Qt Creator Signals And Slots Editor
- Qt offers signals and slots as the interface it uses to manage events. Like an event, the sending component generates a signal—in Qt parlance, the object emits a signal, which is an occurrence of an event—which recipient objects may execute a slot for the purpose.
- Qt Creator must be in Design mode. Make sure that the Action Editor and Signal and Slots Editor are visible. Do this from the top menu as follows. Select Window → Views and then click the check box next to each of the desired editors.
Use Signals and Slots Editing Mode for connecting predefined Qt signals directly to predefined Qt slots. So for 'Close' button on a simple dialog, you can just drag a connection from the button to the dialog, select the clicked signal and the reject slot, click 'OK', and there would be nothing more to do.
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.
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.
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.
| 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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Signals and slots is a language construct introduced also in Qt[1] 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.[citation needed]
The signal/slot system fits well with the way graphical user interfaces are designed.[citation needed] 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. 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.
Qt Creator Signals And Slots Editor
A commonly used metaphor[according to whom?] 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.
Qt Creator Signals And Slots Editor Free
Alternative implementations[edit]
Qt Creator Signals And Slots Editor Online
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.
See also[edit]
Qt5 Designer Signal Slot Editor
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.
References[edit]
- ^'Signals & Slots - QtCore 5.1'. Qt Project. 2013-07-04. Retrieved 2013-07-04.