xpc_events(3) Library Functions Manual xpc_events(3)

xpc_events — launch-on-demand for high-level events

#include <xpc/xpc.h>

void
xpc_set_event_stream_handler(const char *name, dispatch_queue_t targetq, xpc_handler_t handler);

XPC provides a mechanism by which launchd jobs may launch on-demand for certain higher-level events, such as IOKit events or BSD Notifications. These events are delivered to the job through a handler that is set early in its execution. The period between when the event is delivered to the job and when a handler is set is race-free, and any pending events will be queued up for consumption by the job. An event is consumed when it is delivered to the handler.

IMPORTANT: If a launchd job registers for an event, it check in for the event with () during its initialization (e.g., in ()). Failure to do so is a and may result in failure to deliver future events, unexpected relaunching of the job when it exits, or other system performance problems.

Providers of events are known as streams. Two example event streams are the IOKit stream and the BSD Notifications stream. Streams are denoted by a reverse-DNS naming scheme. For the aforementioned examples, the stream names are "com.apple.iokit.matching" and "com.apple.notifyd.matching", respectively. These are currently the only two supported event streams.

A launchd job may be interested in multiple events from different event streams. Each of these events has a name provided by the job in the launchd.plist(5).

The occurrence of any of these events will launch the job on-demand if it is not already running.

Events are specified through the launchd.plist(5) with the LaunchEvents key. The value for this key is a dictionary. Each value of this dictionary is itself a dictionary corresponding to an event stream. The values of this inner dictionary are events that may cause the job to be launched on-demand. The keys of this inner dictionary are names chosen by the developer to identify the event.

<key>LaunchEvents</key>
<dict>
	<key>com.apple.iokit.matching</key>
	<dict>
		<key>Device was attached</key>
		<dict>
			<key>idProduct</key>
			<integer>2794</integer>
			<key>idVendor</key>
			<integer>725</integer>
			<key>IOProviderClass</key>
			<string>IOUSBDevice</string>
			<key>IOMatchLaunchStream</key>
			<true/>
		</dict>
	</dict>
	<key>com.apple.notifyd.matching</key>
	<dict>
		<key>interesting-notification was posted</key>
		<dict>
			<key>Notification</key>
			<string>com.apple.interesting-notification</string>
		</dict>
		<key>other-notification was posted</key>
		<dict>
			<key>Notification</key>
			<string>com.apple.other-interesting-notification</string>
		</dict>
	</dict>
</dict>

The above specifies that the job will be launched when a node matching the given matching dictionary appears in the IORegistry, when a notification named "com.apple.interesting-notification" is posted using notify_post(3), or a notification named "com.apple.other-interesting-notification" is posted.

: The IOMatchLaunchStream key is required to be present and be a Boolean set to true for use with XPC Events. It will be filtered out of the rest of the dictionary when given to IOKit to match. The reasons for this are historical and not applicable to other event streams.

Each event stream has a different plist schema.

Events are consumed with the () API. The stream argument specifies from which event stream the given handler will receive events. The targetq parameter specifies on which queue the handler will be synchronized. The handler will only ever receive dictionaries. Each dictionary is guaranteed to have the XPC_EVENT_KEY_NAME key set. The value for this key is the string that was given as the name for the event in the launchd.plist(5). So if the IOKit event in the above example was received, the value of this key would be "Device was attached". The name can be an arbitrary string, so that in the case of several events on the same stream (like notifications in the example above), the event handler can know which specific event fired.

In addition to the standard payload, events from the IOKit stream also have the "IOMatchLaunchServiceID" key set to a uint64_t which specifies the unique IORegistry ID of the node which matched the given dictionary as obtained by (). This value may be given to () to obtain the registry entry which caused the event to fire.

BSD Notfication events have no additional payload.

xpc_set_event_stream_handler("com.apple.iokit.matching", q, ^(xpc_object_t event) {
	const char *name = xpc_dictionary_get_string(event, XPC_EVENT_KEY_NAME);
	uint64_t id = xpc_dictionary_get_uint64(event, "IOMatchLaunchServiceID");

	CFMutableDictionaryRef matching = IORegistryEntryIDMatching(id);
	// Pass to IOServiceGetMatchingServices() or IOServiceAddNotification().
});

IMPORTANT: () is NOT shareable. Two different subsystems in a process cannot safely both register for events from the same event stream. Therefore, libraries and frameworks should call this API.

xpc_object(3), xpc_dictionary_create(3), xpc_array_create(3), notify(3)

1 July, 2011 Darwin