# Poco

# class PriorityNotificationQueue

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

## Description

A PriorityNotificationQueue 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 PriorityNotificationQueue is quite similar to the NotificationQueue class. The only difference to NotificationQueue is that each Notification is tagged with a priority value. When inserting a Notification into the queue, the Notification is inserted according to the given priority value, with lower priority values being inserted before higher priority values. Therefore, the lower the numerical priority value, the higher the actual notification priority.

Notifications are dequeued in order of their priority.

The PriorityNotificationQueue 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, hasIdleThreads, size, waitDequeueNotification, wakeUpAll

## Constructors

### PriorityNotificationQueue

PriorityNotificationQueue();

Creates the PriorityNotificationQueue.

## Destructor

### ~PriorityNotificationQueue

~PriorityNotificationQueue();

Destroys the PriorityNotificationQueue.

## Member Functions

### clear

void clear();

Removes all notifications from the queue.

### defaultQueue

static PriorityNotificationQueue & defaultQueue();

Returns a reference to the default PriorityNotificationQueue.

### dequeueNotification

Notification * dequeueNotification();

Dequeues the next pending notification. 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.

### dispatch

void dispatch(
NotificationCenter & notificationCenter
);

Dispatches all queued notifications to the given notification center.

### empty

bool empty() const;

Returns true if and only if the queue is empty.

### enqueueNotification

void enqueueNotification(
Notification::Ptr pNotification,
int priority
);

Enqueues the given notification by adding it to the queue according to the given priority. Lower priority values are inserted before higher priority values. The queue takes ownership of the notification, thus a call like

    notificationQueue.enqueueNotification(new MyNotification, 1);

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.

### 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 0 (null) if wakeUpAll() 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();