A better way to build OS X preferences

Behind the interface

Intelligent flash storage arrays

To use these classes, you’ll first need to get the corresponding header files NSPreferences.h and NSPreferencesModule.h. The simplest way of doing this is to use Steve Nygard’s excellent class-dump utility, which you can find here. Run it on the actual executable inside Appkit.framework and redirect the output to a temporary text file. You can then edit this file to extract the class declarations you’re interested in.

Create a new, empty Cocoa application to serve as a test bed for this stuff. You’ll typically want to invoke the preferences window in response to a menu option, but in my test application, I just used a button press in the main window. Here’s what the action procedure (inside AppController.m) looks like:

- (IBAction) showPreferences: (id) sender
    [NSPreferences setDefaultPreferencesClass: [AppPreferences class]];
    [[NSPreferences sharedPreferences] showPreferencesPanel];

Here, we’ve set up a subclass of NSPreferences called AppPreferences. This code tells NSPreferences to use that class, and then displays the preferences dialog. It’s inside AppPreferences.m that the interesting stuff happens:

- (id) init
    _nsBeginNSPSupport();    // MUST come before [super init]
    [super init];
    [self addPreferenceNamed: @"General" 
             owner: [GeneralPreferences sharedInstance]];
    [self addPreferenceNamed: @"HotKeys" 
             owner: [HotKeyPreferences sharedInstance]];
    return self;

- (BOOL) usesButtons
    return NO;

In this case, the init method (called from AppKit) adds two pages to our preference dialogs: GeneralPreferences and HotKeyPreferences. Both of these are sub classed from NSPreferencesModule.

Two points: first, in real-world code you’d typically localize the page names, “General” and “HotKeys” in this case. Second, by default, NSPreferences will assume the needed NIB files and image files have the same name as the module class, such as GeneralPreferences.nib or GeneralPreferences.png for example. Image files should be 32×32 pixels in size.

You’ll also note that evil-looking call to _nsBeginNSPSupport. This is a fairly recent addition by Apple that is designed, I suspect, to prevent third-party use of NSPreferences. Without a preceding call to this extern void routine, the call to super init will return nil.

A very simple demo project can be downloaded here.

There’s more that can be said about NSPreferences and NSPreferencesModule, of course, but I’ll mention just one other interesting feature of NSPreferences for now. When you call the init method for this class, it will look in your application’s resource path for a file called PreferencePanels.plist. If found, this file is used to add all your application’s preference panels in one go. For more on this, have a peek at the aforementioned PreferencePanels.plist file inside your Mail.app.

Until next time, have fun! ®

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