A common cause of problems in iOS applications can be memory warnings. In the simplest terms, when the device is running low on available memory, iOS issues a memory warning which tells all running applications to stop being so greedy and to free up any memory they don’t absolutely need (and if that doesn’t free up enough, it will just start ruthlessly slaughtering them one by one). If an application doesn’t handle this properly and does something like release an object from memory without unlinking it, you can get a crash. Often times this will arise when a user has drilled down several views deep, a memory warning occurs, and as the user navigates back up the stack… kaboom! Let’s say a user is in an application, which has a photo albums view, and from there the user selects an album, and in that album they select a photo. So we have something like: Photo Albums view > Album view > Photo view.
Normally when you’d come back to something like the Photo Albums view, it would have a reference stored of which photo album you tapped on so that it could do a fancy deselect animation when you came back up out of your album view. So let’s say you’re on Photo Details, a memory warning gets issued and in an attempt to free up memory, the application releases the contents of the Photo Albums view, including the cell that you tapped and which it is holding a reference to. As you come back out of the stack to the Photo Albums view, the application will try to reference the cell for the album that you tapped, and be like “WTF? That cell doesn’t exist.” Naturally the application will throw a temper tantrum at this point, or in more technical terms, it will crash. This is just one type of scenario that can cause a crash from a memory warning. (I might cover more later, but for now it’s just important to know that nested views are where to look for these issues). Memory warnings, like all relationships, occur naturally in the wild all the time, but it’s so much easier if you can just force them to happen when it’s convenient for you. Fortunately for us, iOS Simulator provides this functionality. You can simulate a memory warning by going to the Hardware menu in iOS Simulator and selecting Simulate Memory Warning (who knows why they named it something so cryptic).
You should go ahead and set a keyboard shortcut for this now because you’ll be using it a lot from now on. For this just go to System Preferences, then select the Keyboard preference pane. Then select the Keyboard Shortcuts tab, select Application Shortcuts in the left pane, then click + at the bottom. Set Application to iOS Simulator, Menu Title to Simulator Memory Warning and your shortcut to whatever you want (I went with ⌘-1 because I think memory warnings are A #1). Also remember as you’re finding these crashes to note the path you took through the views to make it happen; no need to give developers another reason to curse your name.
So now you can open up some bugs for your developers with all the crashes you’ve found. Sadly, your developers will get these crashes fixed and resolve your tickets just as you’re getting the hang of cruising around the app jamming on ⌘-1. You’ve spent your penny on the mechanical horse ride, but now it has stopped and Mom’s telling you to get off. Fear not! Crashes are only the most obvious bug to spot with memory warnings. So… what else can we look at?
Using apps should be an immersive, consistent, fluid experience for your users. One consistency that a user expects in an application is that however they leave a view, when they come back, it will be the same. For example, if you’re using the music app and you scroll down the artist list to V and tap on Vanilla Ice, when you come back to the list of artists, you expect the view to still be down in the Vs. If every time a memory warning happened while you were listening to music it meant that the Artists scroll view got reset to the top, that’s a frustrating experience. It’s important to note the different properties of your views and make sure that they are being preserved accordingly after memory warnings.
A couple of other things that are sometimes lost are the currently selected tab if you get a memory warning while a modal view is up as well as tapped cells being deselected when you come back to the view. Sometimes your developers have made sure the app won’t crash, but overlook proper behavior like deselecting a cell. Many users may not notice these details on a conscious level, but this kind of polish is what can make the difference between a good app and a great app.
Sometimes you’ll find yourself needing to test particular views on a device that don’t show up in the simulator. The easiest and quickest option for this is to launch some other applications that will eat up memory. So first plug your device in, then open Xcode and from the Window menu select Organizer. You should see your device over in the left pane with a Console option below it, select that. This is where you can get a behind-the-scenes peek at what’s going on with your device and where we’ll see when a memory warning occurs. Now that you know where to look to see when a memory warning happens, let’s figure out how to make one happen without our simulator.
One way to eat up some memory is to start a recording with Voice Memos. This option has the added luxury of allowing you to run your app in the foreground while the recording is still going and eating up memory. (It can also help you test to make sure your app resizes views properly, but we’ll go into that another time). Safari and Camera are two more apps that act like they’re trying to win Hungry Hungry Hippos when it comes to memory. In fact, for a while (I think on iOS 5.0 or 5.0.1), if you had Voice Memos recording and tried to take a photo, both apps would crash from using too much memory. What you’ll be looking for in the console output is something like this:
unknown Camera <Warning>: Received memory warning.
When you see your app receive a memory warning like this, you can then go back to your app and make sure everything still looks okay. One downfall to this method is depending on how low your device is on memory, you might find that your app has freed up more memory than you wanted it to. Instead of being taken back to the view you were expecting, you might be greeted by a splash screen indicating your application got completely killed as a result of the memory warnings (playing Infinity Blade tends to have this result). So this might take a little playing around to find the right combination of apps for you.
One final option, if your developers really love you, might be to have them put code in your debug builds that gives you buttons right in the app to generate memory warnings. This is extremely helpful for testing memory warnings on devices, but might not be an option for everybody. If it’s not an option, you might try searching around for somebody who has already written some memory warning code and made it freely available, like this guy. (Note: I haven’t tried this code, just came across it. Let me know if you have luck with it.)
As you can see, memory warning problems can manifest in a number of different ways. Hopefully with the info above, you’ll be catching these before your users do.
(Thanks to Chris Weathers for help with the explanation of memory warnings.)