Custom Completer Example

TextEdit Class Implementation

The constructor for TextEdit constructs a TextEdit with a parent and initializes c. The instructions to use the completer is displayed on the TextEdit object, using the setPlainText() function.

 TextEdit::TextEdit(QWidget *parent)
 : QTextEdit(parent), c(0)
 {
     setPlainText(tr("This TextEdit provides autocompletions for words that have more than"
                     " 3 characters. You can trigger autocompletion using ") +
                     QKeySequence("Ctrl+E").toString(QKeySequence::NativeText));
 }

In addition, TextEdit also includes a default destructor:

 TextEdit::~TextEdit()
 {
 }

The setCompleter() function accepts a completer and sets it up. We use if (c) to check if c has been initialized. If it has been initialized, the QObject::disconnect() function is invoked to disconnect the signal from the slot. This is to ensure that no previous completer object is still connected to the slot.

 void TextEdit::setCompleter(QCompleter *completer)
 {
     if (c)
         QObject::disconnect(c, 0, this, 0);

     c = completer;

     if (!c)
         return;

     c->setWidget(this);
     c->setCompletionMode(QCompleter::PopupCompletion);
     c->setCaseSensitivity(Qt::CaseInsensitive);
     QObject::connect(c, SIGNAL(activated(const QString&)),
                      this, SLOT(insertCompletion(const QString&)));
 }

We then instantiate c with completer and set it as TextEdit's widget. The completion mode and case sensitivity are also set and then we connect the activated() signal to the insertCompletion() slot.

The completer() function is a getter function that returns c.

 QCompleter *TextEdit::completer() const
 {
     return c;
 }

The completer pops up the options available, based on the contents of wordlist.txt, but the text cursor is responsible for filling in the missing characters, according to the user's choice of word.

Suppose the user inputs "ACT" and accepts the completer's suggestion of "ACTUAL". The completion string is then sent to insertCompletion() by the completer's activated() signal.

The insertCompletion() function is responsible for completing the word using a QTextCursor object, tc. It validates to ensure that the completer's widget is TextEdit before using tc to insert the extra characters to complete the word.

 void TextEdit::insertCompletion(const QString& completion)
 {
     if (c->widget() != this)
         return;
     QTextCursor tc = textCursor();
     int extra = completion.length() - c->completionPrefix().length();
     tc.movePosition(QTextCursor::Left);
     tc.movePosition(QTextCursor::EndOfWord);
     tc.insertText(completion.right(extra));
     setTextCursor(tc);
 }

The figure below illustrates this process:

completion.length() = 6

c->completionPrefix().length()=3

The difference between these two values is extra, which is 3. This means that the last three characters from the right, "U", "A", and "L", will be inserted by tc.

The textUnderCursor() function uses a QTextCursor, tc, to select a word under the cursor and return it.

 QString TextEdit::textUnderCursor() const
 {
     QTextCursor tc = textCursor();
     tc.select(QTextCursor::WordUnderCursor);
     return tc.selectedText();
 }

The TextEdit class reimplements focusInEvent() function, which is an event handler used to receive keyboard focus events for the widget.

 void TextEdit::focusInEvent(QFocusEvent *e)
 {
     if (c)
         c->setWidget(this);
     QTextEdit::focusInEvent(e);
 }

The keyPressEvent() is reimplemented to ignore key events like Qt::Key_Enter, Qt::Key_Return, Qt::Key_Escape, Qt::Key_Tab, and Qt::Key_Backtab so the completer can handle them.

