Archive for the ‘NSTask’ category

NSWorkspace Bug in – openFile: ?

December 7, 2007

EDITED TO ADD: I could reproduce this bug in a large project, but not isolate it in a smaller one. It is much likely for my code to have a bug then NSWorkspace. I’m still not 100% certain that this issue wasn’t my fault in some way I don’t understand. But no matter what caused the bug, the work-around I describe here has been working for me. Please let me know if you have this same issue as well, or any insight into what I could have done to cause it.

The Problem:
I need to launch a background-application from inside the application-support directory. The OS is Mac OS X 10.5.1 build 9B18.
[[NSWorkspace sharedWorkspace] openFile:path];
where path is the correct path to the application, in my case “/Users/user/Library/Application Support/IMLocation/”, works about 3 out out 4 times, but intermittently fails. It also reports success without actually launching, but this is far more rare.

When openFile: explicitly fails, it prints, “LSOpenFromURLSpec() returned -600 for application (null) path /Users/user/Library/Application Support/IMLocation/” to stderr.

Error -600 is procNotFound /*no eligible process with specified descriptor*/.

The path is correct, the application exists there. I verified this by
assert([[NSFileManager defaultManager] fileExistsAtPath:path]);
before calling openFile:.

Calling openFile: again immediately after a failure often works

Also, while(![[NSWorkspace sharedWorkspace] openFile:path]);
will terminate, meaning it reported success at some point, but it will not always have actually launched the application at path.

(EDITED TO ADD: At first I thought that, NSWorkspace was probably opening IMLocationHelper, but not blocking until it has finished being launched. But sleeping, to give IMLocationHelper time to finish launching, did not solve the problem.)

performSelectorOnMainThread did not solve anything ether.

I verify that IMLocationHelper has been launched by
assert([[ShellTask executeShellCommandSynchronously:@"ps -axww | grep IMLocationHelper | grep -v grep"] length] > 0);

A Workaround:
[[NSTask launchedTaskWithLaunchPath:@"/usr/bin/open" arguments:[NSArray arrayWithObject:path] waitUntilExit];.

This calls the open command to launch the application. I have not been able to make it fail when path exists.

Calling the command line from Cocoa

June 21, 2007

This article has been updated and moved here.

tell application “YourGreatApp” NSAppleScript is here to save the day

April 21, 2007

Cocoa includes the wonderful NSAppleScript class, which lets you embed a high-level AppleScript code snippets in your Objective-C program. AppleScript is the bee’s knees for interfacing with other applications, particularly Apple programs and OS X itself. EDITED TO ADD: Unless you use threads in your program. Here is a more sobering perspective, written several months after this article.

For example, say you want to get the user’s current status message from iChat. You can mess around with the InstantMessaging.framework — only to discover that it can’t do this — or you can wrap the following AppleScript:

tell application “iChat” to get status message

in an NSAppleScript object, like so:
NSAppleScript *iChatGetStatusScript; //this would be in the header.
/*Apple recommends only compiling NSAppleScript objects once, for performance reasons. Be sure to call [iChatGetStatusScript release] in your dealloc method.*/
  iChatGetStatusScript = [[NSAppleScript alloc] initWithSource:
    @”tell application \”iChat\” to get status message”
NSString *statusString =
[[iChatGetStatusScript executeAndReturnError:&errorDict] stringValue];

Skeptical readers have probably noticed that I cheated by picking functionality that wasn’t in a framework for the last example. Unfortunately this situation is more of a rule then an exception. AppleEvents have been on the Apple software development scene longer then Cocoa. Lots of programs only expose functionality through AppleScript/AppleEvents. Using NSAppleScript is the best way to interact with these programs from Cocoa.

AppleScript is a big part of how FrontRow integrates with iTunes, even though they are both modern programs.
grok the ouptut of:
strings /System/Library/CoreServices/Front\\ Row
(hint: look for “tell”), and you’ll see a lot of gems like:
tell application “iTunes” to get (data of artwork 1 of current track) as picture
tell application “Finder”
    set myCD to first disk whose format is audio format

Now I am the first to admit that a well-written Objective-C library trumps duck-taping one language on-top of another. But we don’t have the luxury of always being given well-written Objective-C interfaces.

As of version 0.24 IMLocation uses AppleScript to: interface with iChat and Adium; install/uninstall itself; mute/unmute sound; perform inter-process communication between the helper and GUI; and to get the MAC address of the first router connected to the ethernet port. NSAppleScript has also been heavily used during development for prototyping.

EDITED TO ADD: NSAppleScript is pathologically un-thread-safe. That is to say, you can only use it on from the main thread. No amount of @synchronized blocks will keep you from crashing if you use NSAppleScript in any other way. There is a workaround, but the cure is worse then the disease. You can use an NSTask to have the osascript program interpret the script for you. This won’t block your thread, but it will be take a long time, and is resource un-friendly.

NSTasks are the way to call the command line, or embed snippets of a different language in cocoa. I was originally using the do shell command AppleScript construct. If you are using it, please switch to NSTasks. They are much more powerful, still easy to use, and prevent all sorts of bugs caused by quote-marks in AppleScript.