Register as Login Item with Cocoa?

This works on xcode 5.

- (BOOL)isLaunchAtStartup {
    // See if the app is currently in LoginItems.
    LSSharedFileListItemRef itemRef = [self itemRefInLoginItems];
    // Store away that boolean.
    BOOL isInList = itemRef != nil;
    // Release the reference if it exists.
    if (itemRef != nil) CFRelease(itemRef);

    return isInList;
}

- (void)toggleLaunchAtStartup {
    // Toggle the state.
    BOOL shouldBeToggled = ![self isLaunchAtStartup];
    // Get the LoginItems list.
    LSSharedFileListRef loginItemsRef = LSSharedFileListCreate(NULL, kLSSharedFileListSessionLoginItems, NULL);
    if (loginItemsRef == nil) return;
    if (shouldBeToggled) {
        // Add the app to the LoginItems list.
        CFURLRef appUrl = (__bridge CFURLRef)[NSURL fileURLWithPath:[[NSBundle mainBundle] bundlePath]];
        LSSharedFileListItemRef itemRef = LSSharedFileListInsertItemURL(loginItemsRef, kLSSharedFileListItemLast, NULL, NULL, appUrl, NULL, NULL);
        if (itemRef) CFRelease(itemRef);
    }
    else {
        // Remove the app from the LoginItems list.
        LSSharedFileListItemRef itemRef = [self itemRefInLoginItems];
        LSSharedFileListItemRemove(loginItemsRef,itemRef);
        if (itemRef != nil) CFRelease(itemRef);
    }
    CFRelease(loginItemsRef);
}

- (LSSharedFileListItemRef)itemRefInLoginItems {
    LSSharedFileListItemRef res = nil;

    // Get the app's URL.
    NSURL *bundleURL = [NSURL fileURLWithPath:[[NSBundle mainBundle] bundlePath]];
    // Get the LoginItems list.
    LSSharedFileListRef loginItemsRef = LSSharedFileListCreate(NULL, kLSSharedFileListSessionLoginItems, NULL);
    if (loginItemsRef == nil) return nil;
    // Iterate over the LoginItems.
    NSArray *loginItems = (__bridge NSArray *)LSSharedFileListCopySnapshot(loginItemsRef, nil);
    for (id item in loginItems) {
        LSSharedFileListItemRef itemRef = (__bridge LSSharedFileListItemRef)(item);
        CFURLRef itemURLRef;
        if (LSSharedFileListItemResolve(itemRef, 0, &itemURLRef, NULL) == noErr) {
            // Again, use toll-free bridging.
            NSURL *itemURL = (__bridge NSURL *)itemURLRef;
            if ([itemURL isEqual:bundleURL]) {
                res = itemRef;
                break;
            }
        }
    }
    // Retain the LoginItem reference.
    if (res != nil) CFRetain(res);
    CFRelease(loginItemsRef);
    CFRelease((__bridge CFTypeRef)(loginItems));

    return res;
}

I stumbled across Ben Clark-Robinson's LaunchAtLoginController. A very elegant solution to a very common problem.

There's an API that's new in Leopard called LSSharedFileList. One of the things it lets you do is view and edit the Login Items list (called Session Login Items in that API).

BTW, I'm the lead developer of Growl. We haven't switched away from AE yet because we still require Tiger, but I'm thinking of dropping that for 1.2 (haven't talked it over with the other developers yet). When we do drop Tiger, we'll drop LoginItemsAE as well, and switch to the Shared File List API.

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EDIT from the year 2012: Since 2009, when I originally wrote this answer, Growl has switched to LSSharedFileList and I've left the project.