A Swift tool designed to capture and analyze Objective-C message sends during code execution.

When calling methods on Objective-C objects, i.e. [NSString alloc], the Objective-C runtime uses the objc_msgSend function to lookup the method implementation and call it. Using "swizzling" the implementation called by objc_msgSend can be replaced with a custom implementation.

Swift itself does not use the objc_msgSend function, so it does not support swizzling. But many Swift frameworks use Objective-C runtime features, e.g. @objc methods, NSObject methods, etc.

This tool leverages the Objective-C runtime's message logging capability to help debug and understand Objective-C message patterns in your code.

Before using this tool, you need to enable Objective-C message logging by setting the following environment variable NSObjCMessageLoggingEnabled to YES. This will create a file at /tmp/msgSends-<PID> where the messages will be logged (see Apple Technote TN2124 for more details).

Objective-C message logging is only available in debug builds for macOS. It appears to not be available on iOS or the simulator.

1. Clone this repository
2. Open Package.swift with Xcode
3. Edit the main.swift file to add your code block to execute, i.e.:

let logs = try collectLogs {
    let _ = NSString("Hello World")
}

4. Make sure the environment variable for the Run scheme is set:

NSObjCMessageLoggingEnabled=YES

5. Run the analyzer

The analyzer performs the following steps:

1. Opens the message log file at /tmp/msgSends-<PID>
2. Executes your code block
3. Captures all Objective-C messages sent during execution
4. Prints the collected logs

The tool is designed to minimize its own impact on the logged messages by:

• Using C-level file operations instead of higher-level APIs
• Preparing data structures before the analysis
• Capturing only new messages during the execution block

This is an example code block that will be executed (see main.swift):

let logs = try collectLogs {
    let _ = NSString("Hello World")
}

The output will something look like this:

+-----------------+
| COLLECTED LOGS: |
+-----------------+

+ NSString NSString allocWithZone:
- NSPlaceholderString NSPlaceholderString initWithBytesNoCopy:length:encoding:freeWhenDone:
+ NSString NSString allocWithZone:
- NSPlaceholderString NSPlaceholderString initWithString:
- __NSCFString __NSCFString release
- __NSCFString __NSCFString release

Based on this output, you can see that the NSString class is allocated twice, and the NSPlaceholderString class is allocated once. In addition string literals are allocated as __NSCFString instances and released twice.