I get asked or see people ask how to either get front application switched events in Cocoa or how to get global hot keys in Cocoa applications. It takes some extra work, but it is possible. I'll show you how.

Hot Keys in Cocoa

This is easy. Subclass NSApplication with your own version. Set the Principal class in the target settings window to your class then use the following code in your subclass.


enum {
// NSEvent subtypes for hotkey events (undocumented).
kEventHotKeyPressedSubtype = 6,
kEventHotKeyReleasedSubtype = 9,
};

- (void)sendEvent:(NSEvent *)theEvent
{

if ([theEvent type] == NSSystemDefined && [theEvent subtype] ==kEventHotKeyPressedSubtype)
{
// a hot key has been pressed.
// do something here.
}

[super sendEvent:theEvent]; // YOU MUST CALL THIS OR YOU WILL EAT EVENTS!
}

Yeah, that helps, right? [theEvent data1] is the registered hot key's identifier. You register hot keys using the normal Carbon code.


err=RegisterEventHotKey(keyCode,inMods,hotKeyID, GetApplicationEventTarget(), 0, hotKeyRef);
//hotKeyRef is the ID returned by [theEvent data1]


There's very little to it. You just need some way to find out what is supposed to happen when the hotkey is pressed. I use a dictionary to store the hotKeyRef in of each hot key I registered. I then add the key command pressed to the dictionary and execute that key command to my app's menu when the hotkey is pressed.

It really is that simple. Always check for errors.

Application switched events for Cocoa.

Painfully easy once you know what to do. In your application's delegate, register for the distributed notification com.apple.HIToolbox.menuBarShownNotification in -awakeFromNib. This is notification is sent whenever the front application changes. I also register for app launched and app died events "just in case" the menu bar notification fails. You also need to prepare your code in case it receives two notifications in succession. It will when launching or quitting applications.


// The below registrations are made to check if the front application has changed
[[[NSWorkspace sharedWorkspace] notificationCenter] addObserver:self selector:@selector(checkFrontApp:) name:NSWorkspaceDidLaunchApplicationNotification object:nil];
[[[NSWorkspace sharedWorkspace] notificationCenter] addObserver:self selector:@selector(checkFrontApp:) name:NSWorkspaceDidTerminateApplicationNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(checkFrontApp:) name:NSApplicationDidBecomeActiveNotification object:NSApp];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(checkFrontApp:) name:NSApplicationDidResignActiveNotification object:NSApp];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(checkFrontApp:) name:NSApplicationDidHideNotification object:NSApp];
[[NSDistributedNotificationCenter defaultCenter] addObserver:self selector:@selector(checkFrontApp:) name:@"com.apple.HIToolbox.menuBarShownNotification" object:nil];


checkFrontApp: is the selector that gets called when the front application changes. That is, the method called when one of the notifications is received.

Make sense? Any questions? Special thanks to Evan Gross and NoisyApp.