The application tells the file system events daemon to start sending events by calling FSEventStreamStart. The application services events as they arrive. The API posts events by calling the callback function specified in step 1. Before you use the file system event stream API, you must include the Core Services framework as follows:. When you compile, you must include the Core Services Framework by adding it to your target in Xcode or by adding the flag -framework CoreServices to your linker flags on the command line or in a Makefile.
The file system events API supports two types of event streams: Before you can create a stream, you must decide which type of stream to create: A per-host event stream consists of events whose IDs are increasing with respect to other events on that host. These IDs are guaranteed to be unique with one exception: Any new events will automatically start after the highest-numbered historical ID for any attached drive.
A per-disk event stream, by contrast, consists of events whose IDs are increasing with respect to previous events on that disk. It does not have any relationship with other events on other disks, and thus you must create a separate event stream for each physical device that you wish to monitor. In general, if you are writing software that requires persistence, you should use per-disk streams to avoid any confusion due to ID conflicts. By contrast, per-host streams are most convenient if you are monitoring for changes in a directory or tree of directories during normal execution, such as watching a queue directory.
If you are monitoring files on the root file system, either stream mechanism will behave similarly. For more information about run loops, read Run Loops. This function tells the event stream to begin sending events. Its sole parameter is the event stream to start. Once the event stream has been created and scheduled, if your run loop is not already running, you should start it by calling CFRunLoopRun.
Your event handler receives three lists: In effect, these represent a list of events. The first event consists of the first entry taken from each of the arrays, and so on. Your handler must iterate through these lists, processing the events as needed. For each event, you should scan the directory at the specified path, processing its contents as desired.
Normally, you need to scan only the exact directory specified by the path. However, there are three situations in which this is not the case:. If an event in a directory occurs at about the same time as one or more events in a subdirectory of that directory, the events may be coalesced into a single event. When you receive such an event, you must recursively rescan the path listed in the event. The additional changes are not necessarily in an immediate child of the listed path.
In either case, you must do a full scan of any directories that you are monitoring because there is no way to determine what may have changed. If the root directory that you are watching is deleted, moved, or renamed or if any of its parent directories are moved or renamed , the directory may cease to exist.
File System Events Programming Guide
In this case, you must rescan the entire directory because it may not exist. See the manual page for fcntl for more information. However, you should still check for this flag and take appropriate action if you receive it. As part of your handler, you may sometimes need to obtain a list of paths being watched by the current event stream. Sometimes, you may wish to monitor where you are in the stream.
You might, for example, choose to do less processing if your code is slipping significantly behind.
Creating the OS X File Watcher with JNIWrapper
You can find out the latest event included in the current batch of events by calling FSEventStreamGetLatestEventId or by examining the last event in the list. You can then compare this with the value returned by FSEventsGetCurrentEventId , which returns the highest numbered event in the system. 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. On a per-device basis, you can also easily use a time stamp to determine which events to include.
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This process is described further in Building a Directory Hierarchy Snapshot. File system events tell you that something in a given directory changed. In some cases, this is sufficient—for example, if your application is a print or mail spooler, all it needs to know is that a file has been added to the directory. In some cases, however, this is not enough, and you need to know precisely what changed within the directory.
The simplest way to solve this problem is to take a snapshot directory hierarchy, storing your own copy of the state of the system at a given point in time. You might, for example, store a list of filenames and last modified dates, thus allowing you to determine which files have been modified since the last time you performed a backup. You do this by iterating through the hierarchy and building up a data structure of your choice. As you cache this metadata, if you see changes during the caching process, you can reread the directory or directories that changed to obtain an updated snapshot.
Once you have a cached tree of metadata that accurately reflects the current state of the hierarchy you are concerned with, you can then determine what file or files changed within a directory or hierarchy after a file system event notification by comparing the current directory state with your snapshot. The scandir function returns an array of directory entries that you can quickly iterate through.
This is somewhat easier than reading a directory manually with opendir , readdir , and so on, and is slightly more efficient since you will always iterate through the entire directory while caching anyway. The polling thread only launches when at least one path needs to be polled.
Increasing the cache size will improve the reliability of rename correlation and the entry kinds of deleted entries, but will consume more RAM. The default is The throttle defaults to The interval defaults to The returned Promise resolves to a PathWatcher instance when the watcher is fully installed and events are flowing.
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The Promise may reject if the path does not exist, is not a directory, or if an operating system error prevented the watcher from successfully initializing, like a thread failing to launch or memory being exhausted. The path argument specifies the root directory to watch.
This must be an existing directory, but may be relative, contain symlinks, or contain. Multiple independent calls to watchPath may result in PathWatcher instances backed by the same native event source or polling root, so it is relatively cheap to create many watchers within the same directory hierarchy across your codebase. The options argument configures the nature of the watch.
Available options are:. The callback argument will be called repeatedly with each batch of filesystem events that are delivered until the. Event batches are Arrays containing objects with the following keys:. The callback may be invoked for filesystem events that occur before the promise is resolved, but it will be invoked for any changes that occur after it resolves. All three arguments are mandatory. This is especially true on MacOS, where timestamp resolution can cause you to receive events that occurred before you even issued the watchPath call! Returns a Disposable that clients should dispose to release the subscription.
The callback argument will be invoked with an Error with a stack trace that likely isn't very helpful and a message that hopefully is. Release an event subscription. The event callback associated with this PathWatcher will not be called after the watcher has been disposed, synchronously. Note that the native resources or polling root used to feed events to this watcher may remain, if another active PathWatcher is consuming events from it, and even if they are freed as a result of this disposal they will be freed asynchronously.
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How to monitor file system in MAC
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Launching Xcode Launching Visual Studio Latest commit fb79cd3 Feb 20, It prioritizes: High fidelity to the native filesystem watching system calls provided by each operating system. Graceful degredation to polling when native events are unavailable. Be gentle on your CPU even while polling large directory trees by throttling system calls and never using spinlocks. Respect your RAM by capping the amount of persisted information.