If there is an active completer, we cannot process the shortcut, Ctrl+E.

 void TextEdit::keyPressEvent(QKeyEvent *e)
 {
     if (c && c->popup()->isVisible()) {
         // The following keys are forwarded by the completer to the widget
        switch (e->key()) {
        case Qt::Key_Enter:
        case Qt::Key_Return:
        case Qt::Key_Escape:
        case Qt::Key_Tab:
        case Qt::Key_Backtab:
             e->ignore();
             return; // let the completer do default behavior
        default:
            break;
        }
     }

     bool isShortcut = ((e->modifiers() & Qt::ControlModifier) && e->key() == Qt::Key_E); // CTRL+E
     if (!c || !isShortcut) // dont process the shortcut when we have a completer
         QTextEdit::keyPressEvent(e);

We also handle other modifiers and shortcuts for which we do not want the completer to respond to.

     const bool ctrlOrShift = e->modifiers() & (Qt::ControlModifier | Qt::ShiftModifier);
     if (!c || (ctrlOrShift && e->text().isEmpty()))
         return;

     static QString eow("~!@#$%^&*()_+{}|:\"<>?,./;'[]\\-="); // end of word
     bool hasModifier = (e->modifiers() != Qt::NoModifier) && !ctrlOrShift;
     QString completionPrefix = textUnderCursor();

     if (!isShortcut && (hasModifier || e->text().isEmpty()|| completionPrefix.length() < 3
                       || eow.contains(e->text().right(1)))) {
         c->popup()->hide();
         return;
     }

     if (completionPrefix != c->completionPrefix()) {
         c->setCompletionPrefix(completionPrefix);
         c->popup()->setCurrentIndex(c->completionModel()->index(0, 0));
     }
     QRect cr = cursorRect();
     cr.setWidth(c->popup()->sizeHintForColumn(0)
                 + c->popup()->verticalScrollBar()->sizeHint().width());
     c->complete(cr); // popup it up!
 }

Finally, we pop up the completer.

 class MainWindow : public QMainWindow
 {
     Q_OBJECT

 public:
     MainWindow(QWidget *parent = 0);

 private slots:
     void about();

 private:
     void createMenu();
     QAbstractItemModel *modelFromFile(const QString& fileName);

     QCompleter *completer;
     TextEdit *completingTextEdit;
 };

 MainWindow::MainWindow(QWidget *parent)
     : QMainWindow(parent), completer(0)
 {
     createMenu();

     completingTextEdit = new TextEdit;
     completer = new QCompleter(this);
     completer->setModel(modelFromFile(":/resources/wordlist.txt"));
     completer->setModelSorting(QCompleter::CaseInsensitivelySortedModel);
     completer->setCaseSensitivity(Qt::CaseInsensitive);
     completer->setWrapAround(false);
     completingTextEdit->setCompleter(completer);

     setCentralWidget(completingTextEdit);
     resize(500, 300);
     setWindowTitle(tr("Completer"));
 }

 void MainWindow::createMenu()
 {
     QAction *exitAction = new QAction(tr("Exit"), this);
     QAction *aboutAct = new QAction(tr("About"), this);
     QAction *aboutQtAct = new QAction(tr("About Qt"), this);

     connect(exitAction, SIGNAL(triggered()), qApp, SLOT(quit()));
     connect(aboutAct, SIGNAL(triggered()), this, SLOT(about()));
     connect(aboutQtAct, SIGNAL(triggered()), qApp, SLOT(aboutQt()));

     QMenu* fileMenu = menuBar()->addMenu(tr("File"));
     fileMenu->addAction(exitAction);

     QMenu* helpMenu = menuBar()->addMenu(tr("About"));
     helpMenu->addAction(aboutAct);
     helpMenu->addAction(aboutQtAct);
 }

 QAbstractItemModel *MainWindow::modelFromFile(const QString& fileName)
 {
     QFile file(fileName);
     if (!file.open(QFile::ReadOnly))
         return new QStringListModel(completer);

     QApplication::setOverrideCursor(QCursor(Qt::WaitCursor));
     QStringList words;

     while (!file.atEnd()) {
         QByteArray line = file.readLine();
         if (!line.isEmpty())
             words << line.trimmed();
     }

     QApplication::restoreOverrideCursor();
     return new QStringListModel(words, completer);
 }

 void MainWindow::about()
 {
     QMessageBox::about(this, tr("About"), tr("This example demonstrates the "
         "different features of the QCompleter class."));
 }

 int main(int argc, char *argv[])
 {
     QApplication app(argc, argv);
     MainWindow window;
     window.show();
     return app.exec();
 }