Introduce QgsScreenHelper

A utility class for dyanmic handling of changes to screen properties

python/gui/auto_generated/qgsscreenhelper.sip.in

@@ -0,0 +1,69 @@
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/gui/qgsscreenhelper.h                                            *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/
+
+
+
+
+class QgsScreenHelper : QObject
+{
+%Docstring(signature="appended")
+A utility class for dynamic handling of changes to screen properties.
+
+.. versionadded:: 3.28
+%End
+
+%TypeHeaderCode
+#include "qgsscreenhelper.h"
+%End
+  public:
+
+    QgsScreenHelper( QWidget *parent /TransferThis/ );
+%Docstring
+Constructor for QgsScreenHelper for the specified parent ``widget``.
+%End
+
+    QScreen *screen();
+%Docstring
+Returns the screen that the parent widget appears on, or ``None``.
+%End
+
+    QWindow *windowHandle();
+%Docstring
+Returns the window handle for the window the parent widget is associated with, or ``None``.
+%End
+
+    double screenDpi() const;
+%Docstring
+Returns the current screen DPI for the screen that the parent widget appears on.
+
+.. seealso:: :py:func:`screenDpiChanged`
+%End
+
+  signals:
+
+    void screenDpiChanged( double dpi );
+%Docstring
+Emitted whenever the screen ``dpi`` associated with the widget is changed.
+
+.. seealso:: :py:func:`screenDpi`
+%End
+
+  protected:
+    virtual bool eventFilter( QObject *watched, QEvent *event );
+
+
+};
+
+
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/gui/qgsscreenhelper.h                                            *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/

python/gui/gui_auto.sip

@@ -196,6 +196,7 @@
 %Include auto_generated/qgsscalerangewidget.sip
 %Include auto_generated/qgsscalevisibilitydialog.sip
 %Include auto_generated/qgsscalewidget.sip
+%Include auto_generated/qgsscreenhelper.sip
 %Include auto_generated/qgsscrollarea.sip
 %Include auto_generated/qgssearchquerybuilder.sip
 %Include auto_generated/qgssubsetstringeditorinterface.sip

src/gui/CMakeLists.txt

@@ -629,6 +629,7 @@ set(QGIS_GUI_SRCS
   qgsscalerangewidget.cpp
   qgsscalevisibilitydialog.cpp
   qgsscalewidget.cpp
+  qgsscreenhelper.cpp
   qgsscrollarea.cpp
   qgssearchquerybuilder.cpp
   qgssubsetstringeditorinterface.cpp
@@ -892,6 +893,7 @@ set(QGIS_GUI_HDRS
   qgsscalerangewidget.h
   qgsscalevisibilitydialog.h
   qgsscalewidget.h
+  qgsscreenhelper.h
   qgsscrollarea.h
   qgssearchquerybuilder.h
   qgssubsetstringeditorinterface.h

