Class Poco::AbstractEvent
Author: Applied Informatics Software Engineering GmbH and Contributors

# Poco

# template < class TArgs, class TStrategy, class TDelegate, class TMutex = FastMutex >

# class AbstractEvent

Library: Foundation
Package: Events
Header: Poco/AbstractEvent.h

## Description

An AbstractEvent is the base class of all events. It works similar to the way C# handles notifications (aka events in C#).

Events can be used to send information to a set of delegates which are registered with the event. The type of the data is specified with the template parameter TArgs. The TStrategy parameter must be a subclass of NotificationStrategy. The parameter TDelegate can either be a subclass of AbstractDelegate or of AbstractPriorityDelegate.

Note that AbstractEvent should never be used directly. One ought to use one of its subclasses which set the TStrategy and TDelegate template parameters to fixed values. For most use-cases the BasicEvent template will be sufficient:

    #include "Poco/BasicEvent.h"
    #include "Poco/Delegate.h"

Note that as of release 1.4.2, the behavior of BasicEvent equals that of FIFOEvent, so the FIFOEvent class is no longer necessary and provided for backwards compatibility only.

BasicEvent works with a standard delegate. They allow one object to register one or more delegates with an event. In contrast, a PriorityDelegate comes with an attached priority value and allows one object to register for one priority value one or more delegates. Note that PriorityDelegates only work with PriorityEvents:

    #include "Poco/PriorityEvent.h"
    #include "Poco/PriorityDelegate.h"

Use events by adding them as public members to the object which is throwing notifications:

    class MyData
    {
    public:
        Poco::BasicEvent<int> dataChanged;

        MyData();
        ...
        void setData(int i);
        ...
    private:
        int _data;
    };

Firing the event is done either by calling the event's notify() or notifyAsync() method:

    void MyData::setData(int i)
    {
        this->_data = i;
        dataChanged.notify(this, this->_data);
    }

Alternatively, instead of notify(), operator () can be used.

    void MyData::setData(int i)
    {
        this->_data = i;
        dataChanged(this, this->_data);
    }

Note that operator (), notify() and notifyAsync() do not catch exceptions, i.e. in case a delegate throws an exception, notifying is immediately aborted and the exception is propagated back to the caller.

Delegates can register methods at the event. In the case of a BasicEvent the Delegate template is used, in case of an PriorityEvent a PriorityDelegate is used. Mixing of delegates, e.g. using a PriorityDelegate with a BasicEvent is not allowed and can lead to compile-time and/or run-time errors. The standalone delegate() functions can be used to construct Delegate objects.

Events require the observers to have one of the following method signatures:

    void onEvent(const void* pSender, TArgs& args);
    void onEvent(TArgs& args);
    static void onEvent(const void* pSender, TArgs& args);
    static void onEvent(void* pSender, TArgs& args);
    static void onEvent(TArgs& args);

For performance reasons arguments are always sent by reference. This also allows observers to modify the event argument. To prevent that, use `const TArg` as template parameter. A non-conformant method signature leads to compile errors.

Assuming that the observer meets the method signature requirement, it can register this method with the += operator:

    class MyController
    {
    protected:
        MyData _data;

        void onDataChanged(void* pSender, int& data);
        ...
    };

    MyController::MyController()
    {
        _data.dataChanged += delegate(this, &MyController::onDataChanged);
    }

In some cases it might be desirable to work with automatically expiring registrations. Simply add to delegate as 3rd parameter a expireValue (in milliseconds):

    _data.dataChanged += delegate(this, &MyController::onDataChanged, 1000);

This will add a delegate to the event which will automatically be removed in 1000 millisecs.

Unregistering happens via the -= operator. Forgetting to unregister a method will lead to segmentation faults later, when one tries to send a notify to a no longer existing object.

    MyController::~MyController()
    {
        _data.dataChanged -= delegate(this, &MyController::onDataChanged);
    }

Working with PriorityDelegate's as similar to working with BasicEvent. Instead of delegate(), the priorityDelegate() function must be used to create the PriorityDelegate.

## Member Summary

Member Functions: add, clear, disable, empty, enable, executeAsyncImpl, hasDelegates, isEnabled, notify, notifyAsync, operator (), operator +=, operator -=, remove

## Nested Classes

### struct NotifyAsyncParams

## Types

### Args

typedef TArgs Args;

### DelegateHandle

typedef TDelegate * DelegateHandle;

## Constructors

### AbstractEvent

AbstractEvent();

### AbstractEvent

AbstractEvent(
const TStrategy & strat
);

## Destructor

### ~AbstractEvent

virtual ~AbstractEvent();

## Member Functions

### add

DelegateHandle add(
const TDelegate & aDelegate
);

Adds a delegate to the event.

Exact behavior is determined by the TStrategy.

Returns a DelegateHandle which can be used in call to remove() to remove the delegate.

### clear

void clear();

Removes all delegates.

### disable

void disable();

Disables the event. notify and notifyAsnyc will be ignored, but adding/removing delegates is still allowed.

### empty

bool empty() const;

Checks if any delegates are registered at the delegate.

### enable

void enable();

Enables the event.

### hasDelegates

bool hasDelegates() const;

### isEnabled

bool isEnabled() const;

### notify

void notify(
const void * pSender,
TArgs & args
);

Sends a notification to all registered delegates. The order is determined by the TStrategy. This method is blocking. While executing, the list of delegates may be modified. These changes don't influence the current active notifications but are activated with the next notify. If a delegate is removed during a notify(), the delegate will no longer be invoked (unless it has already been invoked prior to removal). If one of the delegates throws an exception, the notify method is immediately aborted and the exception is propagated to the caller.

### notifyAsync

ActiveResult < TArgs > notifyAsync(
const void * pSender,
const TArgs & args
);

Sends a notification to all registered delegates. The order is determined by the TStrategy. This method is not blocking and will immediately return. The delegates are invoked in a seperate thread. Call activeResult.wait() to wait until the notification has ended. While executing, other objects can change the delegate list. These changes don't influence the current active notifications but are activated with the next notify. If a delegate is removed during a notify(), the delegate will no longer be invoked (unless it has already been invoked prior to removal). If one of the delegates throws an exception, the execution is aborted and the exception is propagated to the caller.

### operator ()

void operator () (
const void * pSender,
TArgs & args
);

Shortcut for notify(pSender, args);

### operator ()

void operator () (
TArgs & args
);

Shortcut for notify(args).

### operator +=

void operator += (
const TDelegate & aDelegate
);

Adds a delegate to the event.

Exact behavior is determined by the TStrategy.

### operator -=

void operator -= (
const TDelegate & aDelegate
);

Removes a delegate from the event.

If the delegate is not found, this function does nothing.

### remove

void remove(
DelegateHandle delegateHandle
);

Removes a delegate from the event using a DelegateHandle returned by add().

If the delegate is not found, this function does nothing.

### executeAsyncImpl

TArgs executeAsyncImpl(
const NotifyAsyncParams & par
);

## Variables

### _enabled

bool _enabled;

Stores if an event is enabled. Notfies on disabled events have no effect but it is possible to change the observers.

### _executeAsync

ActiveMethod < TArgs, NotifyAsyncParams, AbstractEvent > _executeAsync;

### _mutex

mutable TMutex _mutex;

### _strategy

TStrategy _strategy;

The strategy used to notify observers.