How can I use Cocoa's Accessibility API to detect that a window is brought to front?

Question

I'm using the Accessibility API to detect when a certain application opens windows, closes windows, when the windows are moved or resized, or made main and/or focused. However the client app seems to move a window to front without an Accessibility API notification being fired.

How can my application detect when another application brings a window to front, without making it key?

I'm hoping to find a solution that works on OS X 10.4 and 10.5

More info: I'm using these statements at the moment. They work fine when the user manually selects a window to bring it to front. But it doens't work when the app itself is bringing the window to the front.

AXObserverAddNotification(observer, element, kAXMainWindowChangedNotification, 0);
AXObserverAddNotification(observer, element, kAXFocusedWindowChangedNotification, 0);

Nick Haddad Nick Haddad · Accepted Answer · 2009-03-09T02:53:45

I've been unable to subscribe to current window changes, but you can ask the accessibility API for the current application, and the current applications most foreground window.

Imagine you have a class called CurrentAppData, with the following data:

@interface CurrentAppData : NSObject {
    NSString* _title;
    AXUIElementRef _systemWide;
    AXUIElementRef _app;
    AXUIElementRef _window;
}

The code to find the current application looks something like this:

-(void) updateCurrentApplication {
   // get the currently active application  
   _app = (AXUIElementRef)[CurrentAppData
                           valueOfExistingAttribute:kAXFocusedApplicationAttribute 
                                        ofUIElement:_systemWide];

   // Get the window that has focus for this application
   _window = (AXUIElementRef)[CurrentAppData 
                              valueOfExistingAttribute:kAXFocusedWindowAttribute 
                                           ofUIElement:_app];

   NSString* appName = [CurrentAppData descriptionOfValue:_window
                                             beingVerbose:TRUE];    

   [self setTitle:appName];
}

In this example the _systemWide variable is initialized in the classes init function as: _system = AXUIElementCreateSystemWide();

The class function valueOfExistingAttribute looks like this:

// -------------------------------------------------------------------------------
//  valueOfExistingAttribute:attribute:element
//
//  Given a uiElement and its attribute, return the value of an accessibility
//  object's attribute.
// -------------------------------------------------------------------------------
+ (id)valueOfExistingAttribute:(CFStringRef)attribute ofUIElement:(AXUIElementRef)element
{
    id result = nil;
    NSArray *attrNames;

    if (AXUIElementCopyAttributeNames(element, (CFArrayRef *)&attrNames) == kAXErrorSuccess) 
    {
        if ( [attrNames indexOfObject:(NSString *)attribute] != NSNotFound
                &&
            AXUIElementCopyAttributeValue(element, attribute, (CFTypeRef *)&result) == kAXErrorSuccess
        ) 
        {
            [result autorelease];
        }
        [attrNames release];
    }
    return result;
}

The previous function was taken from the Apple UIElementInspector example, which is also a great resource for learning about the Accessibility API.

How can I use Cocoa's Accessibility API to detect that a window is brought to front?

4 Answers