Enhancing User Experience: A Guide to QColumnView::setPreviewWidget() in Qt Widgets

Purpose

• In QColumnView (like QListView or QTreeView), this function allows you to customize the appearance of a widget that's displayed when the user hovers over an item. This preview widget provides a more informative or visually appealing representation of the item's data compared to the standard item delegate's rendering.

Functionality

1. • You pass a pointer to a QWidget subclass as the argument to setPreviewWidget(). This widget will be displayed as the preview when the user hovers over an item.
   • The ownership of the widget is transferred to the QColumnView. It will be destroyed when the column view is deleted or when you set a new preview widget.

2. Updating the Preview
   

   • While the basic preview displays the widget you set, you can further customize its content based on the item being hovered over using the updatePreviewWidget() function.
   • This function takes two arguments: the index of the hovered item and the preview widget itself. You can access the item's data through the model using the index, and then modify the preview widget's properties (like text, layout, etc.) accordingly.

Example Usage

#include <QApplication>
#include <QListView>
#include <QLabel>

class MyModel : public QAbstractListModel {
    Q_OBJECT

public:
    MyModel(const QStringList &data, QObject *parent = nullptr) : QAbstractListModel(parent), items(data) {}

    int rowCount(const QModelIndex &parent = QModelIndex()) const override { return items.size(); }
    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override {
        if (role == Qt::DisplayRole) {
            return items.at(index.row());
        }
        return QVariant();
    }

private:
    QStringList items;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    MyModel model({"Item 1", "Item 2", "Item 3"});
    QListView listView;
    listView.setModel(&model);

    QLabel *previewLabel = new QLabel;
    listView.setPreviewWidget(previewLabel);

    QObject::connect(&listView, &QListView::currentIndexChanged,
                      [&model, previewLabel](const QModelIndex &index) {
                          if (index.isValid()) {
                              previewLabel->setText(model.data(index, Qt::DisplayRole).toString() + " (preview)");
                          }
                      });

    listView.show();

    return app.exec();
}

In this example:

• The currentIndexChanged signal is connected to update the preview label's text with the hovered item's data and a "(preview)" string.
• setPreviewWidget() sets the QLabel as the preview.
• A QLabel is created to be the preview widget.
• A custom MyModel provides data for the QListView.

---

Image Preview in QListView

This example showcases a preview widget that displays an image associated with each item in a QListView:

#include <QApplication>
#include <QListView>
#include <QLabel>
#include <QHBoxLayout>
#include <QPixmap>

class ImageItem {
public:
    ImageItem(const QString &text, const QString &imagePath) : text(text), imagePath(imagePath) {}

    QString getText() const { return text; }
    QString getImagePath() const { return imagePath; }

private:
    QString text;
    QString imagePath;
};

class ImageModel : public QAbstractListModel {
    Q_OBJECT

public:
    ImageModel(const QList<ImageItem> &items, QObject *parent = nullptr) : QAbstractListModel(parent), items(items) {}

    int rowCount(const QModelIndex &parent = QModelIndex()) const override { return items.size(); }
    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override {
        if (role == Qt::DisplayRole) {
            return items.at(index.row()).getText();
        } else if (role == Qt::UserRole) {
            return items.at(index.row()).getImagePath();
        }
        return QVariant();
    }

private:
    QList<ImageItem> items;
};

class ImagePreview : public QWidget {
    Q_OBJECT

public:
    ImagePreview(QWidget *parent = nullptr) : QWidget(parent) {
        layout = new QHBoxLayout(this);
        label = new QLabel;
        imageLabel = new QLabel;
        layout->addWidget(label);
        layout->addWidget(imageLabel);
        setLayout(layout);
    }

    void setText(const QString &text) { label->setText(text); }
    void setImage(const QString &imagePath) {
        imageLabel->setPixmap(QPixmap(imagePath));
        imageLabel->setScaledContents(true);
    }

private:
    QHBoxLayout *layout;
    QLabel *label;
    QLabel *imageLabel;
};

int main(int argc, char *argv[]) {
    QApplication app(argc, argv);

    QList<ImageItem> imageItems = {
        ImageItem("Item 1", "path/to/image1.jpg"),
        ImageItem("Item 2", "path/to/image2.png"),
        ImageItem("Item 3", "path/to/image3.bmp")
    };
    ImageModel model(imageItems);
    QListView listView;
    listView.setModel(&model);

    ImagePreview *previewWidget = new ImagePreview;
    listView.setPreviewWidget(previewWidget);

    QObject::connect(&listView, &QListView::currentIndexChanged,
                      [&model, previewWidget](const QModelIndex &index) {
                          if (index.isValid()) {
                              previewWidget->setText(model.data(index, Qt::DisplayRole).toString());
                              previewWidget->setImage(model.data(index, Qt::UserRole).toString());
                          }
                      });

    listView.show();

    return app.exec();
}

• The currentIndexChanged signal is used to update the preview widget's text and image based on the hovered item's data.
• The ImagePreview widget is a custom widget with a layout to display both text and an image.
• The ImageModel provides data for the QListView, including an additional user role for the image path.
• An ImageItem class holds text and image path information.

Rich Text Preview in QTreeView

This example demonstrates a preview widget that displays rich text content associated with tree nodes in a QTreeView:

#include <QApplication>
#include <QTreeView>
#include <QLabel>
#include <QHBoxLayout>
#include <QTextDocument>

class RichTextItem {
public:
    RichTextItem(const QString &text, const QString &richText) : text(text), richText(richText) {}

    QString getText() const { return text; }
    QString getRichText() const { return richText; }

private:
    QString text;
    QString richText;
};

class RichTextModel : public QAbstractItemModel {
    Q_OBJECT

public:
    RichTextModel(const QList<RichTextItem> &

---

Custom Item Delegate

• You can override the paint() function of the delegate to draw the standard item representation and then conditionally draw a preview element when hovering is detected using event() or hasFocus() methods.
• A more flexible approach is to create a custom item delegate. This allows you to completely control the rendering of each item in the QColumnView, including potential hover effects that display a preview.

QToolTip

• Qt will automatically display a tooltip when the user hovers over the item.
• Set the toolTip() property of the model for each item with the desired preview text.
• If your preview content is simpler and doesn't require complex layouts, you can leverage the QToolTip class.

Custom Overlay Widget

• It allows for more flexibility in positioning the preview and customizing its appearance.
• This widget can be shown or hidden based on hover state using signals or mouse tracking events.
• For more advanced scenarios, you might consider creating a separate overlay widget that sits on top of your QColumnView.

Choosing the Right Approach

The best approach for you depends on:

• Level of customization: Custom delegates and overlay widgets offer the most control.
• The complexity of your preview content: If it's simple text, a tooltip might suffice. For richer layouts, a custom delegate or overlay widget is needed.

Approach	Pros	Cons
QColumnView::setPreviewWidget()	Built-in, easy to use	Limited customization, widget ownership managed by Qt
Custom Item Delegate	Highly customizable, full control over rendering	Requires more implementation effort
QToolTip	Simple, no additional widgets needed	Limited content space, less customization
Custom Overlay Widget	Highly customizable, flexible positioning	Requires managing a separate widget, potential performance impact