- Qt 5.15
- Qt Widgets
- Completer Example
- Qt 5.15.19 Reference Documentation

# Completer Example

The Completer example shows how to provide string-completion facilities for an input widget based on data provided by a model.

This example uses a custom item model, `FileSystemModel`, and a QCompleter object. QCompleter is a class that provides completions based on an item model. The type of model, the completion mode, and the case sensitivity can be selected using combo boxes.

#### The Resource File

The Completer example requires a resource file in order to store the countries.txt and words.txt. The resource file contains the following code:

     <!DOCTYPE RCC><RCC version="1.0">
     <qresource prefix="/">
        <file>resources/countries.txt</file>
        <file>resources/wordlist.txt</file>
     </qresource>
     </RCC>

#### FileSystemModel Class Definition

The `FileSystemModel` class is a subclass of QFileSystemModel, which provides a data model for the local filesystem.

     class FileSystemModel : public QFileSystemModel
     {
     public:
         FileSystemModel(QObject *parent = nullptr);
         QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;
     };

This class only has a constructor and a `data()` function as it is only created to enable `data()` to return the entire file path for the display role, unlike QFileSystemModel's `data()` function that only returns the folder and not the drive label. This is further explained in `FileSystemModel`'s implementation.

#### FileSystemModel Class Implementation

The constructor for the `FileSystemModel` class is used to pass parent to QFileSystemModel.

     FileSystemModel::FileSystemModel(QObject *parent)
         : QFileSystemModel(parent)
     {
     }

As mentioned earlier, the `data()` function is reimplemented in order to get it to return the entire file parth for the display role. For example, with a QFileSystemModel, you will see "Program Files" in the view. However, with `FileSystemModel`, you will see "C:\Program Files".

     QVariant FileSystemModel::data(const QModelIndex &index, int role) const
     {
         if (role == Qt::DisplayRole && index.column() == 0) {
             QString path  = QDir::toNativeSeparators(filePath(index));
             if (path.endsWith(QDir::separator()))
                 path.chop(1);
             return path;
         }

         return QFileSystemModel::data(index, role);
     }

The screenshots below illustrate this difference:

The Qt::EditRole, which QCompleter uses to look for matches, is left unchanged.

#### MainWindow Class Definition

The `MainWindow` class is a subclass of QMainWindow and implements five private slots - `about()`, `changeCase()`, `changeMode()`, `changeModel()`, and `changeMaxVisible()`.

     class MainWindow : public QMainWindow
     {
         Q_OBJECT

     public:
         MainWindow(QWidget *parent = nullptr);

     private slots:
         void about();
         void changeCase(int);
         void changeMode(int);
         void changeModel();
         void changeMaxVisible(int);

Within the `MainWindow` class, we have two private functions: `createMenu()` and `modelFromFile()`. We also declare the private widgets needed - three QComboBox objects, a QCheckBox, a QCompleter, a QLabel, and a QLineEdit.

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

         QComboBox *caseCombo = nullptr;
         QComboBox *modeCombo = nullptr;
         QComboBox *modelCombo = nullptr;
         QSpinBox *maxVisibleSpinBox = nullptr;
         QCheckBox *wrapCheckBox = nullptr;
         QCompleter *completer = nullptr;
         QLabel *contentsLabel = nullptr;
         QLineEdit *lineEdit = nullptr;
     };

#### MainWindow Class Implementation

The constructor of `MainWindow` constructs a `MainWindow` with a parent widget and initializes the private members. The `createMenu()` function is then invoked.

We set up three QComboBox objects, `modelComb`, `modeCombo` and `caseCombo`. By default, the `modelCombo` is set to QFileSystemModel, the `modeCombo` is set to "Filtered Popup" and the `caseCombo` is set to "Case Insensitive".

     MainWindow::MainWindow(QWidget *parent)
         : QMainWindow(parent)
     {
         createMenu();

         QWidget *centralWidget = new QWidget;

         QLabel *modelLabel = new QLabel;
         modelLabel->setText(tr("Model"));

         modelCombo = new QComboBox;
         modelCombo->addItem(tr("QFileSystemModel"));
         modelCombo->addItem(tr("QFileSystemModel that shows full path"));
         modelCombo->addItem(tr("Country list"));
         modelCombo->addItem(tr("Word list"));
         modelCombo->setCurrentIndex(0);

         QLabel *modeLabel = new QLabel;
         modeLabel->setText(tr("Completion Mode"));
         modeCombo = new QComboBox;
         modeCombo->addItem(tr("Inline"));
         modeCombo->addItem(tr("Filtered Popup"));
         modeCombo->addItem(tr("Unfiltered Popup"));
         modeCombo->setCurrentIndex(1);

         QLabel *caseLabel = new QLabel;
         caseLabel->setText(tr("Case Sensitivity"));
         caseCombo = new QComboBox;
         caseCombo->addItem(tr("Case Insensitive"));
         caseCombo->addItem(tr("Case Sensitive"));
         caseCombo->setCurrentIndex(0);

The `maxVisibleSpinBox` is created and determines the number of visible item in the completer

The `wrapCheckBox` is then set up. This `checkBox` determines if the `completer`'s setWrapAround() property is enabled or disabled.

