Qt Signals And Slots Observer Pattern

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Qt Connect Signal Slot
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Comment: 2010-08-12

Back when I made KSignals version 1 (dynamic memory) and version 2 (static memory for embedded systems) I had not read GotW_83: Generic Callbacks but I'm still kind of happy that I unbeknowst of much still managed to on my own come up with something very similar, although lacking some of the finer points Herb makes.

Either way I strongly recommend you to read Herbs Gotw article since it's very, very easy read and explains better than any other function callback or function pointer text that I've read how to setup the basics that are needed for Signals and Slots. If you think this is something good, then please go ahead and use my versions and tailor make them as you please (which should be easy since they're only few lines with bare bone code) https://luckyrate.netlify.app/paydirt-slots-play-for-fun.html.

Cheers
Kjell Hedström
https://luckyrate.netlify.app/planetside-2-infiltrator-ability-slot.html.

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The Observer pattern is a mechanism for notifying changes of state in one object to one or more other objects. In Qt programming, it is seldom used, because signals and slots fulfill that role very well. Instead of emitting signals, an observable class calls virtual functions on a list of 'observers' (or 'listeners').

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Signal and slots is a concept developed from Qt. It is basically a generalized implementation of the Observer pattern (see also publisher/subscriber) The purpose of the KjellKod signal-n-slot is to have the power of Observer pattern - but made with generic function callback. The most famous implementations of Signals and Slots are made by Qt and Boost (maybe libsigc++ is worth mentioning also?)

Meeting 13: Qt Signals and Slots¶. Today we will learn about a variation of the Observer design pattern that is used prominently within Qt, called signals and slots. Observer pattern ¶ The idea of the observer pattern is that observers keep track (the state of) of an object, and that an object is agnostic about what it’s tracked. Signals and slots¶ The Qt GUI toolkit makes use of a mechanism called “signals and slots” as an easy way to connect different components of an application.

My own implementation of signals and slots were made when I wanted to learn more about C++ generic function callbacks. Now it's a fully functional library that is in use in multiple projects. KSignals (1 & 2) are very much simplistic signal and slot implementations, but just for that reason it is easy to use it and modify it to your own needs.
Below I've desribed two flavors of the KSignals. The first version was the initial one with dynamic memory (slot) allocation. The second version was made for a barbone embedded system - which disallowed dynamic memory allocation on the heap.

A comparison between the much more advanced and feature rich Boost signals and my own KSignals (version 2) can be found at the end of this article.

Example of Signals (other than KSignals can be found in this article by Scott Collins)

My own signal-n-slot definition

The KjellKod signal-slot mechanism is a C++, cross platform compatible implementation of the Observer design pattern.

Signals are basically notifications of an (observable) event. Signals are connected to Slots which each store a function callback to an object. A signal can be connected to many slots and all slots/receivers are notified when the signal is emitted.

A signal can be just a notification, or it can pass along information. This make's it very handy when creating loosely coupled software systems. Normally such systems are made with a Publisher-Subscriber pattern (a.k.a. Observer) but with signals you have less virtual call overhead and you don't have to implement a given interface to have the power of loose coupling.

The only requirement on an objects function callback that is to be stored within a slot is that it must be able to receive the same argument(s) as the publishing signal is sending.

I.e. If it is a void signal, then the slot (stored callback function) must have a zero argument list. Likewise, if the signal sends out an argument, the receiving function must have that argument type, and only that argument in its argument list. If this requirement is not adhered to, the compiler will generate an error message. I.e. signal/slot is typesafe.

Let show this with some examples. Since the syntax of my two flavors of KSignals is different I show both versions.
Note: All code below written with formatting to be easy to read on a webpage, not necessarily good 'c++ coding style' formatting.

