# Poco

# class NotificationQueue

Library: Foundation
Package: Notifications
Header: Poco/NotificationQueue.h

## Description

A NotificationQueue object provides a way to implement asynchronous notifications. This is especially useful for sending notifications from one thread to another, for example from a background thread to the main (user interface) thread.

The NotificationQueue can also be used to distribute work from a controlling thread to one or more worker threads. Each worker thread repeatedly calls waitDequeueNotification() and processes the returned notification. Special care must be taken when shutting down a queue with worker threads waiting for notifications. The recommended sequence to shut down and destroy the queue is to

1. set a termination flag for every worker thread
2. call the wakeUpAll() method
3. join each worker thread
4. destroy the notification queue.

## Member Summary

Member Functions: clear, defaultQueue, dequeueNotification, dequeueOne, dispatch, empty, enqueueNotification, enqueueUrgentNotification, hasIdleThreads, remove, size, waitDequeueNotification, wakeUpAll

## Constructors

### NotificationQueue

NotificationQueue();

Creates the NotificationQueue.

## Destructor

### ~NotificationQueue

~NotificationQueue();

Destroys the NotificationQueue.

## Member Functions

### clear

void clear();

Removes all notifications from the queue.

### defaultQueue

static NotificationQueue & defaultQueue();

Returns a reference to the default NotificationQueue.

### dequeueNotification

Notification * dequeueNotification();

Dequeues the next pending notification. Returns nullptr if no notification is available. The caller gains ownership of the notification and is expected to release it when done with it.

It is highly recommended that the result is immediately assigned to a Notification::Ptr, to avoid potential memory management issues.

### dispatch

void dispatch(
NotificationCenter & notificationCenter
);

Dispatches all queued notifications to the given notification center.

### empty

bool empty() const;

Returns true iff the queue is empty.

### enqueueNotification

void enqueueNotification(
Notification::Ptr pNotification
);

Enqueues the given notification by adding it to the end of the queue (FIFO). The queue takes ownership of the notification, thus a call like

    notificationQueue.enqueueNotification(new MyNotification);

does not result in a memory leak.

### enqueueUrgentNotification

void enqueueUrgentNotification(
Notification::Ptr pNotification
);

Enqueues the given notification by adding it to the front of the queue (LIFO). The event therefore gets processed before all other events already in the queue. The queue takes ownership of the notification, thus a call like

    notificationQueue.enqueueUrgentNotification(new MyNotification);

does not result in a memory leak.

### hasIdleThreads

bool hasIdleThreads() const;

Returns true if the queue has at least one thread waiting for a notification.

### remove

bool remove(
Notification::Ptr pNotification
);

Removes a notification from the queue. Returns true if remove succeeded, false otherwise

### size

int size() const;

Returns the number of notifications in the queue.

### waitDequeueNotification

Notification * waitDequeueNotification();

Dequeues the next pending notification. If no notification is available, waits for a notification to be enqueued. The caller gains ownership of the notification and is expected to release it when done with it. This method returns nullptr if wakeUpWaitingThreads() has been called by another thread.

It is highly recommended that the result is immediately assigned to a Notification::Ptr, to avoid potential memory management issues.

### waitDequeueNotification

Notification * waitDequeueNotification(
long milliseconds
);

Dequeues the next pending notification. If no notification is available, waits for a notification to be enqueued up to the specified time. Returns 0 (null) if no notification is available. The caller gains ownership of the notification and is expected to release it when done with it.

It is highly recommended that the result is immediately assigned to a Notification::Ptr, to avoid potential memory management issues.

### wakeUpAll

void wakeUpAll();

Wakes up all threads that wait for a notification.

### dequeueOne

Notification::Ptr dequeueOne();