# WatchKey

public interface **WatchKey** A token representing the registration of a `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/Watchable.html` object
with a `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchService.html`.

A watch key is created when a watchable object is registered with a watch
service. The key remains `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#isValid()` until:

1. It is cancelled, explicitly, by invoking its `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#cancel()` method, or
2. Cancelled implicitly, because the object is no longer accessible, or
3. By `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchService.html#close()` the watch service.

A watch key has a state. When initially created the key is said to be
*ready* . When an event is detected then the key is *signalled*
and queued so that it can be retrieved by invoking the watch service's `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchService.html#poll()` or `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchService.html#take()` methods. Once
signalled, a key remains in this state until its `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#reset()` method
is invoked to return the key to the ready state. Events detected while the
key is in the signalled state are queued but do not cause the key to be
re-queued for retrieval from the watch service. Events are retrieved by
invoking the key's `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#pollEvents()` method. This method
retrieves and removes all events accumulated for the object. When initially
created, a watch key has no pending events. Typically events are retrieved
when the key is in the signalled state leading to the following idiom:

```
     for (;;) {
         // retrieve key
         WatchKey key = watcher.take();

         // process events
         for (WatchEvent<?> event: key.pollEvents()) {
             :
         }

         // reset the key
         boolean valid = key.reset();
         if (!valid) {
             // object no longer registered
         }
     }
 
```

Watch keys are safe for use by multiple concurrent threads. Where there
are several threads retrieving signalled keys from a watch service then care
should be taken to ensure that the `reset` method is only invoked after
the events for the object have been processed. This ensures that one thread
is processing the events for an object at any time.

### Public Method Summary

|---|---|
| abstract void | [cancel](https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#cancel())() Cancels the registration with the watch service. |
| abstract boolean | [isValid](https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#isValid())() Tells whether or not this watch key is valid. |
| abstract [List](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/List.html)\<[WatchEvent](https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchEvent.html)\<?\>\> | [pollEvents](https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#pollEvents())() Retrieves and removes all pending events for this watch key, returning a `List` of the events that were retrieved. |
| abstract boolean | [reset](https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#reset())() Resets this watch key. |
| abstract [Watchable](https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/Watchable.html) | [watchable](https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#watchable())() Returns the object for which this watch key was created. |

## Public Methods

#### public abstract void
**cancel**
()

Cancels the registration with the watch service. Upon return the watch key
will be invalid. If the watch key is enqueued, waiting to be retrieved
from the watch service, then it will remain in the queue until it is
removed. Pending events, if any, remain pending and may be retrieved by
invoking the `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#pollEvents()` method after the key is
cancelled.

If this watch key has already been cancelled then invoking this
method has no effect. Once cancelled, a watch key remains forever invalid.

#### public abstract boolean
**isValid**
()

Tells whether or not this watch key is valid.

A watch key is valid upon creation and remains until it is cancelled,
or its watch service is closed.

##### Returns

- `true` if, and only if, this watch key is valid

#### public abstract [List](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/List.html)\<[WatchEvent](https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchEvent.html)\<?\>\>
**pollEvents**
()

Retrieves and removes all pending events for this watch key, returning
a `List` of the events that were retrieved.

Note that this method does not wait if there are no events pending.

##### Returns

- the list of the events retrieved; may be empty

#### public abstract boolean
**reset**
()

Resets this watch key.

If this watch key has been cancelled or this watch key is already in
the ready state then invoking this method has no effect. Otherwise
if there are pending events for the object then this watch key is
immediately re-queued to the watch service. If there are no pending
events then the watch key is put into the ready state and will remain in
that state until an event is detected or the watch key is cancelled.

##### Returns

- `true` if the watch key is valid and has been reset, and `false` if the watch key could not be reset because it is no longer `https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/WatchKey.html#isValid()`

#### public abstract [Watchable](https://developers.google.com/j2objc/javadoc/jre/reference/java/nio/file/Watchable.html)
**watchable**
()

Returns the object for which this watch key was created. This method will
continue to return the object even after the key is cancelled.

As the `WatchService` is intended to map directly on to the
native file event notification facility (where available) then many of
details on how registered objects are watched is highly implementation
specific. When watching a directory for changes for example, and the
directory is moved or renamed in the file system, there is no guarantee
that the watch key will be cancelled and so the object returned by this
method may no longer be a valid path to the directory.

##### Returns

- the object for which this watch key was created