检测何时卸下可移动存储

Detect when removable storage is unmounted

I am working on an app which should detect events that happen when removable storage is unmounted or forcefully unplugged from the USB. How can I receive these events?

I have seen NSWorkspace for the first possibility of smoothly unmounting the device but this class has methods like -unmountAndEjectDeviceAtPath: to unmount a device. Can someone point me to some sample code that detects unmounted volumes?

A pice of code from HardwareGrowler:

NSWorkspace *workspace = [NSWorkspace sharedWorkspace];
NSNotificationCenter *center = [workspace notificationCenter];

[center addObserver:[VolumeNotifier class] selector:@selector(volumeDidMount:) name:NSWorkspaceDidMountNotification object:nil];
[center addObserver:[VolumeNotifier class] selector:@selector(volumeDidUnmount:) name:NSWorkspaceDidUnmountNotification object:nil];
[center addObserver:[VolumeNotifier class] selector:@selector(volumeWillUnmount:) name:NSWorkspaceWillUnmountNotification object:nil];

You then need to implement the methods to react on the notifications ala

+ (void) volumeDidUnmount:(NSNotification *)aNotification;
{
...
}

For the whole implementation check out http://growl.info/source.php In the Source bundle go to Extras/HardwareGrowler and there check out VolumeNotifier.h/m

UPDATE:

Peters answer is superior to this. Please consider using the Disk Arbitration framework if you come about this problem.

Make sure to make it a background app as well ;)
Man, and I work on HardwareGrowler! I would not make those class methods, though. Someday, when other things aren't so pressing, I intend to go through and change the notifiers to instances (among other clean-ups).

Use the DARegisterDiskDisappearedCallback function in the Disk Arbitration framework.

Documentation can now be found at developer.apple.com/documentation/Darwin/Reference/…
@QuinnTaylor: Documentation links added. Thanks!
I believe this is the correct answer.
@PacMan--: Sort of. There are certain non-trivial cases in which a volume can be unmounted without disappearing (particularly when it's not the only volume on that disk). You also want DescriptionChanged; a volume whose disk description doesn't have a mount path is not mounted.
It is described in Amit Singh's book "Mac OS X Internals: A Systems Approach" too.