Wiring up signals and slots

Qt uses a signals and slots system to process events. There are at least four different ways to make signal→slot connections:

• Create an "event handler"
• Do it "visually"
• Do it in the *.ui file
• Do it in the constructor

Each of the above listed approaches and when they are available are discussed below.

Availability: Qt Creator and hand-coded projects (apparently)

Users of Visual Studio, Delphi, and Lazarus will be most familiar with this approach. In Qt Creator, right click on a control and select Go to slot …. A dialog box giving you a list of signals available for the control will appear; select the signal for which you wish to create a handler. If the handler doesn't already exist, Qt Creator will create it and take you to it. If it does, then Qt Creator will just take you to it.

A handler you make this way for a button_quit widget might look like:

void MainWindow::on_button_quit_clicked()
{
    this->close();
}

The handlers you create this way are actually a private slots. You can confirm this by looking at the header file for the class you are editing:

private slots:
    void on_button_quit_clicked();

How does the build system know that that button_quit's clicked() signal connects to MainWindow's on_button_quit_event_clicked() slot? It appears that the connection between signals and slots created this way is done purely via a naming convention along the following lines:

<widget-name> <signal-name>()

automatically connects to any slot of the form

<ClassName>::on_<widget-name>_<signal-name>()

If the above is true, then it should be possible to create event handlers just by writing code. Two tests are indicated:

• Write the handlers manually in Qt Creator.
• Write the handlers manually in a project managed by something other than Qt Creator.

I've done both and assuming I didn't overlook something, indeed you can hand-add handlers as suggested.

Availability: Qt Creator, Qt Designer

This is the drag-and-drop-gosh-golly way of making connections that at first might seem more cute than useful but actually turns out to be more useful that you might have first thought.

In Qt Creator or Qt Designer, open the Edit Signal/Slots mode. In either app, this is done by any of the following:

• menu bar: Edit→Edit Signal/Slots
• function key: F4
• toolbar icon: (figure it out from the menu icon)

In this mode, you will notice that when you roll over widgets, they turn red and get a thick border. This tells you they are providers of signals. To make a signal/slot connection:

1. Roll over the widget that will provide the signal.
2. Click and drag to the widget whose slot you want to connect to.
3. Release the mouse button.
4. From the dialog that appears, select the desired signal from the left column and the desired slot from the right and click OK.

If you don't see a slot that you expect, try clicking on the “Show slots inherited from …” checkbox. IMHO, this should be checked by default, but it isn't.

When you set signal/slot connections this way, you will see arrows connecting widgets indicating signal/slot relationships.

There are a couple special cases that are worth mentioning.

Connecting to the window If you want to connect a signal to the slot of a window, just drag onto the window rather than onto another widget. You will know you are connecting to the main window because the slot end of the connector will turn into an electrical earth ground symbol rather than an arrow.

Connecting a widget to itself You can also connect widgets to themselves. If you do this, you can set up all sorts of hilarity. Sometimes you can even do useful things.

You can edit existing signal/slot connections made in Edit Signal/Slots mode by using the Signal/Slot Editor that appears at the bottom of the screen in Qt Creator and the left bottom on Qt Designer. Each column of the editor is actually a drop-down list that you need to double click on to delete.

You can add new relationships and delete existing ones using the + and – icons. The – icon seems a bit temperamental, so if it isn't activated for you, try clicking on various bits on your form until it activates. (And if you figure out what the logic that going on here is, please let me know.)

Connections made this way are stored in a form's <form-name>.ui file in the <connections> section. For example, here is the snippet from a file mainwindow.ui that connects button_quit's clicked() signal to the window's close() slot:

 <connections>
  <connection>
   <sender>button_quit</sender>
   <signal>clicked()</signal>
   <receiver>MainWindow</receiver>
   <slot>close()</slot>
   <hints>
    <hint type="sourcelabel">
     <x>121</x>
     <y>149</y>
    </hint>
    <hint type="destinationlabel">
     <x>105</x>
     <y>221</y>
    </hint>
   </hints>
  </connection>
 </connections>

Note: If you are using Qt Creator to develop your project, the Qt gods discourage you from manually editing *.ui files.

Availability: any kind of project

If your project uses *.ui files to specify UI layout, then you can specify signal→slot connections here in the same way that Qt Creator and Qt Designer do–by specifying the connections in the <connections> section:

 <connections>
  <connection>
   <sender>pushButton</sender>
   <signal>clicked()</signal>
   <receiver>HelloForm</receiver>
   <slot>close()</slot>
  </connection>
 </connections>

Availability: any project

You can explicitly connect signals and slots programatically. This is typically done in a form's constructor.

HelloForm::HelloForm()
{
    widget.setupUi(this);
 
    connect(widget.button_quit, SIGNAL(clicked()), this, SLOT(close()));
}

While the Create an "event handler" approach is the fastest, my current thinking is that using the Qt Designer approach of Do it "visually" is actually the best way to do this–using the Signal/Slot editor panel if needed to add custom slots to the forms. The downside to this is that you have to hand-code the custom slots. The upsides are that the connections are well documented and that this approach simplifies moving your program from C++ to some other language (Python, Ruby, Java, etc.).

I don't know. But it should be possible.