Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt's meta-object system. This section can be skipped for now if you only want to program with Qt. Just know that you need to put SIGNAL and SLOT around the signals and slots while calling connect. If you want to know how Qt works, it is better to read this. The Meta Object. Qt provides a meta-object system. Meta-object (literally 'over the object') is a way to achieve.
Quite a frequent problem when working with signals with slots in Qt5, according to my observations on the forum, is the connection of slots in the syntax on the pointers to signals having an overload of the signature. The same applies to slots that have an overload.
Let's take a test class that has overloaded signals.
Here there is a signal, with an overload of the signature. Connect this signal will also be to the slots that are declared in the Widget class, and which also have an overload of the signature.
How it was in Qt4
Within Qt4, everything was solved quite simply by specifying the signature of the signal and the slot in the SIGNAL and SLOT macros.
How it became in Qt5
But in Qt5, when writing in the new syntax of signals and slots, there are some problems. Because you need to make the static_cast of the method signature.
By the way, the new syntax also allows you to connect signals to slots with a smaller signature, as it was in Qt4.
Advantages of the new syntax
And now a stumbling block. Why use the new syntax of signals and slots? I still hear this question from time to time. Especially when people see such terrible castes of signatures.
- Therefore, I will list potential advantages:The ability to track errors in the connection of signals and slots at the compilation stage, rather than in the runtime
- Reducing compilation time by excluding macros from the code
- The ability to connect lambda functions, it's quite an important bun
- We protect ourselves from errors when we try to connect from the outside to a private slot. Yes!! Yes!! The SIGNAL and SLOT macros ignore the access levels of methods, violating OOP.
In general, for me this is enough, but for you?
In Qt Designer's signals and slots editing mode, you can connect objects in a form together using Qt's signals and slots mechanism. Both widgets and layouts can be connected via an intuitive connection interface, using the menu of compatible signals and slots provided by Qt Designer. When a form is saved, all connections are preserved so that they will be ready for use when your project is built.
For more information on Qt's signals and sltos mechanism, refer to the Signals and Slots document.
Connecting Objects
To begin connecting objects, enter the signals and slots editing mode by opening the Edit menu and selecting Edit Signals/Slots, or by pressing the F4 key.
All widgets and layouts on the form can be connected together. However, spacers just provide spacing hints to layouts, so they cannot be connected to other objects.
| Highlighted Objects When the cursor is over an object that can be used in a connection, the object will be highlighted. |
To make a connectionn, press the left mouse button and drag the cursor towards the object you want to connect it to. As you do this, a line will extend from the source object to the cursor. If the cursor is over another object on the form, the line will end with an arrow head that points to the destination object. This indicates that a connection will be made between the two objects when you release the mouse button.
Qt Signal Slot C++ Qml
You can abandon the connection at any point while you are dragging the connection path by pressing Esc.
| Making a Connection The connection path will change its shape as the cursor moves around the form. As it passes over objects, they are highlighted, indicating that they can be used in a signal and slot connection. Release the mouse button to make the connection. |
The Configure Connection dialog (below) is displayed, showing signals from the source object and slots from the destination object that you can use.
Qt C++ Signal Slot
To complete the connection, select a signal from the source object and a slot from the destination object, then click OK. Click Cancel if you wish to abandon the connection.
Note: If the Show all signals and slots checkbox is selected, all available signals from the source object will be shown. Otherwise, the signals and slots inherited from QWidget will be hidden.
You can make as many connections as you like between objects on the form; it is possible to connect signals from objects to slots in the form itself. As a result, the signal and slot connections in many dialogs can be completely configured from within Qt Designer.
| Connecting to a Form To connect an object to the form itself, simply position the cursor over the form and release the mouse button. The end point of the connection changes to the electrical 'ground' symbol. |
Editing and Deleting Connections
By default, connection paths are created with two labels that show the signal and slot involved in the connection. These labels are usually oriented along the line of the connection. You can move them around inside their host widgets by dragging the red square at each end of the connection path.
Signal And Slot Qt C++
| The Signal/Slot Editor The signal and slot used in a connection can be changed after it has been set up. When a connection is configured, it becomes visible in Qt Designer's signal and slot editor where it can be further edited. You can also edit signal/slot connections by double-clicking on the connection path or one of its labels to display the Connection Dialog. |
| Deleting Connections The whole connection can be selected by clicking on any of its path segments. Once selected, a connection can be deleted with the Delete key, ensuring that it will not be set up in the UI file. |
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