Qt Widgets: Controlling Horizontal Spacing in QGraphicsAnchorLayout

Purpose

• It controls the amount of space inserted horizontally between anchored items within the layout.
• This function sets the default horizontal spacing between items managed by a QGraphicsAnchorLayout object.

Parameters

• spacing (qreal): A floating-point value specifying the desired horizontal spacing in pixels.

How it Works

1. Setting Default Spacing
   When you call setHorizontalSpacing(spacing), the layout internally stores the provided spacing as its default horizontal spacing.
2. Anchor Relationships
   This default spacing is used as a guideline when laying out items based on the anchor relationships you've established using methods like addAnchor(), addCornerAnchors(), or addAnchors().
3. Layout Calculation
   During layout calculation, the anchor layout considers the following:
   • Anchors you've defined between items.
   • The size hints of the items themselves.
   • The default horizontal and vertical spacings (set using setHorizontalSpacing() and setVerticalSpacing(), respectively).
   • Any margins set around the layout using setContentsMargins().

Flexibility

• For more granular control over specific anchor pairs, you can directly set spacing values on individual anchors using the setSpacing() method of the returned QGraphicsAnchor object from anchor creation methods.
• While the default spacing provides a baseline for item placement, it's not absolute. Anchor relationships can override the default spacing if necessary to satisfy the specified anchor constraints.

Example

#include <QtWidgets>

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

    // Create a graphics scene and view
    QGraphicsScene scene;
    QGraphicsView view(&scene);
    view.show();

    // Create some widgets to be laid out
    QWidget *widget1 = new QWidget;
    widget1->resize(100, 50);
    QWidget *widget2 = new QWidget;
    widget2->resize(80, 60);

    // Add widgets to the scene
    scene.addItem(widget1);
    scene.addItem(widget2);

    // Create an anchor layout and set it as the layout for both widgets
    QGraphicsAnchorLayout *layout = new QGraphicsAnchorLayout;
    widget1->setLayout(layout);
    widget2->setLayout(layout);

    // Add anchors to position the widgets horizontally
    QGraphicsAnchor *leftAnchor = layout->addAnchor(widget1, Qt::AnchorLeft, widget2, Qt::AnchorLeft);
    QGraphicsAnchor *rightAnchor = layout->addAnchor(widget1, Qt::AnchorRight, widget2, Qt::AnchorRight);

    // Set the default horizontal spacing between the widgets
    layout->setHorizontalSpacing(20);

    // Update the layout to apply the changes
    layout->update();

    return app.exec();
}

In this example, the setHorizontalSpacing(20) call establishes a 20-pixel gap between widget1 and widget2 horizontally, while the anchors ensure they remain aligned on the left and right edges.

• Use anchor spacing for more precise control over specific anchor pairs.
• It's a guideline and might be adjusted based on anchor relationships and item size hints.
• setHorizontalSpacing() sets the default horizontal spacing for the entire QGraphicsAnchorLayout.

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Overriding Default Spacing with Anchor Spacing

#include <QtWidgets>

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

    // ... (code to create scene, view, widgets, and layout)

    // Add anchors to position the widgets horizontally with some overlap
    QGraphicsAnchor *leftAnchor = layout->addAnchor(widget1, Qt::AnchorLeft, widget2, Qt::AnchorLeft);
    leftAnchor->setSpacing(-10); // Negative spacing for 10-pixel overlap

    QGraphicsAnchor *rightAnchor = layout->addAnchor(widget1, Qt::AnchorRight, widget2, Qt::AnchorRight);

    // Set the default horizontal spacing (will be overridden for left anchor)
    layout->setHorizontalSpacing(20);

    // Update the layout to apply the changes
    layout->update();

    return app.exec();
}

In this example, the leftAnchor->setSpacing(-10) line sets a negative spacing of -10 pixels between widget1 and widget2's left edges, effectively creating a 10-pixel overlap that overrides the default spacing of 20 pixels set for the layout.

Using Size Hints and Spacing

#include <QtWidgets>

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

    // ... (code to create scene, view, widgets, and layout)

    // Set preferred sizes for the widgets
    widget1->setFixedWidth(120);
    widget2->setFixedWidth(100);

    // Add anchors to position the widgets horizontally with some spacing
    QGraphicsAnchor *leftAnchor = layout->addAnchor(widget1, Qt::AnchorLeft, scene.sceneRect(), Qt::AnchorLeft);
    QGraphicsAnchor *rightAnchor = layout->addAnchor(widget2, Qt::AnchorRight, scene.sceneRect(), Qt::AnchorRight);

    // Set the horizontal spacing between the widgets
    layout->setHorizontalSpacing(30);

    // Update the layout to apply the changes
    layout->update();

    return app.exec();
}

Here, widget1 and widget2 have fixed widths set using setFixedWidth(). The anchors position them at the left and right edges of the scene (scene.sceneRect()), and the setHorizontalSpacing(30) line creates a 30-pixel gap between their right edges, considering both widget sizes and the spacing.

---

1. • Instead of a layout-wide default, you can directly set the spacing on individual anchors using the setSpacing() method of the returned QGraphicsAnchor object from anchor creation methods like addAnchor(), addCornerAnchors(), or addAnchors().
   • This approach provides finer control over the spacing between specific item pairs.

   // ... (code to create scene, view, widgets, and layout)
   
   // Add anchors with custom spacing between widgets
   QGraphicsAnchor *leftAnchor = layout->addAnchor(widget1, Qt::AnchorLeft, widget2, Qt::AnchorLeft);
   leftAnchor->setSpacing(10); // Set spacing for the left anchor pair
   
   QGraphicsAnchor *rightAnchor = layout->addAnchor(widget1, Qt::AnchorRight, widget2, Qt::AnchorRight);
   rightAnchor->setSpacing(20); // Set spacing for the right anchor pair
   

2. QHBoxLayout with Margins
   

   • If you don't need the advanced anchoring capabilities of QGraphicsAnchorLayout and just want a simple horizontal layout with spacing, consider using QHBoxLayout.
   • Set the layout for both widgets to a QHBoxLayout object.
   • Use the setContentsMargins() method on the layout to set the spacing between the widgets and the layout's edges.

   // ... (code to create scene, view, widgets)
   
   // Create a horizontal box layout
   QHBoxLayout *hLayout = new QHBoxLayout;
   
   // Set the layout for both widgets
   widget1->setLayout(hLayout);
   widget2->setLayout(hLayout);
   
   // Add widgets to the layout
   hLayout->addWidget(widget1);
   hLayout->addWidget(widget2);
   
   // Set spacing between widgets and layout edges
   hLayout->setContentsMargins(20, 10, 20, 10); // Adjust margins as needed
   

3. Custom Layout Class
   

   • For highly customized layouts, you can create your own subclass of QLayout and implement its layout() method to define the desired layout behavior, including horizontal spacing.
   • This approach gives you maximum control but requires more development effort.

The best choice depends on your specific needs:

• Consider a custom layout class only if you require highly customized layout behavior beyond the capabilities of existing layout managers.
• Use QHBoxLayout with margins for a simpler horizontal layout with uniform spacing between widgets.
• Use manual anchor spacing if you need fine-grained control over gaps between specific item pairs within a flexible anchor-based layout.