Customizing Column Widths in Qt TreeView with QTreeView::sizeHintForColumn()

• Output
  It returns a QSize object. The width component of this QSize represents the recommended minimum width for the column.
• Input
  It takes an integer argument, column, which specifies the column index for which the size hint is being requested.
• Purpose
  This function provides a hint to the QHeaderView (which manages the column headers) about the preferred width of a particular column.

By overriding this function in your custom tree view subclass, you can define logic for calculating the ideal width based on the data displayed in that column.

Here are some common use cases for overriding sizeHintForColumn():

• Custom Logic
  You can implement more complex logic based on your specific data format or display requirements.
• Icon Size
  If your data includes icons, you can base the width on the size of the icons to provide a visually consistent view.
• Fitting Text Content
  You can iterate through the items in the column and calculate the width required to display the longest text string. This ensures all content is visible without truncation.

• You can use functions from the underlying data model like data() and rowCount() to access the data for calculating the size hint.
• Qt provides the setResizeContentsPrecision function on QHeaderView to control the accuracy of automatic resizing based on size hints.
• sizeHintForColumn() is just a suggestion. The QHeaderView might adjust the width based on other factors like available space and user interaction.

---

#include <QTreeView>
#include <QHeaderView>
#include <QFontMetrics>

class MyTreeView : public QTreeView {
  Q_OBJECT

public:
  MyTreeView(QWidget* parent = nullptr) : QTreeView(parent) {}

protected:
  QSize sizeHintForColumn(int column) const override {
    int width = 0;
    QFontMetrics fm(font());

    // Get the root model index
    QModelIndex root = model()->rootIndex();

    // Iterate through all top-level items
    for (int row = 0; row < model()->rowCount(root); ++row) {
      QModelIndex index = model()->index(row, column, root);

      // Get the text data for the current item
      QString text = model()->data(index, Qt::DisplayRole).toString();

      // Update width if current text is longer
      width = qMax(width, fm.width(text));
    }

    // Add some padding for aesthetics
    return QSize(width + 10, 0);
  }
};

1. We create a custom MyTreeView class that inherits from QTreeView.
2. We override the sizeHintForColumn() function.
3. Inside the function:
   • We initialize a width variable to store the maximum width encountered.
   • We get a QFontMetrics object to measure text width.
   • We access the model's root index.
   • We loop through all top-level items (rows) using rowCount().
   • For each item, we get the model index for the specific column using index().
   • We retrieve the text data using data() with Qt::DisplayRole.
   • We compare the current text width with the current width and update the maximum if needed.
4. Finally, we return a QSize object with the calculated width and some padding for aesthetics.

---

QHeaderView::setStretchLastSection(bool)

This function allows you to stretch the last column of the QHeaderView to fill any remaining space after resizing other columns. This can be useful for a simple view where the last column should take up any extra space.

QHeaderView::setSectionResizeMode(int column, QHeaderView::ResizeMode mode)

This function sets the resize mode for a specific column in the QHeaderView. Here are some relevant resize modes:

• QHeaderView::ResizeToContents: The column automatically resizes to fit its content.
• QHeaderView::Interactive: The user can resize the column manually.
• QHeaderView::Fixed: The column maintains a fixed width.

This approach gives you more control over individual column behavior, but it might require more manual intervention compared to using sizeHintForColumn().

QHeaderView::resizeSections(QHeaderView::ResizeMode mode)

This function resizes all columns in the QHeaderView based on the specified mode. Similar to setSectionResizeMode(), you can use ResizeToContents to automatically adjust all columns, but this might not be desirable if you need specific control over some columns.

Using a Custom Layout

If you have very specific requirements for column layout and behavior, you can consider abandoning the QTreeView altogether and implementing a custom layout using widgets like QLabel and QWidget for displaying your data. This approach offers maximum control but requires more development effort.

• If you have very specific needs and are comfortable with more complex development, consider a custom layout.
• If you want all columns to automatically fit their content initially, use resizeSections(QHeaderView::ResizeToContents).
• If you need fixed widths or user control over resizing for specific columns, use setSectionResizeMode().
• If you just need the last column to fill remaining space, use setStretchLastSection(true).