Qt Signal Slot Register Type

Posted By admin On 10.06.20

Overview

Qt Signal Slots
Qt Signal Slot Parameter
Qt Signal Slot With 2 Arguments
Qt Signals And Slots Tutorial
Qt Signal Slot Example
Signal And Slot
Qt Signal Slot Thread

Qt Signal Slots

When creating user interfaces with Qt, particularly those with specialized controls and features, developers sometimes need to create new data types that can be used alongside or in place of Qt's existing set of value types.

Standard types such as QSize, QColor and QString can all be stored in QVariant objects, used as the types of properties in QObject-based classes, and emitted in signal-slot communication.

Qt Signal Slot Parameter

In this document, we take a custom type and describe how to integrate it into Qt's object model so that it can be stored in the same way as standard Qt types. We then show how to register the custom type to allow it to be used in signals and slots connections.

Creating a Custom Type

How to use enums in Qt signals and slots. The main function simply starts the thread, the thread then creates an instance of the StateMachine and processes signals from the StateMachine. I am pretty new to Qt, so I was a bit puzzled when I realised that you have to enclose the enum with QENUMS and register it with the type system. So It's fully possible that I've made some rookie mistake.
How to Register your C Class as a QML Type. The second possibility to use C components in QML is to register the class as a QML type. This allows to create objects (= instances) of your type directly in QML instead of C. And the best thing is, the concepts with signals, slots and properties we used in the previous example still apply.
Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type.
How to Register your C Class as a QML Type. The second possibility to use C components in QML is to register the class as a QML type. This allows to create objects (= instances) of your type directly in QML instead of C. And the best thing is, the concepts with signals, slots and properties we used in the previous example still apply.

Before we begin, we need to ensure that the custom type we are creating meets all the requirements imposed by QMetaType. In other words, it must provide:

a public default constructor,
a public copy constructor, and
a public destructor.

The following Message class definition includes these members:

The onBootStrapDone slot is never called but the onKeyModelChanged slot is. I tried putting the slot in a connections block in qml, using a QObject::connect in main (although I may have not had the syntax correct) and using a jscript function in qml all without success. Again I know the signal is being emitted in c. Please help Thanks. Signals and slots is a language construct introduced in Qt for communication between objects 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.

Qt Signal Slot With 2 Arguments

The class also provides a constructor for normal use and two public member functions that are used to obtain the private data.

Declaring the Type with QMetaType

The Message class only needs a suitable implementation in order to be usable. However, Qt's type system will not be able to understand how to store, retrieve and serialize instances of this class without some assistance. For example, we will be unable to store Message values in QVariant.

The class in Qt responsible for custom types is QMetaType. To make the type known to this class, we invoke the Q_DECLARE_METATYPE() macro on the class in the header file where it is defined:

This now makes it possible for Message values to be stored in QVariant objects and retrieved later. See the Custom Type Example for code that demonstrates this.

The Q_DECLARE_METATYPE() macro also makes it possible for these values to be used as arguments to signals, but only in direct signal-slot connections. To make the custom type generally usable with the signals and slots mechanism, we need to perform some extra work.

Creating and Destroying Custom Objects

Although the declaration in the previous section makes the type available for use in direct signal-slot connections, it cannot be used for queued signal-slot connections, such as those that are made between objects in different threads. This is because the meta-object system does not know how to handle creation and destruction of objects of the custom type at run-time.

To enable creation of objects at run-time, call the qRegisterMetaType() template function to register it with the meta-object system. This also makes the type available for queued signal-slot communication as long as you call it before you make the first connection that uses the type.

The Queued Custom Type Example declares a Block class which is registered in the main.cpp file:

This type is later used in a signal-slot connection in the window.cpp file:

If a type is used in a queued connection without being registered, a warning will be printed at the console; for example:

Making the Type Printable

It is often quite useful to make a custom type printable for debugging purposes, as in the following code:

This is achieved by creating a streaming operator for the type, which is often defined in the header file for that type:

The implementation for the Message type in the Custom Type Example goes to some effort to make the printable representation as readable as possible:

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Qt Signals And Slots Tutorial

The output sent to the debug stream can, of course, be made as simple or as complicated as you like. Note that the value returned by this function is the QDebug object itself, though this is often obtained by calling the maybeSpace() member function of QDebug that pads out the stream with space characters to make it more readable.

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