SPLMessageLogger can intercept and log all objc messages right to your console:

-> <UINavigationBar:0x167a9960>: -[UIView setCenter:NSPoint: {384, 42}]
=> void
-> <UIView:0x167b7e40>: -[UIView setCenter:NSPoint: {384, 512}]
=> void
-> <UIImageView:0x167c56c0>: -[UIView setCenter:NSPoint: {0, 0}]
=> void

Installation

SPLMessageLogger is available through CocoaPods, to install it simply add the following line to your Podfile:

pod "SPLMessageLogger"

Usage

SPLMessageLogger can intercept and log all objc messages. It therefore defines the following interface on NSObject

@interface NSObject (SPLMessageLogger)

+ (instancetype)messageLogger;

@end

where the messageLogger records every message it receives and hooks into the runtime to intercept and log these recorded messages. Let's take a look at an example:

@interface SPLView : UIView

- (CGPoint)setCenter:(CGPoint)center atIndex:(NSInteger)index forObject:(id)object;

@end

@implementation SPLView

- (CGPoint)setCenter:(CGPoint)center atIndex:(NSInteger)index forObject:(id)object
{
    [super setCenter:center];
    return self.center;
}

@end

To now intercept and log all messages to -[SPLView setCenter:atIndex:forObject:] and -[UIView setCenter:], simply run

SPLView *firstMessageLogger = [SPLView messageLogger];
[firstMessageLogger setCenter:CGPointZero atIndex:0 forObject:nil];

UIView *secondMessageLogger = [UIView messageLogger];
[secondMessageLogger setCenter:CGPointZero];

At runtime, this can result in the following output

-> <SPLView:0x166c41f0>: -[SPLView setCenter:NSPoint: {0.5, 0.5} atIndex:5 forObject:<ICAppDelegate: 0x166b0570>]
--> <SPLView:0x166c41f0>: -[UIView setCenter:NSPoint: {0.5, 0.5}]
==> void
=> NSPoint: {0.5, 0.5}
-> <SPLView:0x166c41f0>: -[UIView setCenter:NSPoint: {0.5, 0.5}]
=> void
-> <UINavigationBar:0x167a9960>: -[UIView setCenter:NSPoint: {384, 42}]
=> void
-> <UIView:0x167b7e40>: -[UIView setCenter:NSPoint: {384, 512}]
=> void
-> <UIImageView:0x167c56c0>: -[UIView setCenter:NSPoint: {0, 0}]
=> void

How it works

Message forwarding

For this library, I have written a custom trampoline (you can read about trampolines here) which can forward any objc message to a new selector.

IMP imp_implementationForwardingToSelector(SEL forwardingSelector, BOOL returnsAStructValue);

Here is an example

IMP forwardingImplementation = imp_implementationForwardingToSelector(@selector(setCenter:), NO);
class_addMethod([UIView class], @selector(thisSetCenterDoesNotExistYet:), forwardingImplementation, typeEndoding);

and suddenly every instance of UIView responds to -[UIView thisSetCenterDoesNotExistYet:] and forwards this message to -[UIView setCenter:]. If you would like some more information about trampolines and maybe a blog post like Writing custom trampolines for beginners and all the pitfalls, hit me up on Twitter.

Runtime hacking

+[NSObject messageLogger] returns an instance of SPLMessageLoggerRecorder which implements -[NSObject methodSignatureForSelector:] and then uses -[NSObject forwardInvocation:] (similar to what UIAppearance is using) as an entry point to hook into the runtime. The runtime hook is probably best explained with the example from above:

SPLView *firstMessageLogger = [SPLView messageLogger];
[firstMessageLogger setCenter:CGPointZero atIndex:0 forObject:nil];

Once firstMessageLogger receives the setCenter:atIndex:forObject: message, it does the following things:

• Store the original implementation of -[SPLView setCenter:atIndex:forObject:] at the new location -[SPLView __SPLMessageLogger_original_SPLView_setCenter:atIndex:forObject:]
• Replace the implementation of -[SPLView setCenter:atIndex:forObject:] with a new implementation obtained by imp_implementationForwardingToSelector which forwards to __SPLMessageLogger_SPLView_setCenter:atIndex:forObject:.

The trick here is that no instance every is responding to the forwarded -[SPLView __SPLMessageLogger_SPLView_setCenter:atIndex:forObject:]. In these cases, the objc runtime is asking -[NSObject forwardingTargetForSelector:], which for this selector returns an instance of the actual SPLMessageLogger class. This instance then receives the message __SPLMessageLogger_SPLView_setCenter:atIndex:forObject:. It implements -[NSObject methodSignatureForSelector:] and then uses -[NSObject forwardInvocation:] to log the invocation and then forwards the invocation back to the original object to the original implementation:

-[SPLView setCenter:] // gets forwarded
↓
-[SPLView __SPLMessageLogger_SPLView_setCenter:atIndex:forObject:] // no implementation is found
↓
-[SPLView forwardingTargetForSelector:] // return an instance of SPLMessageLogger
↓
-[SPLMessageLogger methodSignatureForSelector:] // lets the runtime construct an invocation
↓
-[SPLMessageLogger forwardInvocation:]
// 1) log the invocation
// 2) forward invocation
↓
--->    `-[SPLView __SPLMessageLogger_original_SPLView_setCenter:atIndex:forObject:]` // execute original implementation
<---    `-[SPLView __SPLMessageLogger_original_SPLView_setCenter:atIndex:forObject:]`
// 3) log return argument of invocation

Limitations

This approach uses a custom trampoline written in raw assembly which is currently only available on i386, armv7, armv7s and arm64.

Author

Oliver Letterer

• http://github.com/OliverLetterer
• http://twitter.com/oletterer
• [email protected]

License

SPLMessageLogger is available under the MIT license. See the LICENSE file for more info.