Avoid flicker when invalidate is propagated in a widget/layout hierarchy

* Do not call invalidate from activateRecursive().
  This resulted in that a layout was invalidated as many times as there
  were items in the layout.

* Several improvements. Do not call resize(size()) too often.
  Calling resize() from the widgetEvent() is not very nice though...

* Remove commented out code

* make sure layout is activated even if the widget does not change size

* activate the layout if the resize is same as size()

* In order to not break existing apps, make this an opt-in feature
  with QGraphicsLayout::setInstantInvalidatePropagation(true);

Reviewed-by: Frederik Gladhorn
Reviewed-by: John Tapsell

5 files changed, 190 insertions, 76 deletions

diff --git a/src/gui/graphicsview/qgraphicslayout.cpp b/src/gui/graphicsview/qgraphicslayout.cpp
index 5bd298061d..a67ae48cc7 100644
--- a/src/gui/graphicsview/qgraphicslayout.cpp
+++ b/src/gui/graphicsview/qgraphicslayout.cpp
@@ -269,12 +269,20 @@ void QGraphicsLayout::activate()
         return;
     Q_ASSERT(!parentItem->isLayout());
 
-    setGeometry(parentItem->contentsRect());    // relayout children
+    if (QGraphicsLayout::instantInvalidatePropagation()) {
+        QGraphicsWidget *parentWidget = static_cast<QGraphicsWidget*>(parentItem);
+        if (!parentWidget->parentLayoutItem()) {
+            // we've reached the topmost widget, resize it
+            bool wasResized = parentWidget->testAttribute(Qt::WA_Resized);
+            parentWidget->resize(parentWidget->size());
+            parentWidget->setAttribute(Qt::WA_Resized, wasResized);
+        }
 
-    // ### bug, should be parentItem ?
-    parentLayoutItem()->updateGeometry();            // bubble up; will set activated to false
-    // ### too many resizes? maybe we should walk up the chain to the
-    // ### top-level layouted layoutItem and call activate there.
+        setGeometry(parentItem->contentsRect());    // relayout children
+    } else {
+        setGeometry(parentItem->contentsRect());    // relayout children
+        parentLayoutItem()->updateGeometry();
+    }
 }
 
