Using a Component in Your Application

Widgets and Dialogs with Auto-Connect

Although it is easy to implement a custom slot in the dialog and connect it in the constructor, we could instead use QMetaObject's auto-connection facilities to connect the OK button's clicked() signal to a slot in our subclass. uic automatically generates code in the dialog's setupUi() function to do this, so we only need to declare and implement a slot with a name that follows a standard convention:

     void on_<widget name>_<signal name>(<signal parameters>);

Using this convention, we can define and implement a slot that responds to mouse clicks on the OK button:

 class ImageDialog : public QDialog, private Ui::ImageDialog
 {
     Q_OBJECT

 public:
     ImageDialog(QWidget *parent = 0);

 private slots:
     void on_okButton_clicked();
 };

Another example of automatic signal and slot connection would be the Text Finder with its on_findButton_clicked() slot.

We use QMetaObject's system to enable signal and slot connections:

     QMetaObject::connectSlotsByName(this);

This enables us to implement the slot, as shown below:

 void TextFinder::on_findButton_clicked()
 {
     QString searchString = ui_lineEdit->text();
     QTextDocument *document = ui_textEdit->document();

     bool found = false;

     if (isFirstTime == false)
         document->undo();

     if (searchString == "") {
         QMessageBox::information(this, tr("Empty Search Field"),
                 "The search field is empty. Please enter a word and click Find.");
     } else {

         QTextCursor highlightCursor(document);
         QTextCursor cursor(document);

         cursor.beginEditBlock();
     ...
         cursor.endEditBlock();
         isFirstTime = false;

         if (found == false) {
             QMessageBox::information(this, tr("Word Not Found"),
                 "Sorry, the word cannot be found.");
         }
     }
 }

Automatic connection of signals and slots provides both a standard naming convention and an explicit interface for widget designers to work to. By providing source code that implements a given interface, user interface designers can check that their designs actually work without having to write code themselves.

[Previous: Using Custom Widgets with Qt Designer] [Contents] [Next: Creating Custom Widgets for Qt Designer]