Discussion

The rendering of a GRMustache template can lead to NSUndefinedKeyExceptions to be raised, because of the usage of the valueForKey: method. Those exceptions are nicely handled by GRMustache, and are part of the regular rendering of a template.

Unfortunately, Objective-C exceptions have several drawbacks, particularly:

1. they play badly with autorelease pools, and are reputed to leak memory.
2. they usually stop your debugger when you are developping your application.

The first point is indeed a matter of worry: Apple does not guarantee that exceptions raised by valueForKey: do not leak memory. However, I never had any evidence of such a leak from NSObject’s implementation.

Should you still worry, we recommend that you avoid the valueForKey: method altogether. Instead, implement the keyed subscripting objectForKeyedSubscript: method on objects that you provide to GRMustache.

The second point is valid also: NSUndefinedKeyException raised by template rendering may become a real annoyance when you are debugging your project, because it’s likely you’ve told your debugger to stop on every Objective-C exceptions.

You can avoid them as well: make sure you invoke once, early in your application, the preventNSUndefinedKeyExceptionAttack method.

Depending on the number of NSUndefinedKeyException that get prevented, you will experience a slight performance hit, or a performance improvement.

Since the main use case for this method is to avoid Xcode breaks on rendering exceptions, the best practice is to conditionally invoke this method, using the NS_BLOCK_ASSERTIONS#documentation/Cocoa/Reference/Foundation/Miscellaneous/Foundation_Functions/Reference/reference.html) that helps identifying the Debug configuration of your targets:

#if !defined(NS_BLOCK_ASSERTIONS)
// Debug configuration: keep GRMustache quiet
[GRMustache preventNSUndefinedKeyExceptionAttack];
#endif

Companion guide: https://github.com/groue/GRMustache/blob/master/Guides/runtime.md