 /*!
@@ -300,32 +308,36 @@ bool QGraphicsLayout::isActivated() const
 */
 void QGraphicsLayout::invalidate()
 {
-    // only mark layouts as invalid (activated = false) if we can post a LayoutRequest event.
-    QGraphicsLayoutItem *layoutItem = this;
-    while (layoutItem && layoutItem->isLayout()) {
-        // we could call updateGeometry(), but what if that method
-        // does not call the base implementation? In addition, updateGeometry()
-        // does more than we need.
-        layoutItem->d_func()->sizeHintCacheDirty = true;
-        layoutItem->d_func()->sizeHintWithConstraintCacheDirty = true;
-        layoutItem = layoutItem->parentLayoutItem();
-    }
-    if (layoutItem) {
-        layoutItem->d_func()->sizeHintCacheDirty = true;
-        layoutItem->d_func()->sizeHintWithConstraintCacheDirty = true;
-    }
-
-    bool postIt = layoutItem ? !layoutItem->isLayout() : false;
-    if (postIt) {
-        layoutItem = this;
-        while (layoutItem && layoutItem->isLayout()
-                && static_cast<QGraphicsLayout*>(layoutItem)->d_func()->activated) {
-            static_cast<QGraphicsLayout*>(layoutItem)->d_func()->activated = false;
+    if (QGraphicsLayout::instantInvalidatePropagation()) {
+        updateGeometry();
+    } else {
+        // only mark layouts as invalid (activated = false) if we can post a LayoutRequest event.
+        QGraphicsLayoutItem *layoutItem = this;
+        while (layoutItem && layoutItem->isLayout()) {
+            // we could call updateGeometry(), but what if that method
+            // does not call the base implementation? In addition, updateGeometry()
+            // does more than we need.
+            layoutItem->d_func()->sizeHintCacheDirty = true;
+            layoutItem->d_func()->sizeHintWithConstraintCacheDirty = true;
             layoutItem = layoutItem->parentLayoutItem();
         }
-        if (layoutItem && !layoutItem->isLayout()) {
-            // If a layout has a parent that is not a layout it must be a QGraphicsWidget.
-            QApplication::postEvent(static_cast<QGraphicsWidget *>(layoutItem), new QEvent(QEvent::LayoutRequest));
+        if (layoutItem) {
+            layoutItem->d_func()->sizeHintCacheDirty = true;
+            layoutItem->d_func()->sizeHintWithConstraintCacheDirty = true;
+        }
+
+        bool postIt = layoutItem ? !layoutItem->isLayout() : false;
+        if (postIt) {
+            layoutItem = this;
+            while (layoutItem && layoutItem->isLayout()
+                    && static_cast<QGraphicsLayout*>(layoutItem)->d_func()->activated) {
+                static_cast<QGraphicsLayout*>(layoutItem)->d_func()->activated = false;
+                layoutItem = layoutItem->parentLayoutItem();
+            }
+            if (layoutItem && !layoutItem->isLayout()) {
+                // If a layout has a parent that is not a layout it must be a QGraphicsWidget.
+                QApplication::postEvent(static_cast<QGraphicsWidget *>(layoutItem), new QEvent(QEvent::LayoutRequest));
+            }
         }
     }
 }
@@ -335,12 +347,27 @@ void QGraphicsLayout::invalidate()
 */
 void QGraphicsLayout::updateGeometry()
 {
-    QGraphicsLayoutItem::updateGeometry();
-    if (QGraphicsLayoutItem *parentItem = parentLayoutItem()) {
-        if (parentItem->isLayout()) {
+    Q_D(QGraphicsLayout);
+    if (QGraphicsLayout::instantInvalidatePropagation()) {
+        d->activated = false;
+        QGraphicsLayoutItem::updateGeometry();
+
+        QGraphicsLayoutItem *parentItem = parentLayoutItem();
+        if (!parentItem)
+            return;
+
+        if (parentItem->isLayout())
+            static_cast<QGraphicsLayout *>(parentItem)->invalidate();
+        else
             parentItem->updateGeometry();
-        } else {
-            invalidate();
+    } else {
+        QGraphicsLayoutItem::updateGeometry();
+        if (QGraphicsLayoutItem *parentItem = parentLayoutItem()) {
+            if (parentItem->isLayout()) {
+                parentItem->updateGeometry();
+            } else {
+                invalidate();
+            }
         }
     }
 }
@@ -446,6 +473,50 @@ void QGraphicsLayout::addChildLayoutItem(QGraphicsLayoutItem *layoutItem)
     d->addChildLayoutItem(layoutItem);
 }
 
+static bool g_instantInvalidatePropagation = false;
+
+/*!
+    \internal
+    \since 4.8
+    \see instantInvalidatePropagation
+
+    Calling this function with \a enable set to true will enable a feature that
+    makes propagation of invalidation up to ancestor layout items to be done in
+    one go. It will propagate up the parentLayoutItem() hierarchy until it has
+    reached the root. If the root item is a QGraphicsWidget, it will *post* a
+    layout request to it. When the layout request is consumed it will traverse
+    down the hierarchy of layouts and widgets and activate all layouts that is
+    invalid (not activated). This is the recommended behaviour.
+
+    If not set it will also propagate up the parentLayoutItem() hierarchy, but
+    it will stop at the \i first \i widget it encounters, and post a layout
+    request to the widget. When the layout request is consumed, this might
+    cause it to continue propagation up to the parentLayoutItem() of the
+    widget. It will continue in this fashion until it has reached a widget with
+    no parentLayoutItem(). This strategy might cause drawing artifacts, since
+    it is not done in one go, and the consumption of layout requests might be
+    interleaved by consumption of paint events, which might cause significant
+    flicker.
+    Note, this is not the recommended behavior, but for compatibility reasons
+    this is the default behaviour.
+*/
+void QGraphicsLayout::setInstantInvalidatePropagation(bool enable)
+{
+    g_instantInvalidatePropagation = enable;
+}
+
+/*!
+    \internal
+    \since 4.8
+    \see setInstantInvalidatePropagation
+
+    returns true if the complete widget/layout hierarchy is rearranged in one go.
+*/
+bool QGraphicsLayout::instantInvalidatePropagation()
+{
+    return g_instantInvalidatePropagation;
+}
+
 QT_END_NAMESPACE
 
