I’m new to Xcode and iOS and have been experimenting today with the Instruments tool. There are some things I’m struggling to get my head around:
Scenario:
I have created a simple app that filters a UITableView with a UISearchBar (target iOS5 with ARC). Simple code, nothing too fancy.
For my own learning, I was watching the allocations as I perform various actions – in this particular instance when I’m typing in the search bar.
Using the ‘Allocations’ instrument tool, I get the following:
As expected, I see a sudden spike in memory allocated when I first start typing in the search bar. After that, any further searching makes no significant difference to the allocation graph.
However, when I look at the memory allocations using the ‘Zombie’ instrument tool, the allocation graph continues to rise whenever I type in the search bar.
At first, I thought this may have been to what I was tracking – and I tried to ensure all the settings were matched. However, it still shows a rising graph when searching.
Does anyone have an explanation of this? No doubt I have some conceptual misunderstanding about what the allocation tool is tracking in each of these instrument modes.
The Zombies instrument works by telling your app not to free objects. Instead, when the object would be deallocated, the app instead leaves the object allocated but changes the object’s class to a special zombie class that handles any message by raising a zombie error.
Thus, allocations keep rising under the Zombie instrument because nothing is really deallocated.
Incidentally, since using Zombies prevents the app from deallocating objects, the app usually requires much more memory to run under the Zombies instrument. This is ok on a Mac (and in the iOS Simulator running on a Mac), since the Mac probably has several gigabytes of RAM and also supports paging to disk. But iOS devices only have between 256 MB and 1 GB of RAM, and don’t support paging. This is probably why Instruments won’t let you use Zombies on an iOS device.