KSignal flavor with no dynamic memory allocation necessary. One benefit of this way of doing it is that the slot function clearly is shown in the signal receiving object,. making it easier to follow in code which functions may be recipients of signal calling. To distinguish between the KSignal flavors I use in this example the namespace Rtfor lack of better name (anyone with a suggestion?)

void hello(std::string msg) { std

::cout << 'Hello ' << msg.c_str() << std::endl; }
Rt::Slot1<std::string, HelloWorld> slotStr;
};}

Let's try it again but this time with the original KSignal flavor

Using dynamic memory allocation (slots) and no explicit declaration of slots in the source code. Another difference is that the functions cannot be named identically,. this version of the KSignals cannot work out the template arguments for the ambiguous calls.

What more

Yes - I can also mention that both flavors of KSignal detects recursive calling of the signals and asserts if that happens. This is something that COULD be disallowed,. but in my experience it is usually by mistake such behaviors and thereby by default not allowed.

Example:

Future version of KSignals are already thought of, but with a recent addition to the family :-) it's on hold for now ..

Anyhow, in the next version I had planned to use with dynamic number of slots, but using syntax of explicit declaration of the slots. Preferably as they are now (up to the programmer) i.e. explicitly declared next to the recipient function.
Another improvement would be to remove the Signal0and Signal1<Type> syntax difference. It would be better to always use the syntax of Signal<Type>where Type could be eithervoid or a non-void type argument.
Further using the same concept as Boost in the area of shared_ptr and weak_ptr and their inner workings of shared_cntr and weak_cntr it is feasible to have auto disconnect on when objects go out of scope. .. However this would move KSignal towards more feature rich areas and flirting with the idea of thread safety.

.. and I have not planned to make the next KSignal version threadsafe. Well it all depends what you put into the word thread-safe. But for communication between threads I would much rather then use the concept of Active objects (I hope to write more about that later) queues and maybe futures. Using signals of any type (boost, sigslot, libsig or ksignal) to communicate between threads must be done VERY carefully but preferably not at all since it essentially will call 'unknown code' from the thread space perspective at least.

Cons

Having too many signal - slot calls instead of normal function calls would made the code incomprehensible. Using the explicit slot declaration somewhat rectifies this.
KSignals are not threadsafe in that they do not have atomic disconnect/connect. They are not reentrant since a signal member variable is used to verify whether recursive calling is used (recursive calling is considered an error). Changing it to making signals reentrant and allowing recursive calling is an small and easy change.
No auto-disconnect if an object stored in a slot is deleted. Emitting a signal to a slot in a destroyed object is obviously an error (crash). Fixing this (a la boost) requires some work but is definitely doable.
Will take more time/space than a direct function call but less time than a virtual function call. Which would be the case if you instead used the Publisher-Subscriber (a.k.a. Observer) solution.

Pros

Qt Signals And Slots Observer Pattern Download

Easy to understand syntax and usage. A signal is an object and you call emit just like you call another function. It is also type safe and argument list safe.
It is a very valuable tool when putting together software pieces that doesn't or shouldn't be closely coupled.
It is a definite improvement of the Observer design pattern - and should in my opinion be used instead.
KSignals doesn't require a lot of libraries to install, it's just a few files and with a license that put basically no limits on the use of the software whether or not commercial or open source.
KSignals is free, open source, do with it what you like - and the code is so bare boned and simple that it should be easy to modify it to your specific needs or to study it for understanding of signals and generic function callback.
Slot functions can be inlined (I put this note here since I read in Scott Collins articlethat wasn't the case for all signal/slot libraries)

Lets make a Boost and KSignal comparison. Differences are marked.

I haven't explained below why I believe some things are better than other but if I get flamed I might to this :-)

Boost.Signals is a beautiful made library. But the ambiguous way in which it can be used and that it copies the object with the slot receiving function is in my opinion a bad design decision (correct me if I'm wrong about the copying part)

In contrast, KSignals, by explicitly declaring the slot in the class with the slot-function it is abundantly clear for the developer (future maintenance etc) that a signal may connect to a specific function.
In my own biased opinion I would say that Simple is Better :)

