How to do lengthy operations without being killed

Question

I have an important operation that is executed rarely. In some cases, it might take minutes to execute. My app is getting killed after a 50 second operation. How to avoid that?

Should I put it in a background thread? Could anyone please point me in the right direction here. I have not found any useful information about the so called watchdog. Is a background thread the way to go?

Answer 1

Yes, you need to move this task to a background thread. You should never jam up the main thread with any task that takes longer than a fraction of a second to perform. Ignoring the watchdog timer, which only kicks in under extreme conditions, your application is completely unresponsive to touch or other events during this lengthy operation, and you're unable to provide feedback to the user as to the progression of this operation.

The watchdog timer will kill an application that jams up the main thread for an extremely long period of time, making the application unresponsive to input (I believe this duration is currently 20 seconds on startup, but I'm not sure what it is for when the application is running). You should never let your application get to the point where the watchdog is killing it, because that's pointing to a real problem in the way your application is handling things.

Moving a long-running task to a background thread is a lengthy topic by itself, which is why I recommend reading Apple's Concurrency Programming Guide (updated) as well as watching some of their WWDC videos on the subject before starting.

However, in my opinion the most elegant way to deal with long-running tasks is to use Grand Central Dispatch, where something like

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // Do your long-running task here

    dispatch_async(dispatch_get_main_queue(), ^{
           // Do callbacks to any UI updates here, like for a status indicator
    });
});

will fire off your task to be performed in a background thread on one of the global concurrent queues. The little section of code within the main block shows how you might update any UI elements, such as a progress bar, from within this background task. Generally, UI updates must be performed on the main thread (there are some exceptions as of iOS 4.0, but it's still a good practice in general).

I also highly recommend adding some kind of visual indication of the status of this long-running task as it proceeds. Your users will really appreciate this, and it will make your application appear faster, even though it may run for the same duration.

Answer 2

Can you occasionally hit the watchdog during your process? Watchdog timers are just there to detect whether something crashed. They aren't really concerned with the system being busy.

Is the phone still able to respond to the user doing stuff like pressing the home button during your process?

EDIT: This guys recommends using a background thread