Understanding QAbstractItemView::updateGeometries() in Qt Widgets

updateGeometries(): This is a protected function within QAbstractItemView. Protected functions are meant for internal use by the class and its subclasses. In general, they are not intended to be called directly from outside the class.
QAbstractItemView: This is a base class for various item view widgets like QListView, QTableView, and QTreeView in Qt Widgets. It provides the core functionality for these widgets to interact with data models.

Based on this information, we can infer that QAbstractItemView::updateGeometries() likely handles the task of recalculating the geometry (position and size) of items within the view. This becomes necessary whenever the underlying data model changes, or the view itself is resized.

Data Model Change: When the data model associated with the item view changes (e.g., new items added, items removed, or data updated), the item view needs to adjust the layout of its items to reflect these changes.
View Resize: If the item view itself is resized (the window containing the view is resized), the positions and sizes of the items need to be recalculated to fit within the new boundaries.

updateGeometries() likely gets triggered in these scenarios and performs the necessary calculations to update the item geometries. This ensures that the items are displayed correctly within the view based on the current state of the data and the available space.

It's important to note that this is a high-level understanding. The actual implementation of updateGeometries() might involve more complex logic depending on the specific type of item view and the layout it uses.

Here are some resources that might be helpful for further understanding:

Qt Model/View Framework
The model/view framework is a core concept in Qt that separates data (model) from its presentation (view). Understanding this framework will provide more context to the role of QAbstractItemView.

Model Class (SimpleStringListModel)

#include <QStringList>
#class SimpleStringListModel : public QAbstractListModel {
    Q_OBJECT

public:
    explicit SimpleStringListModel(const QStringList &data, QObject *parent = nullptr);

    int rowCount(const QModelIndex &parent = QModelIndex()) const override;
    QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;

private:
    QStringList m_data;
};

Main Window Class

#include <QApplication>
#include <QMainWindow>
#include <QListView>
#include <QVBoxLayout>
#include "SimpleStringListModel.h"

class MainWindow : public QMainWindow {
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);

private:
    QListView *m_listView;
    SimpleStringListModel *m_model;
    QVBoxLayout *m_layout;

public slots:
    void onItemAdded();
    void onWindowResized(const QSize &newSize);
};

SimpleStringListModel: This is a simple model class that inherits from QAbstractListModel and stores a list of strings (QStringList). It provides methods for retrieving the number of rows (rowCount) and data for a specific index (data).
MainWindow: This class represents the main window of the application. It creates a QListView, a SimpleStringListModel, and a layout. It also defines slots for handling item addition and window resize events.
onItemAdded(): This slot gets triggered when a button is clicked (implementation not shown) to add a new item to the data model. It updates the model's data list and emits the dataChanged() signal.
onWindowResized(const QSize &newSize): This slot gets triggered when the window is resized. It simply resizes the layout to fit the new window size.
dataChanged() Signal: When the model's data changes (e.g., in onItemAdded()), it emits the dataChanged() signal. This signal is typically connected to the QAbstractItemView (the QListView in this case) to inform it about the changes.
Geometry Updates: When the dataChanged() signal is received by the QListView, it likely triggers internal mechanisms (potentially including updateGeometries()) to recalculate the layout and adjust the geometries of the list items based on the new data and the available space within the resized window.

This is a simplified example, but it demonstrates how changes in the data model and view resize can lead to geometry updates within the QAbstractItemView.

Public Methods

void QLayout::update()
Calling update() on the layout managing the QAbstractItemView can also trigger a layout recalculation, potentially involving geometry updates for the items.
void QWidget::updateGeometry()
This method can be called on the QAbstractItemView itself or its parent widget. It triggers a geometry recalculation, which might involve calling internal functions like updateGeometries().

Signals and Slots

void QMainWindow::resizeEvent(QResizeEvent *event)
This event gets triggered whenever the main window is resized. In the event handler, you can adjust the layout that manages the QAbstractItemView to fit the new window size.
void QAbstractItemView::dataChanged(const QModelIndex &topLeft, const QModelIndex &bottomRight)
This signal gets emitted by the data model whenever data changes. You can connect this signal to a slot in your application that performs the necessary logic to update the view. In the slot, you can adjust item sizes or positions manually, or use other layout management techniques.

Layout Management Techniques

Layout Spacers
Using layout spacers like QSpacerItem can help distribute available space within the layout more effectively. This can be useful for scenarios where you want to have flexible item sizing based on the available space.
Item Size Hints
You can set appropriate size hints for each item in the QAbstractItemView using methods like setItemSizeHint(). This helps the layout manager determine the preferred size of each item, which can influence the overall geometry.

Custom Item Delegate

For more control over item appearance and behavior, you can create a custom item delegate class that inherits from QItemDelegate. This delegate can override methods like sizeHint() and paint() to define the size and appearance of each item.

Choosing the Right Approach