Omggain

Signals and slots is a language construct introduced in Qt, which makes it easy to implement the Observer pattern while avoiding boilerplate code. The concept is that controls (also known as widgets) can send signals containing event information which can be received by other controls using special functions known as slots. As @SGaist said in Signal-slot-mechanism vs. Observer pattern - Good practice: Note that in the case of signals and slots, the emitters don't care about what is connected to the signals they provide, it's not their responsibility. That's the beauty of it, you continue to be flexible, without losing anything.

Copyright (C) 2016-2018 David Capello

Library to use the observer pattern in C++11 programs withobservable/observer classes or signals/slots.

Features

• Generate an observable notification/signal from multiple threads
• Add/remove observers/slots from multiple threads
• Erase/disconnect an observer/slot from the same observable notification/signal
• Reconnect an observer in the same notification

Observable

An observable Widget:

An example

Signal

Tested Compilers

• Visual Studio 2015
• Xcode 7.3.1 (-std=c++11)
• GCC 4.8.4 (-std=c++11)

From Wikipedia, the free encyclopedia

The observer pattern (a subset of theasynchronous publish/subscribe pattern) is a software designpattern in which an object, called the subject,maintains a list of its dependents, called observers, and notifiesthem automatically of any state changes, usually by calling one oftheir methods. It is mainly used toimplement distributed event handling systems.

Examples

Below is an example that takes keyboard input and treats eachinput line as an event. The example is built upon the libraryclasses java.util.Observer and java.util.Observable. When a string issupplied from System.in, the method notifyObservers isthen called, in order to notify all observers of the event'soccurrence, in the form of an invocation of their 'update' methods- in our example, ResponseHandler.update(...).

Java

The file MyApp.java contains a main() method that might be usedin order to run the code.

Here another example how to implement the observer pattern injava.

An observer interface which has an update method.

Two implementations of the Observer interface. Each implementthe update method where they update their own state.

This is the LogSubject which get observed by the Observers. Itmaintains a list of Observers. Everytime its state is changed, itruns the update method of all of the observers in its list changingtheir states as well.

And here the usage of the pattern above:

Python

Here is a very basic implementation of the observer pattern inPython:

In the example above, a Python dictionary is overloaded toaccept functions as keys whose return values are stored incorresponding slots in the dictionary. Below is a simpleillustration. This class could easily be extended with anasynchronous notification method. Although this example storesfunctions as keys, it is conceivable that one could storeuser-defined class instances whose methods are accessed in asimilar fashion.

C++

Implementations

The observer pattern is implemented in numerous programming libraries and systems,including almost all GUI toolkits.

Some of the most notable implementations of this pattern:

• The Java Swinglibrary makes extensive use of the observer pattern for eventmanagement
• Boost.Signals, anextension of the C++ STL providing a signal/slot model
• The Qt C++framework's signal/slot model
• libsigc++ - the C++ signalling template library.
• sigslot - C++ Signal/Slot Library
• Cpp::Events -Template-based C++ implementation that introduces separation ofconnection management interface of the event object from theinvocation interface.
• XLObject - Template-basedC++ signal/slot model patterned after Qt.
• Signals - A lightweightand non-intrusive C++ signal/slot modelimplementation.
• libevent - Multi-threadedCrossplatform Signal/Slot C++ Library
• GObject, in GLib - an implementation of objectsand signals/callbacks in C. (Thislibrary has many bindings to other programming languages.)
• Exploring the Observer DesignPattern - the C# and Visual Basic.NET implementation, using delegates and the Event pattern
• Using the ObserverPattern, a discussion and implementation in REALbasic
• flash.events, a packagein ActionScript3.0 (following from the mx.events package in ActionScript2.0).
• CSP - ObserverPattern using CSP-likeRendezvous (each actor is a process, communication is viarendezvous).
• YUI Event utilityimplements custom events through the observer pattern
• Py-notify, a Pythonimplementation
• Event_Dispatcher, a PHP implementation
• Delphi Observer Pattern, aDelphi implementation
• .NET Remoting, Applyingthe Observer Pattern in .NET Remoting (using C#)
• PerfectJPattern Open SourceProject, Provides a context-free and type-safe implementationof the Observer Pattern in Java.
• Cells, a dataflowextension to CommonLisp that uses meta-programming to hide some of the details ofObserver pattern implementation.
• Publish/Subscribe withLabVIEW, Implementation example of Observer orPublish/Subscribe using G.
• SPL, the Standard PHPLibrary
• EventDispatchersingleton, a JavaScript core API based Signals andslots implementation - an observer concept different from Publish/subscribe - prettylightweighted but still type-safety enforcing.
• Observer, from the RubyStandard Library. Also see Russ Olsen's coverage of this pattern inRuby in Design Patterns inRuby

References

C++ Signals And Slots

• http://www.research.ibm.com/designpatterns/example.htm
• 'Speaking on the Observerpattern' - JavaWorld

See also

• Design Patterns(book), the book which gave rise to the study of designpatterns in computer science
• Design pattern (computerscience), a standard solution to common problems in softwaredesign
• model-view-controller (MVC)

Externallinks

Signals And Slots

• Observer Patternimplementation in JDK 6
• A sample implementation in.NET
• Observer Pattern inJava
• Definition, C# example &UML diagram
• Jt J2EE Pattern Oriented Framework