such a long running process, is there a better way for me to think about the problem->solution, the user experience, the user expectations of an application running on a phone?

Would love to hear from some more experienced android developers out there who have already wrestled with these questions.

Answer 1

In my opinion this is the most tricky/hard part of a mainstream/average Android development. For instance on BlackBerry this is IN TIMES easier.

Definitely you need to use a Service.

AsyncTask does not suit, because it is tightly "bound" to your Activity via a Context handle (otherwise you would not be able to update UI of the Activity from your AsyncTask). However an Activity can be killed by OS once the Activity went in background. An example reason of going to background can be an incoming call - user switches to Phone application so your Activity becomes invisible. In this case (depending on the current RAM state) OS may decide to kill one of the background (invisible to the user) activities.

Some devs workaround this by arranging a static stuff for having a long-running actions inside of. Some recommend to use Application instance. This is because static stuff and Application exist while the whole app process exists. However those are incorrect workarounds. Processes in Android are also may be killed when OS decides it is time to. Android OS have its own considerations about what it can kill and in what order. All processes are devided to 5 levels of "killability". Here is the doc where those levels are specified. It is interesting to read there:

Because a process running a service is ranked higher than one with background activities, an activity that initiates a long-running operation might do well to start a service for that operation, rather than simply spawn a thread — particularly if the operation will likely outlast the activity. Examples of this are playing music in the background and uploading a picture taken by the camera to a web site. Using a service guarantees that the operation will have at least "service process" priority, regardless of what happens to the activity.

Your Activity where users initiate a long-running action should show a ProgressDialog to make sure user does not do anything else while the action is running. The guide is here.

Also, you'd most likely want to use the NotificationManager for notifying the user about your long-running action completion (or failure) if your Activity is currently invisible. Here is the NotificationManager info to start from.

Answer 2

There are multiple considerations that you must weigh in order to best decide how to approach your situation. It sounds like you need a good comparison between the two approaches... So here is a list of similarities, and differences and additional considerations that must be taken into account when working on a handheld device.

A Service is a part of your Application that has no UI. It may be called by a UI(Activity) to be started, or may be started by any other component of your Application. When developing, you have the freedom to place it on a different thread, or even run it in a different Task or Process. This allows you to ultimately separate it from your UI. Additionally, you may start the Service to run independently (startService) or bind your activity to it (bindService) depending upon your needs. By using custom Handlers, you can set callbacks to update the UI with your progress. A Service does not necessarily end if a User changes Activities, but may be ended at ANY time by the OS.

A AsyncTask is always instantiated from the UI thread. It only allows specific callbacks, but simplifies the process of multi-threading for the purposes of relatively short transactions (as compared to dedicated separate threaded services) that are inherently tied to actions performed by an Activity. Whenever a User changes Activities, the AsyncTask is put on "pause" and may even die because there is no UI thread for your Activity any longer.

The thing that I would be most concerned about is if the app is going to take 10-20 minutes the first time, I would ASSUME that the User will either change tasks temporarily or set the phone down until it completes (which can cause all of the same complications if the phone sleeps). Given this consideration, a threaded service bound to your activity may be your best choice. To protect your UI, I would make a Progress Dialog for your Activity that receives your progress callbacks. This limits user input in YOUR app and allows your service to continue the way that it needs to. Then override the Activity onResume to check the status of your Service and if it is running. Then you can reset the Dialog immediately.

Given that this is my preferred method, I would also take into account that the OS may kill the App at any time anyway. So make sure to have some way to detect an incomplete or partial sync. Then you may resume automatically when your Activity or Service restarts.