src/gui/qgsscreenhelper.cpp

@@ -0,0 +1,93 @@
+/***************************************************************************
+     qgsscreenhelper.cpp
+     ---------------
+    Date                 : August 2022
+    Copyright            : (C) 2022 by Nyall Dawson
+    Email                : nyall dot dawson at gmail dot com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+
+#include "qgsscreenhelper.h"
+#include "qgis.h"
+#include <QWidget>
+#include <QEvent>
+#include <QWindow>
+#include <QScreen>
+
+
+QgsScreenHelper::QgsScreenHelper( QWidget *parent )
+  : QObject( parent )
+  , mWidget( parent )
+{
+  mWidget->installEventFilter( this );
+}
+
+QScreen *QgsScreenHelper::screen()
+{
+  if ( QWindow *windowHandle = QgsScreenHelper::windowHandle() )
+  {
+    return windowHandle->screen();
+  }
+  return nullptr;
+}
+
+QWindow *QgsScreenHelper::windowHandle()
+{
+  if ( QWidget *window = mWidget->window() )
+    return window->windowHandle();
+
+  return nullptr;
+}
+
+bool QgsScreenHelper::eventFilter( QObject *watched, QEvent *event )
+{
+  if ( watched != mWidget )
+    return false;
+
+  switch ( event->type() )
+  {
+    case QEvent::Show:
+    {
+      updateDevicePixelFromScreen();
+      // keep device pixel ratio up to date on screen or resolution change
+      if ( QWindow *handle = windowHandle() )
+      {
+        connect( handle, &QWindow::screenChanged, this, [ = ]( QScreen * )
+        {
+          disconnect( mScreenDpiChangedConnection );
+          if ( QWindow *windowHandleInLambda = windowHandle() )
+          {
+            mScreenDpiChangedConnection = connect( windowHandleInLambda->screen(), &QScreen::physicalDotsPerInchChanged, this, &QgsScreenHelper::updateDevicePixelFromScreen );
+            updateDevicePixelFromScreen();
+          }
+        } );
+
+        mScreenDpiChangedConnection = connect( handle->screen(), &QScreen::physicalDotsPerInchChanged, this, &QgsScreenHelper::updateDevicePixelFromScreen );
+      }
+    }
+
+    default:
+      break;
+  }
+
+  return false;
+}
+
+void QgsScreenHelper::updateDevicePixelFromScreen()
+{
+  if ( QScreen *screen = QgsScreenHelper::screen() )
+  {
+    const double newDpi = screen->physicalDotsPerInch();
+    if ( !qgsDoubleNear( newDpi, mScreenDpi ) )
+    {
+      mScreenDpi = newDpi;
+      emit screenDpiChanged( mScreenDpi );
+    }
+  }
+}

src/gui/qgsscreenhelper.h

@@ -0,0 +1,88 @@
+/***************************************************************************
+     qgsscreenhelper.h
+     ---------------
+    Date                 : August 2022
+    Copyright            : (C) 2022 by Nyall Dawson
+    Email                : nyall dot dawson at gmail dot com
+ ***************************************************************************
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ ***************************************************************************/
+#ifndef QGSSCREENHELPER_H
+#define QGSSCREENHELPER_H
+
+#include "qgis_gui.h"
+#include "qgis_sip.h"
+
+#include <QObject>
+
+class QWidget;
+class QScreen;
+class QWindow;
+
+/**
+ * \ingroup gui
+ * \class QgsScreenHelper
+ * \brief A utility class for dynamic handling of changes to screen properties.
+ *
+ * \since QGIS 3.28
+ */
+class GUI_EXPORT QgsScreenHelper : public QObject
+{
+    Q_OBJECT
+
+  public:
+
+    /**
+     * Constructor for QgsScreenHelper for the specified parent \a widget.
+     */
+    QgsScreenHelper( QWidget *parent SIP_TRANSFERTHIS );
+
+    /**
+     * Returns the screen that the parent widget appears on, or NULLPTR.
+     */
+    QScreen *screen();
+
+    /**
+     * Returns the window handle for the window the parent widget is associated with, or NULLPTR.
+     */
+    QWindow *windowHandle();
+
+    /**
+     * Returns the current screen DPI for the screen that the parent widget appears on.
+     *
+     * \see screenDpiChanged()
+     */
+    double screenDpi() const { return mScreenDpi; }
+
+  signals:
+
+    /**
+     * Emitted whenever the screen \a dpi associated with the widget is changed.
+     *
+     * \see screenDpi()
+     */
+    void screenDpiChanged( double dpi );
+
+  protected:
+    bool eventFilter( QObject *watched, QEvent *event ) override;
+
+  private slots:
+
+    void updateDevicePixelFromScreen();
+
+  private:
+
+    QWidget *mWidget = nullptr;
+
+    double mScreenDpi = 96.0;
+    QMetaObject::Connection mScreenDpiChangedConnection;
+
+};
+
+
+#endif // QGSSCREENHELPER_H