         QLabel *maxVisibleLabel = new QLabel;
         maxVisibleLabel->setText(tr("Max Visible Items"));
         maxVisibleSpinBox = new QSpinBox;
         maxVisibleSpinBox->setRange(3,25);
         maxVisibleSpinBox->setValue(10);

         wrapCheckBox = new QCheckBox;
         wrapCheckBox->setText(tr("Wrap around completions"));
         wrapCheckBox->setChecked(true);

We instantiate `contentsLabel` and set its size policy to fixed. The combo boxes' activated() signals are then connected to their respective slots.

         contentsLabel = new QLabel;
         contentsLabel->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);

         connect(modelCombo, QOverload<int>::of(&QComboBox::activated),
                 this, &MainWindow::changeModel);
         connect(modeCombo, QOverload<int>::of(&QComboBox::activated),
                 this, &MainWindow::changeMode);
         connect(caseCombo, QOverload<int>::of(&QComboBox::activated),
                 this, &MainWindow::changeCase);
         connect(maxVisibleSpinBox, QOverload<int>::of(&QSpinBox::valueChanged),
                 this, &MainWindow::changeMaxVisible);

The `lineEdit` is set up and then we arrange all the widgets using a QGridLayout. The `changeModel()` function is called, to initialize the `completer`.

         lineEdit = new QLineEdit;

         QGridLayout *layout = new QGridLayout;
         layout->addWidget(modelLabel, 0, 0); layout->addWidget(modelCombo, 0, 1);
         layout->addWidget(modeLabel, 1, 0);  layout->addWidget(modeCombo, 1, 1);
         layout->addWidget(caseLabel, 2, 0);  layout->addWidget(caseCombo, 2, 1);
         layout->addWidget(maxVisibleLabel, 3, 0); layout->addWidget(maxVisibleSpinBox, 3, 1);
         layout->addWidget(wrapCheckBox, 4, 0);
         layout->addWidget(contentsLabel, 5, 0, 1, 2);
         layout->addWidget(lineEdit, 6, 0, 1, 2);
         centralWidget->setLayout(layout);
         setCentralWidget(centralWidget);

         changeModel();

         setWindowTitle(tr("Completer"));
         lineEdit->setFocus();
     }

The `createMenu()` function is used to instantiate the QAction objects needed to fill the `fileMenu` and `helpMenu`. The actions' triggered() signals are connected to their respective slots.

     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, &QAction::triggered, qApp, &QApplication::quit);
         connect(aboutAct, &QAction::triggered, this, &MainWindow::about);
         connect(aboutQtAct, &QAction::triggered, qApp, &QApplication::aboutQt);

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

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

The `modelFromFile()` function accepts the fileName of a file and processes it depending on its contents.

We first validate the `file` to ensure that it can be opened in QFile::ReadOnly mode. If this is unsuccessful, the function returns an empty QStringListModel.

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

The mouse cursor is then overridden with Qt::WaitCursor before we fill a QStringList object, `words`, with the contents of `file`. Once this is done, we restore the mouse cursor.

     #ifndef QT_NO_CURSOR
         QGuiApplication::setOverrideCursor(QCursor(Qt::WaitCursor));
     #endif
         QStringList words;

         while (!file.atEnd()) {
             QByteArray line = file.readLine();
             if (!line.isEmpty())
                 words << QString::fromUtf8(line.trimmed());
         }

     #ifndef QT_NO_CURSOR
         QGuiApplication::restoreOverrideCursor();
     #endif

As mentioned earlier, the resources file contains two files - countries.txt and words.txt. If the `file` read is words.txt, we return a QStringListModel with `words` as its QStringList and `completer` as its parent.

         if (!fileName.contains(QLatin1String("countries.txt")))
             return new QStringListModel(words, completer);

If the `file` read is countries.txt, then we require a QStandardItemModel with `words.count()` rows, 2 columns, and `completer` as its parent.

