Allow any Item to act as a viewport for any of its children

The children that have the ItemObservesViewport flag set will have
transformChanged() called whenever any parent moves, and can use
clipRect() to avoid creating SG nodes that fall outside the viewport.
So now the viewport can be narrower than the whole QQuickWindow; but the
fallback is the window.

It was always unclear why we had both boundingRect() and clipRect(),
returning the same rectangular areas.  Now clipRect() can actually be
used for a rough pre-clipping, to limit the amount of vertex data going
into the scene graph, which can save a lot of memory in some cases.

setClip() sets the ItemIsViewport flag, for the sake of making it easy
to do viewporting without writing C++; but it's well known that
scissoring has a performance impact, so we recommend that users who
are writing C++ anyway can set the flags in C++ rather than in QML.

In case there are nested items with both flags ItemIsViewport and
ItemObservesViewport, calling clipRect() on something inside the
innermost viewport is recursive: we intersect all the viewports,
going up the hierarchy to the window. So it's possible for the
innermost item to be clipped down to a small "iris" where all the
viewports are letting "light" pass through.

[ChangeLog][QtQuick][QQuickItem] QQuickItem::clipRect() now provides
the region visible in the viewport, and can be used to limit SG node
vertices as an optimization in custom items, at the cost of having
updatePaintNode() called more often. See docs about the new
ItemObservesViewport and ItemIsViewport flags.

Fixes: QTBUG-37364
Task-number: QTBUG-60491
Task-number: QTBUG-65741
Task-number: QTBUG-77521
Task-number: QTBUG-90734
Change-Id: I71a26c2dab4e991d7fb0f6679f1aa0c34e7a14ee
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

1 files changed, 7 insertions, 4 deletions

diff --git a/src/quick/items/qquickitem_p.h b/src/quick/items/qquickitem_p.h
index 0d35cfd48c..51ef850d5f 100644
--- a/src/quick/items/qquickitem_p.h
+++ b/src/quick/items/qquickitem_p.h
@@ -441,7 +441,7 @@ public:
     inline QQuickItem::TransformOrigin origin() const;
 
     // Bit 0
-    quint32 flags:5;
+    quint32 flags:7;
     bool widthValidFlag:1;
     bool heightValidFlag:1;
     bool componentComplete:1;
@@ -451,9 +451,9 @@ public:
     bool smooth:1;
     bool antialiasing:1;
     bool focus:1;
+    // Bit 16
     bool activeFocus:1;
     bool notifiedFocus:1;
-    // Bit 16
     bool notifiedActiveFocus:1;
     bool filtersChildMouseEvents:1;
     bool explicitVisible:1;
@@ -468,9 +468,9 @@ public:
     bool inheritMirrorFromItem:1;
     bool isAccessible:1;
     bool culled:1;
+    // Bit 32
     bool hasCursor:1;
     bool subtreeCursorEnabled:1;
-    // Bit 32
     bool subtreeHoverEnabled:1;
     bool activeFocusOnTab:1;
     bool implicitAntialiasing:1;
@@ -486,6 +486,9 @@ public:
     bool hasCursorHandler:1;
     // set true when this item does not expect events via a subscene delivery agent; false otherwise
     bool maybeHasSubsceneDeliveryAgent:1;
+    // set true if this item or any child wants QQuickItemPrivate::transformChanged() to visit all children
+    // (e.g. when parent has ItemIsViewport and child has ItemObservesViewport)
+    bool subtreeTransformChangedEnabled:1;
 
     enum DirtyType {
         TransformOrigin         = 0x00000001,
@@ -617,7 +620,7 @@ public:
     }
 
     QPointF computeTransformOrigin() const;
-    virtual void transformChanged(QQuickItem *transformedItem);
+    virtual bool transformChanged(QQuickItem *transformedItem);
 
     QPointF adjustedPosForTransform(const QPointF &centroid,
                                     const QPointF &startPos, const QVector2D &activeTranslatation,