How to Monitor iPhone App Memory Usage using Instruments in the iOS 4 SDK

Should you need to observe memory usage for your app, the iOS 4 SDK contains a useful option. The excerpt below from Learning the iOS 4 SDK for Javascript Programmers explains how to use it.

---

Apple supplies a number of extra tools in the iOS SDK. One of them, called Instruments, lets you run your app in the simulator or on a device while observing real-time memory usage and the occurrence of memory leaks. For example, you can watch how many objects are being allocated in memory while a program runs. If a particular user operation triggers a significant repeat loop that does not employ an NSAutoreleasePool object, you will likely observe the number of object allocations and overall memory usage continue to creep upward while the loop works. If so, you’ve spotted a candidate for wrapping inside an autorelease pool.

To run your app through Instruments, first make sure that it is in a usable state on the simulator. Then choose Run→Run with Performance Tool→Leaks from the Xcode menubar. Your app will probably run slower than usual as Instruments gathers runtime system data and displays it both in a running graph and a table.

Instruments is an incredibly rich development and testing tool. Explaining how to exploit all its powers would take a separate book. Although the iOS SDK documentation set includes an Instruments User Guide (search for “Instruments”), the guide is not necessarily for the newcomer to the platform. You can, however, get some individual help by asking questions at Apple’s iOS Developer Center forums (iOS developer membership required).

One other helpful tool is available to all users of the SDK. Xcode includes a static analyzer, which goes through your source code in search of problems the compiler does not always locate. For instance, the analyzer (built from an open source tool called CLANG) will spot potential memory leaks in your code. The first time you run your code through CLANG (choose Build→Build and Analyze), you may be shocked at the number of potential leaks (generally unresolved retain counts) it displays in the Build Results window (choose Build→Build Results to show the window). CLANG can display occasional false positives for problems, but you should study each reported issue carefully.