Boost.Signals	KSignals
a signal is an object	a signal is an object
a signal is emitted by calling it like a function. I.e. Slot function is always the operator ( )	a signal is emitted by calling with the emit keyword.No restriction on function name as long as argument list matches.
signals can be global, local, or member objects	signals can be global, local, or member objects
any code with sufficient access to connect to the signal can also cause it to be emitted	any code with sufficient access to connect to the signal can also cause it to be emitted
a slot is any callable function or function-object	a slot is any callable function within an object
can return values collected from multiple slots	no return
synchronous	synchronous
not thread-safe	not thread-safe
Pro: It is possible to have auto-disconnect on slot destruction : if and only if the slot is trackable Con: I.e. there are several ways to define signals - this can be confusing and give more error prone usage of the library	Currently not any released version with auto-disconnect on slot destruction.
type-safe (compile-time checked). argument-list much match exactly	type-safe (compile-time checked). argument-list must match exactly
allows recursive calls	disallows recursive calls
signals, slots may be templates	signals, slots may be templates
implemented via straight C++	implemented via straight C++
Copies the object that keeps the slot function.	Does not copy the object that keeps the slot function.

Signals and slots are definitely an improvement of the Observer Pattern. It is a great tool for making software with separate components, it promotes loosely coupled design. Signals exist with many flavors of which this text describes a few.

Using KSignals you have a statically type-safe, template-based, bare bone and easy to modify implementation of signals and slots. Signals are used just as any other object with functions. Slots (in the case of Rt::slot) are explicitly declared in the receiving object - making it obvious what functions may be recipient of a signal call.
Compared to the Observer pattern that use virtual calls the KSignal implementation is faster since there is no virtual call overhead (For details I can on request give information about this: I measured on a 300Mhz PPC, bare boned embedded system for comparison reasons).

Signals (no matter what type) should be used in moderation like everything else. But in the right place they promote good loosely coupled software that is easy to understand, flexible, re-usable and easy to test.

Comments?

I appreciate feedback. You can reach me through my blog. This entry can be commented on
http://kjellkod.blogspot.com/2007/10/ksignal-signal-and-slot-observer-design.html

Signals and slots is a language construct introduced in Qt for communication between objects^[1] which makes it easy to implement the observer pattern while avoiding boilerplate code. The concept is that GUI widgets can send signals containing event information which can be received by other widgets / controls using special functions known as slots. This is similar to C/C++ function pointers, but signal/slot system ensures the type-correctness of callback arguments.^{[citation needed]}

The signal/slot system fits well with the way graphical user interfaces are designed. Similarly, the signal/slot system can be used for other non-GUI usages, for example asynchronous I/O (including sockets, pipes, serial devices, etc.) event notification or to associate timeout events with appropriate object instances and methods or functions. La noire vice slot machine. It is easy to use and no registration/deregistration/invocation code need to be written, because Qt's metaobject compiler (MOC) automatically generates the needed infrastructure.

A commonly used metaphor is a spreadsheet. A spreadsheet has cells that observe the source cell(s). When the source cell is changed, the dependent cells are updated from the event.

Alternative implementations[edit]

There are some implementations of signal/slot systems based on C++ templates, which don't require the extra metaobject compiler, as used by Qt, such as libsigc++, sigslot, vdk-signals, nano-signal-slot, neosigslot, Signals, boost.signals2, Synapse, Cpp::Events, Platinum and JBroadcaster. Common Language Infrastructure (CLI) languages such as C# also supports a similar construct although with a different terminology and syntax: events play the role of signals, and delegates are the slots. Another implementation of signals exists for ActionScript 3.0, inspired by C# events and signals/slots in Qt. Additionally, a delegate can be a local variable, much like a function pointer, while a slot in Qt must be a class member declared as such. The C based GObject system also provides similar functionality via GSignal.In D it is implemented by std.signals.

Libraries[edit]

Java: sig4j - multi-threaded, type-safe, based on the FunctionalInterface annotation introduced in Java 8.

C++: vdk-signals - thread-safe, type-safe, written in C++11 with atomic variables.

Qt Connect Signal Slot

References[edit]

Qt Signals And Slots Tutorial

^'Signals & Slots - QtCore 5.1'. Qt Project. 2013-07-04. Retrieved 2013-07-04.

Qt Signals And Slots Observer Pattern Printable

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