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Each PyQt widget, which is derived from QObject class, is designed to emit ‘ signal ’ in response to one or more events. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘ slot ’. The slot can be any callable Python function. Code for this videothis video we will learn How Qt Signals and Slots Wor.

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Qt5 alpha has been released. One of the features which I have been working on is a new syntax for signals and slot.This blog entry will present it.

Here is how you would connect a signal to a slot:

What really happens behind the scenes is that the SIGNAL and SLOT macros will convert their argument to a string. Then QObject::connect() will compare those strings with the introspection data collected by the moc tool.

What's the problem with this syntax?

While working fine in general, we can identify some issues:

No compile time check: All the checks are done at run-time by parsing the strings. That means if you do a typo in the name of the signal or the slot, it will compile but the connection will not be made, and you will only notice a warning in the standard output.
Since it operates on the strings, the type names of the slot must match exactly the ones of the signal. And they also need to be the same in the header and in the connect statement. This means it won't work nicely if you want to use typedef or namespaces

In the upcoming Qt5, an alternative syntax exist. The former syntax will still work. But you can now also use this new way of connecting your signals to your slots:

Which one is the more beautiful is a matter of taste. One can quickly get used to the new syntax.

So apart from the aesthetic point of view, let us go over some of the things that it brings us:

Compile-time checking

You will get a compiler error if you misspelled the signal or slot name, or if the arguments of your slot do not match those from the signal.
This might save you some time while you are doing some re-factoring and change the name or arguments of signals or slots.

An effort has been made, using static_assert to get nice compile errors if the arguments do not match or of you miss a Q_OBJECT

Arguments automatic type conversion

Not only you can now use typedef or namespaces properly, but you can also connect signalsto slots that take arguments of different types if an implicit conversion is possible

In the following example, we connect a signal that has a QString as a parameter to a slot that takes a QVariant. It works because QVariant has an implicit constructor that takes a QString

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Connecting to any function

As you might have seen in the previous example, the slot was just declared as publicand not as slot. Qt will indeed call directly the function pointer of the slot, andwill not need moc introspection anymore. (It still needs it for the signal)

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But what we can also do is connecting to any function or functor:

This can become very powerful when you associate that with boost or tr1::bind.

C++11 lambda expressions

Everything documented here works with the plain old C++98. But if you use compiler that supportsC++11, I really recommend you to use some of the language's new features.Lambda expressions are supportedby at least MSVC 2010, GCC 4.5, clang 3.1. For the last two, you need to pass -std=c++0x asa flag.

You can then write code like:

This allows you to write asynchronous code very easily.

Update: Also have a look what other C++11 features Qt5 offers.

It is time to try it out. Check out the alpha and start playing. Don't hesistate to report bugs.

Introduction

Remember old X-Window call-back system? Generally it isn't type safe and flexible. There are many problems with them. Qt offer new event-handling system - signal-slot connections. Imagine alarm clock. When alarm is ringing, signal is sending (emitting). And you're handling it as a slot.

Every QObject class may have as many signals of slots as you want.
You can emit signal only from that class, where signal is.
You can connect signal with another signal (make chains of signals);
Every signal and slot can have unlimited count of connections with other.
ATTENTION! You can't set default value in slot attributes. e.g.void mySlot(int i = 0);

Connection

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You can connect signal with this template:QObject::connect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method);You have to wrap const char * signal and const char * method into SIGNAL () and SLOT() macros.

And you also can disconnect signal-slot:QObject::disconnect ( const QObject * sender, const char * signal, const QObject * receiver, const char * method);

Deeper

Widgets emit signals when events occur. For example, a button will emit a 'clicked' signal when it is clicked. A developer can choose to connect to a signal by creating a function (a 'slot') and calling the connect() function to relate the signal to the slot. Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes. Since signals and slots are type-safe, type errors are reported as warnings and do not cause crashes to occur.

For example, if a Quit button's clicked() signal is connected to the application's quit() slot, a user's click on Quit makes the application terminate. In code, this is written as

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connect(button, SIGNAL (clicked()), qApp, SLOT (quit()));

Connections can be added or removed at any time during the execution of a Qt application, they can be set up so that they are executed when a signal is emitted or queued for later execution, and they can be made between objects in different threads.

The signals and slots mechanism is implemented in standard C++. The implementation uses the C++ preprocessor and moc, the Meta-Object Compiler, included with Qt. Code generation is performed automatically by Qt's build system. Developers never have to edit or even look at the generated code.

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