 #endif //QT_NO_GRAPHICSVIEW
diff --git a/src/gui/graphicsview/qgraphicslayout.h b/src/gui/graphicsview/qgraphicslayout.h
index c622fb881e..60311744a2 100644
--- a/src/gui/graphicsview/qgraphicslayout.h
+++ b/src/gui/graphicsview/qgraphicslayout.h
@@ -76,6 +76,8 @@ public:
     virtual QGraphicsLayoutItem *itemAt(int i) const = 0;
     virtual void removeAt(int index) = 0;
 
+    static void setInstantInvalidatePropagation(bool enable);
+    static bool instantInvalidatePropagation();
 protected:
     QGraphicsLayout(QGraphicsLayoutPrivate &, QGraphicsLayoutItem *);
     void addChildLayoutItem(QGraphicsLayoutItem *layoutItem);
diff --git a/src/gui/graphicsview/qgraphicslayout_p.cpp b/src/gui/graphicsview/qgraphicslayout_p.cpp
index c325602cc6..05e6cdf599 100644
--- a/src/gui/graphicsview/qgraphicslayout_p.cpp
+++ b/src/gui/graphicsview/qgraphicslayout_p.cpp
@@ -180,9 +180,14 @@ void QGraphicsLayoutPrivate::activateRecursive(QGraphicsLayoutItem *item)
 {
     if (item->isLayout()) {
         QGraphicsLayout *layout = static_cast<QGraphicsLayout *>(item);
-        if (layout->d_func()->activated)
-            layout->invalidate();
-
+        if (layout->d_func()->activated) {
+            if (QGraphicsLayout::instantInvalidatePropagation()) {
+                return;
+            } else {
+                layout->invalidate();   // ### LOOKS SUSPICIOUSLY WRONG!!???
+            }
+        }
+        
         for (int i = layout->count() - 1; i >= 0; --i) {
             QGraphicsLayoutItem *childItem = layout->itemAt(i);
             if (childItem)
diff --git a/src/gui/graphicsview/qgraphicslinearlayout.cpp b/src/gui/graphicsview/qgraphicslinearlayout.cpp
index 5591638395..1229379ee8 100644
--- a/src/gui/graphicsview/qgraphicslinearlayout.cpp
+++ b/src/gui/graphicsview/qgraphicslinearlayout.cpp
@@ -275,17 +275,16 @@ void QGraphicsLinearLayout::insertItem(int index, QGraphicsLayoutItem *item)
         qWarning("QGraphicsLinearLayout::insertItem: cannot insert itself");
         return;
     }
-    Q_ASSERT(item);
 
-    //the order of the following instructions is very important because
-    //invalidating the layout before adding the child item will make the layout happen
-    //before we try to paint the item
-    invalidate();
     d->addChildLayoutItem(item);
 
     d->fixIndex(&index);
     d->engine.insertRow(index, d->orientation);
     new QGridLayoutItem(&d->engine, item, d->gridRow(index), d->gridColumn(index), 1, 1, 0, index);
+    //the order of the following instructions is very important because
+    //invalidating the layout before adding the child item will make the layout happen
+    //before we try to paint the item
+    invalidate();
 }
 
 /*!
diff --git a/src/gui/graphicsview/qgraphicswidget.cpp b/src/gui/graphicsview/qgraphicswidget.cpp
index 675a5c5c77..141e305acd 100644
--- a/src/gui/graphicsview/qgraphicswidget.cpp
+++ b/src/gui/graphicsview/qgraphicswidget.cpp
@@ -354,8 +354,10 @@ void QGraphicsWidget::setGeometry(const QRectF &rect)
         newGeom = rect;
         newGeom.setSize(rect.size().expandedTo(effectiveSizeHint(Qt::MinimumSize))
                                    .boundedTo(effectiveSizeHint(Qt::MaximumSize)));
-        if (newGeom == d->geom)
-            return;
+
+        if (newGeom == d->geom) {
+            goto relayoutChildrenAndReturn;
+        }
 
         // setPos triggers ItemPositionChange, which can adjust position
         wd->inSetGeometry = 1;
@@ -363,8 +365,9 @@ void QGraphicsWidget::setGeometry(const QRectF &rect)
         wd->inSetGeometry = 0;
         newGeom.moveTopLeft(pos());
 
-        if (newGeom == d->geom)
-           return;
+        if (newGeom == d->geom) {
+            goto relayoutChildrenAndReturn;
+        }
 
          // Update and prepare to change the geometry (remove from index) if the size has changed.
         if (wd->scene) {
@@ -375,35 +378,54 @@ void QGraphicsWidget::setGeometry(const QRectF &rect)
     }
 
