Detecting when the active Space changes

Actu­ally, there is a way to do this a bit more ele­gantly (i.e., no polling), but it’s yet another layer of undoc­u­mented API hack­ery. Then again, that’s what’s got­ten us this far, isn’t it?

The Spaces menu extra uses an inter­nal call­back hook to ask the win­dow man­ager to poke it when some­thing hap­pens with Spaces. I haven’t puz­zled out all the details about what’s being sent back and forth, but this proof-​of-​concept code shows how you can at least receive auto­matic noti­fi­ca­tions with­out hav­ing to con­tin­u­ally ask for them.

--------------------
SpacesListener.h
--------------------

// Internal CoreGraphics typedefs

/* Connection ID between the app and the window server (from CGSPrivate.h). */
typedef int CGSConnection;

/* Prototype for the Spaces change notification callback.
 *
 * data1 -- returns whatever value is passed to data1 parameter in CGSRegisterConnectionNotifyProc
 * data2 -- indeterminate (always a large negative integer; seems to be limited to a small set of values)
 * data3 -- indeterminate (always returns the number '4' for me)
 * userParameter -- returns whatever value is passed to userParameter in CGSRegisterConnectionNotifyProc
 */
typedef void (*CGConnectionNotifyProc)(int data1, int data2, int data3, void* userParameter);


// Internal CoreGraphics functions

/* Get the default connection for the current process (from CGSPrivate.h). */
extern CGSConnection _CGSDefaultConnection(void);

/* Retrieve the workspace number associated with the workspace currently
 * being shown (from CGSPrivate.h). 
 *
 * cid -- Current connection
 * workspace -- Pointer to int value to be set to workspace number
 */
extern OSStatus CGSGetWorkspace(const CGSConnection cid, int *workspace);

/* Register a callback function to receive notifications about when the current Space is changing.
 *
 * cid -- Current connection
 * function -- A pointer to the intended callback function (must be in C; cannot be an Objective-C selector)
 * data1 -- indeterminate (this is hard-coded to 0x579 in Spaces.menu...perhpas some kind of event filter code?)
 * userParameter -- pointer to user-defined auxiliary information structure; passed directly to callback proc
 */
extern CGError CGSRegisterConnectionNotifyProc(const CGSConnection cid, CGConnectionNotifyProc function, int data1, void* userParameter);


// Demonstration class declaration

@interface SpacesListener : NSObject {
}
- (void)displayCurrentSpace;
@end


--------------------
SpacesListener.m
--------------------

#import "SpacesListener.h"

// C-language callback function

/* Callback function used to notify app when the Space is changing.
 *
 * data1 -- returns whatever value is passed to data1 parameter in CGSRegisterConnectionNotifyProc
 * data2 -- indeterminate (always a large negative integer; seems to be limited to a small set of values)
 * data3 -- indeterminate (always returns the number '4' for me)
 * userParameter -- returns whatever value is passed to userParameter in CGSRegisterConnectionNotifyProc
 */ 
void spacesCallback(int data1, int data2, int data3, void* userParameter) {
    // Re-construct an instance of the calling class instance and call back into it
    SpacesListener* listenerProxy = (SpacesListener*)userParameter;
    [listenerProxy displayCurrentSpace];
}


// Demonstration class implementation

@implementation SpacesListener

- (id)init {
    self = [super init];
    if (self) {
        // Register our Spaces callback. Not sure why '1401', but that's what Spaces.menu does;
        // userParameter is used to pass along a pointer to this SpacesListener class instance.
        CGSRegisterConnectionNotifyProc(_CGSDefaultConnection(), spacesCallback, 1401, (void*)self);
    }
    return self;
}

- (void)displayCurrentSpace {
    // Get the current space ID
    int spaceID = 0;
    CGSGetWorkspace(_CGSDefaultConnection(), &spaceID);

    // Display it to the console
    if (spaceID == 65538)
        NSLog(@"transitioning (or in overview)");
    else
        NSLog(@"switched to space number %d", spaceID);
}

@end

Thanks for all your awe­some work on VirtueDesk­tops – I wouldn’t have been able to com­plete my dis­ser­ta­tion research with­out it! I’m also look­ing for­ward to see­ing what comes of the new Hyper­Spaces project. Any chance you’d be will­ing to share your new code­base for some aca­d­e­mic research & development?

Cheers,
Steve Voida
Uni­ver­sity of Calgary

Posted by Steve Voida on March 14, 2008 at 18:46

Any chance of beta-​testing Hyper­spaces? I ran into this post after search­ing for “cocoa space change noti­fi­ca­tion”, since that’s the only way I could think to sanely imple­ment a “labels for spaces” fea­ture, which seems to be exactly what you’re doing with Hyperspaces.

Posted by Brian Dono­van on September 24, 2008 at 08:52

In 10.6, NSWorkspace now has the following notification: NSWorkspaceActiveSpaceDidChangeNotification.

Posted by Paul on September 2, 2010 at 06:59