Best Way to Handle Errors with AppleScript-ObjC embedded in Cocoa App?

Question

I'm invoking a small AppleScript handler from within a larger Cocoa/Objective-C app (using the AppleScript-ObjC framework to call the AppleScript methods directly from the Objective-C code). I originally tried using Scripting Bridge, but that didn't work for me due to compatibility problems with the external app. The only purpose of the AppleScript is essentially to send a small string to the external app. This is the first time I've attempted to do something like this in order to control an external application, so please bear with me if I'm making an obvious mistake.

Occasionally, the AppleScript may encounter errors depending on the state of the external application, and I would like to handle those errors appropriately in my Objective-C code.

The way I'm doing this right now is that I have a try block in the AppleScript:

try
    -- Do stuff
    return the number 0
on error the error_message number the error_number
    return the error_number
end try

I return zero if the script finishes normally, and return the error number if it doesn't. Then in the Objective-C code, I test the return value and throw a "normal" exception in cases where there is an error, so that I can handle that in my regular exception-handling code.

This works, but is there a way to trap the AppleScript exception directly from the Objective-C code? When I run the code without the AppleScript try-error code in XCode, it seems that it hits my exception breakpoint before it returns from the AppleScript, but it never invokes the @catch block for NSException. Is there some way to catch that exception that is being hit somehow? I'm guessing the answer is no, but I wanted to check that there isn't some better way to do this than what I'm doing. Apple's documentation on AppleScript-ObjC is rather sparse, and mostly just discusses handling errors within AppleScript.

Update: So, I just tried playing around with the breakpoint setup a little bit, and it turns out that it's a C++ exception being sent from the AppleScript code, not an Objective-C one. That's why it wasn't being caught by the @catch block I was using. It seems that the AppleScript-ObjC mechanism uses exceptions for control flow/recoverable errors, and I had my breakpoint set up to break when they are thrown. So, I think it's probably best to just catch the errors in the AppleScript application, and return an error code as a string or integer to the Objective-C code.

Answer 1

AppleScript uses C++ exceptions to implement its own exceptions, but nothing else; you shouldn’t see them in code that completes successfully. If an AppleScript exception escapes to point of returning to Objective-C, the bridge actually swallows it -- you should get a log message, and a nil/zero/NO return value from the bridged method. The upshot of all this is that you’re already doing the right thing: trap the error in AppleScript and return something that your Objective-C code can detect.

As a side note, AppleScriptObjC is mainly intended for developers writing applications primarily or entirely in AppleScript. If you’re writing a primarily Objective-C application with a few bits that control other applications via scripting, consider using ScriptingBridge instead.