Avoid accidental child widget value changes when scrolling scroll areas

This adds a new QgsScrollArea widget which is a subclass of QScrollArea. QgsScrollArea has extra logic which temporarily blocks wheel events from hitting child widgets for a short period following a scroll of the area. This means that when scrolling a scroll area using the mouse wheel, values won't be accidentally changed if the mouse cursor momentarily lands on top of a widget. QScrollArea should no longer be used in any QGIS code or plugins, instead use QgsScrollArea to benefit from this fix.
qgis · Mar 20, 2017 · 907ad02 · 907ad02
1 parent f60dc81
commit 907ad02
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diff --git a/src/gui/CMakeLists.txt b/src/gui/CMakeLists.txt
@@ -296,6 +296,7 @@ SET(QGIS_GUI_SRCS
   qgsscalerangewidget.cpp
   qgsscalevisibilitydialog.cpp
   qgsscalewidget.cpp
+  qgsscrollarea.cpp
   qgssearchquerybuilder.cpp
   qgsshortcutsmanager.cpp
   qgsslider.cpp
@@ -442,6 +443,7 @@ SET(QGIS_GUI_MOC_HDRS
   qgsscalerangewidget.h
   qgsscalevisibilitydialog.h
   qgsscalewidget.h
+  qgsscrollarea.h
   qgssearchquerybuilder.h
   qgsshortcutsmanager.h
   qgsslider.h

diff --git a/src/gui/qgsscrollarea.cpp b/src/gui/qgsscrollarea.cpp
@@ -0,0 +1,127 @@
+/***************************************************************************
+    qgsscrollarea.cpp
+    -----------------
+    begin                : March 2017
+    copyright            : (C) 2017 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 <QEvent>
+#include <QMouseEvent>
+#include "qgsscrollarea.h"
+
+// milliseconds to swallow child wheel events for after a scroll occurs
+#define TIMEOUT 1000
+
+QgsScrollArea::QgsScrollArea( QWidget *parent )
+  : QScrollArea( parent )
+  , mFilter( new ScrollAreaFilter( this, viewport() ) )
+{
+  viewport()->installEventFilter( mFilter );
+}
+
+void QgsScrollArea::wheelEvent( QWheelEvent *e )
+{
+  //scroll occurred, reset timer
+  scrollOccurred();
+  QScrollArea::wheelEvent( e );
+}
+
+void QgsScrollArea::scrollOccurred()
+{
+  mTimer.setSingleShot( true );
+  mTimer.start( TIMEOUT );
+}
+
+bool QgsScrollArea::hasScrolled() const
+{
+  return mTimer.isActive();
+}
+
+///@cond PRIVATE
+
+ScrollAreaFilter::ScrollAreaFilter( QgsScrollArea *parent, QWidget *viewPort )
+  : QObject( parent )
+  , mScrollAreaWidget( parent )
+  , mViewPort( viewPort )
+{}
+
+bool ScrollAreaFilter::eventFilter( QObject *obj, QEvent *event )
+{
+  switch ( event->type() )
+  {
+    case QEvent::ChildAdded:
+    {
+      // need to install filter on all child widgets as well
+      QChildEvent *ce = static_cast<QChildEvent *>( event );
+      addChild( ce->child() );
+      break;
+    }
+
+    case QEvent::ChildRemoved:
+    {
+      QChildEvent *ce = static_cast<QChildEvent *>( event );
+      removeChild( ce->child() );
+      break;
+    }
+
+    case QEvent::Wheel:
+    {
+      if ( obj == mViewPort )
+      {
+        // scrolling scroll area - kick off the timer to block wheel events in children
+        mScrollAreaWidget->scrollOccurred();
+      }
+      else
+      {
+        if ( mScrollAreaWidget->hasScrolled() )
+        {
+          // swallow wheel events for children shortly after scroll occurs
+          return true;
+        }
+      }
+      break;
+    }
+
+    default:
+      break;
+  }
+  return QObject::eventFilter( obj, event );
+}
+
+void ScrollAreaFilter::addChild( QObject *child )
+{
+  if ( child && child->isWidgetType() )
+  {
+    child->installEventFilter( this );
+
+    // also install filter on existing children
+    Q_FOREACH ( QObject *c, child->children() )
+    {
+      addChild( c );
+    }
+  }
+}
+
+void ScrollAreaFilter::removeChild( QObject *child )
+{
+  if ( child && child->isWidgetType() )
+  {
+    child->removeEventFilter( this );
+
+    // also remove filter on existing children
+    Q_FOREACH ( QObject *c, child->children() )
+    {
+      removeChild( c );
+    }
+  }
+}
+
+///@endcond PRIVATE
diff --git a/src/gui/qgsscrollarea.h b/src/gui/qgsscrollarea.h
@@ -0,0 +1,100 @@
+/***************************************************************************
+    qgsscrollarea.h
+    ---------------
+    begin                : March 2017
+    copyright            : (C) 2017 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 QGSSCROLLAREA_H
+#define QGSSCROLLAREA_H
+
+#include <QScrollArea>
+#include "qgis_gui.h"
+#include <QTimer>
+class ScrollAreaFilter;
+
+/**
+ * \class QgsScrollArea
+ * \ingroup gui
+ * A QScrollArea subclass with improved scrolling behavior.
+ *
+ * QgsScrollArea should be used instead of QScrollArea widgets.
+ * In most cases the use is identical, however QgsScrollArea
+ * has extra logic to avoid wheel events changing child widget
+ * values when the mouse cursor is temporarily located over
+ * a child widget during a scroll event.
+ *
+ * All QGIS code and plugins should use QgsScrollArea in place
+ * of QScrollArea.
+ *
+ * \note added in QGIS 3.0
+ */
+class GUI_EXPORT QgsScrollArea : public QScrollArea
+{
+    Q_OBJECT
+
+  public:
+
+    /**
+     * Constructor for QgsScrollArea.
+     */
+    explicit QgsScrollArea( QWidget *parent = nullptr );
+
+    /**
+     * Should be called when a scroll occurs on with the
+     * QScrollArea itself or its child viewport().
+     */
+    void scrollOccurred();
+
+    /**
+     * Returns true if a scroll recently occurred within
+     * the QScrollArea or its child viewport()
+     */
+    bool hasScrolled() const;
+
+  protected:
+    void wheelEvent( QWheelEvent *event ) override;
+
+  private:
+    QTimer mTimer;
+    ScrollAreaFilter *mFilter = nullptr;
+};
+
+///@cond PRIVATE
+
+/**
+ * \class ScrollAreaFilter
+ * Swallows wheel events for QScrollArea children for a short period
+ * following a scroll.
+ */
+class ScrollAreaFilter : public QObject
+{
+    Q_OBJECT
+  public:
+
+    ScrollAreaFilter( QgsScrollArea *parent = nullptr,
+                      QWidget *viewPort = nullptr );
+
+  protected:
+    bool eventFilter( QObject *obj, QEvent *event ) override;
+
+  private:
+    QgsScrollArea *mScrollAreaWidget = nullptr;
+    QWidget *mViewPort = nullptr;
+
+    void addChild( QObject *child );
+    void removeChild( QObject *child );
+
+};
+
+///@endcond PRIVATE
+
+#endif // QGSSCROLLAREA_H