Large Text: populate only lines visible in Window into SG

Text is rendered top-to-bottom: QQuickTextNode::addTextLayout()
iterates lines of text.  Now it first maps the window size() rectangle
into the Text item's coordinate system, and treats that as the viewport:
lines whose y coordinates fall outside are omitted.
This saves a huge amount of time and memory when the text is large.
So Flickable { Text { } } can actually be used in an ebook reader now,
for example.

QQuickItemPrivate::transformChanged(parent) is now used to inform
all children when a parent moves; so e.g. when a content item is
moved inside a "viewport" item (like a Flickable, or any other item
that is being used as a clipping viewport or just as a holder),
all children of the content item will be notified; and QQuickTextPrivate
uses this occasion to mark itself dirty so that the updatePaintNode()
will be called again.  This happens directly after the actual movement:

  QQuickTextPrivate::transformChanged  qquicktext.cpp        2990
  QQuickItemPrivate::transformChanged  qquickitem.cpp        5200
  QQuickItemPrivate::dirty             qquickitem.cpp        6339
  QQuickItem::setY                     qquickitem.cpp        6830
  QQuickFlickablePrivate::setViewportY qquickflickable.cpp   1800

Task-number: QTBUG-60491
Task-number: QTBUG-90734
Change-Id: I152d23caa725f194a186a604fbc8d0c07f597b16
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/quick/items/qquickitem_p.h b/src/quick/items/qquickitem_p.h
index f55c655723..0d35cfd48c 100644
--- a/src/quick/items/qquickitem_p.h
+++ b/src/quick/items/qquickitem_p.h
@@ -617,7 +617,7 @@ public:
     }
 
     QPointF computeTransformOrigin() const;
-    virtual void transformChanged();
+    virtual void transformChanged(QQuickItem *transformedItem);
 
     QPointF adjustedPosForTransform(const QPointF &centroid,
                                     const QPointF &startPos, const QVector2D &activeTranslatation,