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

1 files changed, 105 insertions, 34 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