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>

6 files changed, 64 insertions, 9 deletions

diff --git a/src/quick/items/qquickflipable.cpp b/src/quick/items/qquickflipable.cpp
index a338985622..27fb02987c 100644
--- a/src/quick/items/qquickflipable.cpp
+++ b/src/quick/items/qquickflipable.cpp
@@ -70,7 +70,7 @@ class QQuickFlipablePrivate : public QQuickItemPrivate
 public:
     QQuickFlipablePrivate() : current(QQuickFlipable::Front), front(nullptr), back(nullptr), sideDirty(false) {}
 
-    void transformChanged() override;
+    void transformChanged(QQuickItem *transformedItem) override;
     void updateSide();
     void setBackTransform();
 
@@ -219,7 +219,7 @@ QQuickFlipable::Side QQuickFlipable::side() const
     return d->current;
 }
 
-void QQuickFlipablePrivate::transformChanged()
+void QQuickFlipablePrivate::transformChanged(QQuickItem *transformedItem)
 {
     Q_Q(QQuickFlipable);
 
@@ -228,7 +228,7 @@ void QQuickFlipablePrivate::transformChanged()
         q->polish();
     }
 
-    QQuickItemPrivate::transformChanged();
+    QQuickItemPrivate::transformChanged(transformedItem);
 }
 
 void QQuickFlipable::updatePolish()
diff --git a/src/quick/items/qquickitem.cpp b/src/quick/items/qquickitem.cpp
index 7c30746df3..0e5b33776e 100644
--- a/src/quick/items/qquickitem.cpp
+++ b/src/quick/items/qquickitem.cpp
@@ -5241,8 +5241,24 @@ QPointF QQuickItemPrivate::computeTransformOrigin() const
     }
 }
 
-void QQuickItemPrivate::transformChanged()
+/*!
+    \internal
+    QQuickItemPrivate::dirty() calls transformChanged(q) to inform this item and
+    all its children that its transform has changed, with \a transformedItem always
+    being the parent item that caused the change.  Override to react, e.g. to
+    call update() if the item needs to re-generate SG nodes based on visible extents.
+*/
+void QQuickItemPrivate::transformChanged(QQuickItem *transformedItem)
 {
+    Q_Q(QQuickItem);
+    if (q != transformedItem)
+        return;
+
+    // Inform the children in paint order: by the time we visit leaf items,
+    // they can see any consequences in their parents
+    for (auto child : paintOrderChildItems())
+        QQuickItemPrivate::get(child)->transformChanged(transformedItem);
+
 #if QT_CONFIG(quick_shadereffect)
     if (extra.isAllocated() && extra->layer)
         extra->layer->updateMatrix();
@@ -6380,7 +6396,7 @@ void QQuickItemPrivate::dirty(DirtyType type)
 {
     Q_Q(QQuickItem);
     if (type & (TransformOrigin | Transform | BasicTransform | Position | Size))
-        transformChanged();
+        transformChanged(q);
 
     if (!(dirtyAttributes & type) || (window && !prevDirtyItem)) {
         dirtyAttributes |= type;
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,
diff --git a/src/quick/items/qquicktext.cpp b/src/quick/items/qquicktext.cpp
index 0f17af1975..05b62a11f2 100644
--- a/src/quick/items/qquicktext.cpp
+++ b/src/quick/items/qquicktext.cpp
@@ -76,6 +76,12 @@ Q_DECLARE_LOGGING_CATEGORY(lcHoverTrace)
 
 const QChar QQuickTextPrivate::elideChar = QChar(0x2026);
 
+#if !defined(QQUICKTEXT_LARGETEXT_THRESHOLD)
+  #define QQUICKTEXT_LARGETEXT_THRESHOLD 10000
+#endif
+// if QString::size() > largeTextSizeThreshold, we render more often, but only visible lines
+const int QQuickTextPrivate::largeTextSizeThreshold = QQUICKTEXT_LARGETEXT_THRESHOLD;
+
 QQuickTextPrivate::QQuickTextPrivate()
     : fontInfo(font), elideLayout(nullptr), textLine(nullptr), lineWidth(0)
     , color(0xFF000000), linkColor(0xFF0000FF), styleColor(0xFF000000)
@@ -2979,6 +2985,18 @@ void QQuickText::hoverLeaveEvent(QHoverEvent *event)
     d->processHoverEvent(event);
 }
 