     // Update the layout item geometry
-    bool moved = oldPos != pos();
-    if (moved) {
-        // Send move event.
-        QGraphicsSceneMoveEvent event;
-        event.setOldPos(oldPos);
-        event.setNewPos(pos());
-        QApplication::sendEvent(this, &event);
-        if (wd->inSetPos) {
-            //set the new pos
-            d->geom.moveTopLeft(pos());
-            emit geometryChanged();
-            return;
+    {
+        bool moved = oldPos != pos();
+        if (moved) {
+            // Send move event.
+            QGraphicsSceneMoveEvent event;
+            event.setOldPos(oldPos);
+            event.setNewPos(pos());
+            QApplication::sendEvent(this, &event);
+            if (wd->inSetPos) {
+                //set the new pos
+                d->geom.moveTopLeft(pos());
+                emit geometryChanged();
+                goto relayoutChildrenAndReturn;
+            }
+        }
+        QSizeF oldSize = size();
+        QGraphicsLayoutItem::setGeometry(newGeom);
+        // Send resize event
+        bool resized = newGeom.size() != oldSize;
+        if (resized) {
+            QGraphicsSceneResizeEvent re;
+            re.setOldSize(oldSize);
+            re.setNewSize(newGeom.size());
+            if (oldSize.width() != newGeom.size().width())
+                emit widthChanged();
+            if (oldSize.height() != newGeom.size().height())
+                emit heightChanged();
+            QGraphicsLayout *lay = wd->layout;
+            if (QGraphicsLayout::instantInvalidatePropagation()) {
+                if (!lay || lay->isActivated()) {
+                    QApplication::sendEvent(this, &re);
+                }
+            } else {
+                QApplication::sendEvent(this, &re);
+            }
         }
     }
-    QSizeF oldSize = size();
-    QGraphicsLayoutItem::setGeometry(newGeom);
-    // Send resize event
-    bool resized = newGeom.size() != oldSize;
-    if (resized) {
-        QGraphicsSceneResizeEvent re;
-        re.setOldSize(oldSize);
-        re.setNewSize(newGeom.size());
-        if (oldSize.width() != newGeom.size().width())
-            emit widthChanged();
-        if (oldSize.height() != newGeom.size().height())
-            emit heightChanged();
-        QApplication::sendEvent(this, &re);
-    }
+
     emit geometryChanged();
+relayoutChildrenAndReturn:
+    if (QGraphicsLayout::instantInvalidatePropagation()) {
+        if (QGraphicsLayout *lay = wd->layout) {
+            if (!lay->isActivated()) {
+                QEvent layoutRequest(QEvent::LayoutRequest);
+                QApplication::sendEvent(this, &layoutRequest);
+            }
+        }
+    }
 }
 
 /*!
@@ -1052,16 +1074,31 @@ void QGraphicsWidget::updateGeometry()
     QGraphicsLayoutItem *parentItem = parentLayoutItem();
 
     if (parentItem && parentItem->isLayout()) {
-        parentItem->updateGeometry();
+        if (QGraphicsLayout::instantInvalidatePropagation()) {
+            static_cast<QGraphicsLayout *>(parentItem)->invalidate();
+        } else {
+            parentItem->updateGeometry();
+        }
     } else {
         if (parentItem) {
+            // This is for custom layouting
             QGraphicsWidget *parentWid = parentWidget();    //###
             if (parentWid->isVisible())
                 QApplication::postEvent(parentWid, new QEvent(QEvent::LayoutRequest));
+        } else {
+            /**
+             * If this is the topmost widget, post a LayoutRequest event to the widget.
+             * When the event is received, it will start flowing all the way down to the leaf
+             * widgets in one go. This will make a relayout flicker-free.
+             */
+            if (QGraphicsLayout::instantInvalidatePropagation())
+                QApplication::postEvent(static_cast<QGraphicsWidget *>(this), new QEvent(QEvent::LayoutRequest));
+        }
+        if (!QGraphicsLayout::instantInvalidatePropagation()) {
+            bool wasResized = testAttribute(Qt::WA_Resized);
+            resize(size()); // this will restrict the size
+            setAttribute(Qt::WA_Resized, wasResized);
         }
-        bool wasResized = testAttribute(Qt::WA_Resized);
-        resize(size()); // this will restrict the size
-        setAttribute(Qt::WA_Resized, wasResized);
     }
 }