I’ve been surprised that iOS 5.1 does not manage memory quite as I expected. When the device is running a lot of apps, it appears that iOS does not kill memory hogging apps in the background, but sends memory warning to my own app as well.
For example showing a UIImagePicker crashed the app on two test devices. Double tapping the home key and killing some of the background apps prevents the app from receiving the memory warning and crashing.
I’m wandering if iOS would not free up memory for me, is it acceptable to show some sort of alert view notifying the user that the memory is low and some of the background tasks have to be killed?
I’m at a loss of how to deal with such events – does it take time for iOS to clean up some memory (while apps respond to memory warnings)?
iOS does a lot of stuff before bothering you with memory warnings, including killing backgrounded Apps. Since iOS 5, iOS is even going to annoy you as less as possible with memory warnings, meaning that you only get one if there really is a need for you to get rid of stuff that is using memory but not needed right now (and that you can safely recreate in the future without taking hours for it). If your App crashes without giving you a memory warning first, chances are that you allocated so much memory that the system can’t tell you that its running out of memory before it decides to kill you, the reason for this is that the memory warning is scheduled on the runloop of the main thread and until you give the runloop time to do another iteration, you won’t receive the warning.
Also, Apple doesn’t like you to tell the user that there is a memory problem; Its your App that has to deal with it, not the user! So its very very likewise that your App gets rejected if a memory warning comes up while the review team is reviewing your app (rumor says that they send these warnings to test how your App reacts to them)
Soo, to sum it up: iOS does work like you expected by killing what it can and even shutting down other system daemons, only after this happening you will be notified that memory is low. The correct way to respond to these warnings is to free up as much memory as you can, start with the big stuff that can be easily recreated in the future (eg. if your app shows loads of pictures but not all are visible at a time, throw away the ones that aren’t visible right now). Telling the user is the wrong way to deal with the problem and Apple doesn’t like it, so try to solve the problem on your end.