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Using Persistent Events

One of the most powerful features of file system events is their persistence across reboots. This means that your application can easily find out what happened since a particular time or a particular event in the distant past. By doing so, you can find out what files have been modified even when your application is not running. This can greatly simplify tasks such as backing up modified files, checking for changed dependencies in multi-file projects, and so on.

To work with persistent events, your application should regularly store the last event ID that it processes. Then, when it needs to go back and see what files have changed, it only needs to look at events that occurred after the last known event. To obtain all events since a particular event in the past, you pass the event ID in the sinceWhen argument to FSEventStreamCreate or FSEventStreamCreateRelativeToDevice.] [On a per-device basis, you can also easily use a timestamp to determine which events to include. To do this, you must first call FSEventsGetLastEventIdForDeviceBeforeTime to obtain the last event ID sinceWhen argument to FSEventStreamCreateRelativeToDevice.

On a per-device basis, you can also easily use a time stamp to determine which events to include. To do this, you must first call FSEventsGetLastEventIdForDeviceBeforeTime to obtain the last event ID for that device prior to the specified time stamp. You then pass the resulting value to FSEventStreamCreateRelativeToDevice. This is described further in “Special Considerations for Per-Device Streams.”

When working with persistent events, a commonly-used technique is to combine file system event notifications with a cached “snapshot” of the metadata of files within the tree. This process is described further in “Building a Directory Hierarchy Snapshot.”

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