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, 22 insertions, 6 deletions

diff --git a/examples/quick/pointerhandlers/fakeFlickable.qml b/examples/quick/pointerhandlers/fakeFlickable.qml
index b6be729e3c..2234a7793a 100644
--- a/examples/quick/pointerhandlers/fakeFlickable.qml
+++ b/examples/quick/pointerhandlers/fakeFlickable.qml
@@ -120,15 +120,31 @@ Rectangle {
     }
 
     LeftDrawer {
-        width: 100
+        width: buttonRow.implicitWidth + 20
         anchors.verticalCenter: parent.verticalCenter
-        Slider {
-            id: slider
-            label: "font\nsize"
+        Column {
             anchors.fill: parent
             anchors.margins: 10
-            maximumValue: 36
-            value: 14
+            Slider {
+                id: slider
+                width: parent.width
+                height: parent.height - buttonRow.implicitHeight
+                label: "font\nsize"
+                maximumValue: 36
+                value: 14
+            }
+            Row {
+                id: buttonRow
+                spacing: 4
+                TapHandlerButton {
+                    text: "⭯"
+                    onClicked: ff.rotation -= 45
+                }
+                TapHandlerButton {
+                    text: "⭮"
+                    onClicked: ff.rotation += 45
+                }
+            }
         }
     }
 }