         QStandardItemModel *m = new QStandardItemModel(words.count(), 2, completer);

A standard line in countries.txt is:

> Norway NO

Hence, to populate the QStandardItemModel object, `m`, we have to split the country name and its symbol. Once this is done, we return `m`.

         for (int i = 0; i < words.count(); ++i) {
             QModelIndex countryIdx = m->index(i, 0);
             QModelIndex symbolIdx = m->index(i, 1);
             QString country = words.at(i).mid(0, words[i].length() - 2).trimmed();
             QString symbol = words.at(i).right(2);
             m->setData(countryIdx, country);
             m->setData(symbolIdx, symbol);
         }

         return m;
     }

The `changeMode()` function sets the `completer`'s mode, depending on the value of `index`.

     void MainWindow::changeMode(int index)
     {
         QCompleter::CompletionMode mode;
         if (index == 0)
             mode = QCompleter::InlineCompletion;
         else if (index == 1)
             mode = QCompleter::PopupCompletion;
         else
             mode = QCompleter::UnfilteredPopupCompletion;

         completer->setCompletionMode(mode);
     }

The `changeModel()` function changes the item model used based on the model selected by the user.

A `switch` statement is used to change the item model based on the index of `modelCombo`. If `case` is 0, we use an unsorted QFileSystemModel, providing us with a file path excluding the drive label.

     void MainWindow::changeModel()
     {
         delete completer;
         completer = new QCompleter(this);
         completer->setMaxVisibleItems(maxVisibleSpinBox->value());

         switch (modelCombo->currentIndex()) {
         default:
         case 0:
             { // Unsorted QFileSystemModel
                 QFileSystemModel *fsModel = new QFileSystemModel(completer);
                 fsModel->setRootPath(QString());
                 completer->setModel(fsModel);
                 contentsLabel->setText(tr("Enter file path"));
             }
             break;

Note that we create the model with `completer` as the parent as this allows us to replace the model with a new model. The `completer` will ensure that the old one is deleted the moment a new model is assigned to it.

If `case` is 1, we use the `DirModel` we defined earlier, resulting in full paths for the files.

         case 1:
             {   // FileSystemModel that shows full paths
                 FileSystemModel *fsModel = new FileSystemModel(completer);
                 completer->setModel(fsModel);
                 fsModel->setRootPath(QString());
                 contentsLabel->setText(tr("Enter file path"));
             }
             break;

When `case` is 2, we attempt to complete names of countries. This requires a QTreeView object, `treeView`. The country names are extracted from countries.txt and set the popup used to display completions to `treeView`.

         case 2:
             { // Country List
                 completer->setModel(modelFromFile(":/resources/countries.txt"));
                 QTreeView *treeView = new QTreeView;
                 completer->setPopup(treeView);
                 treeView->setRootIsDecorated(false);
                 treeView->header()->hide();
                 treeView->header()->setStretchLastSection(false);
                 treeView->header()->setSectionResizeMode(0, QHeaderView::Stretch);
                 treeView->header()->setSectionResizeMode(1, QHeaderView::ResizeToContents);
                 contentsLabel->setText(tr("Enter name of your country"));
             }
             break;

The screenshot below shows the Completer with the country list model.

If `case` is 3, we attempt to complete words. This is done using a QStringListModel that contains data extracted from words.txt. The model is sorted case insensitively.

The screenshot below shows the Completer with the word list model.

Once the model type is selected, we call the `changeMode()` function and the `changeCase()` function and set the wrap option accordingly. The `wrapCheckBox`'s clicked() signal is connected to the `completer`'s setWrapAround() slot.

         case 3:
             { // Word list
                 completer->setModel(modelFromFile(":/resources/wordlist.txt"));
                 completer->setModelSorting(QCompleter::CaseInsensitivelySortedModel);
                 contentsLabel->setText(tr("Enter a word"));
             }
             break;
         }

         changeMode(modeCombo->currentIndex());
         changeCase(caseCombo->currentIndex());
         completer->setWrapAround(wrapCheckBox->isChecked());
         lineEdit->setCompleter(completer);
         connect(wrapCheckBox, &QAbstractButton::clicked, completer, &QCompleter::setWrapAround);
     }

The `changeMaxVisible()` update the maximum number of visible items in the completer.

     void MainWindow::changeMaxVisible(int max)
     {
         completer->setMaxVisibleItems(max);
     }

The `about()` function provides a brief description about the example.

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

#### `main()` Function

The `main()` function instantiates QApplication and `MainWindow` and invokes the show() function.

     int main(int argc, char *argv[])
     {
         Q_INIT_RESOURCE(completer);

         QApplication app(argc, argv);
         MainWindow window;
         window.show();
         return app.exec();
     }

Example project @ code.qt.io