+void QQuickTextPrivate::transformChanged(QQuickItem *transformedItem)
+{
+    QQuickItemPrivate::transformChanged(transformedItem);
+
+    // If there's a lot of text, we may need QQuickText::updatePaintNode() to call
+    // QQuickTextNode::addTextLayout() again to populate a different range of lines
+    if (text.size() > largeTextSizeThreshold) {
+        updateType = UpdatePaintNode;
+        dirty(QQuickItemPrivate::Content);
+    }
+}
+
 /*!
     \qmlproperty int QtQuick::Text::renderTypeQuality
 
diff --git a/src/quick/items/qquicktext_p_p.h b/src/quick/items/qquicktext_p_p.h
index 7d8744671a..c34eca7824 100644
--- a/src/quick/items/qquicktext_p_p.h
+++ b/src/quick/items/qquicktext_p_p.h
@@ -89,6 +89,7 @@ public:
     void clearFormats();
 
     void processHoverEvent(QHoverEvent *event);
+    void transformChanged(QQuickItem *transformedItem) override;
 
     QRectF layedOutTextRect;
     QSizeF advance;
@@ -180,6 +181,7 @@ public:
     bool updateSizeRecursionGuard:1;
 
     static const QChar elideChar;
+    static const int largeTextSizeThreshold;
 
     qreal getImplicitWidth() const override;
     qreal getImplicitHeight() const override;
diff --git a/src/quick/items/qquicktextnode.cpp b/src/quick/items/qquicktextnode.cpp
index 2ee8d08fff..87cae92565 100644
--- a/src/quick/items/qquicktextnode.cpp
+++ b/src/quick/items/qquicktextnode.cpp
@@ -53,6 +53,7 @@
 #include <qabstracttextdocumentlayout.h>
 #include <qxmlstream.h>
 #include <private/qquickstyledtext_p.h>
+#include <private/qquicktext_p_p.h>
 #include <private/qfont_p.h>
 #include <private/qfontengine_p.h>
 
@@ -74,6 +75,8 @@ namespace {
 
 }
 
+Q_DECLARE_LOGGING_CATEGORY(lcSgText)
+
 /*!
   Creates an empty QQuickTextNode
 */
@@ -261,10 +264,18 @@ void QQuickTextNode::addTextLayout(const QPointF &position, QTextLayout *textLay
     QVarLengthArray<QTextLayout::FormatRange> colorChanges;
     engine.mergeFormats(textLayout, &colorChanges);
 
+    // If there's a lot of text, transform the window's bounds into the Text item's space
+    // and then insert only the range of lines that can possibly be visible within that viewport.
+    QRectF viewport;
+    if (textLayout->text().size() > QQuickTextPrivate::largeTextSizeThreshold && m_ownerElement->window()) {
+        viewport = m_ownerElement->mapRectFromScene(m_ownerElement->window()->contentItem()->boundingRect());
+        qCDebug(lcSgText) << "text viewport" << viewport;
+    }
     lineCount = lineCount >= 0
             ? qMin(lineStart + lineCount, textLayout->lineCount())
             : textLayout->lineCount();
 
+    bool inViewport = false;
     for (int i=lineStart; i<lineCount; ++i) {
         QTextLine line = textLayout->lineAt(i);
 
@@ -279,9 +290,17 @@ void QQuickTextNode::addTextLayout(const QPointF &position, QTextLayout *textLay
             end += preeditLength;
         }
 #endif
-
-        engine.setCurrentLine(line);
-        engine.addGlyphsForRanges(colorChanges, start, end, selectionStart, selectionEnd);
+        if (viewport.isNull() || (line.y() + line.height() > viewport.top() && line.y() < viewport.bottom())) {
+            if (!inViewport && !viewport.isNull())
+                qCDebug(lcSgText) << "first line in viewport" << i << "@" << line.y();
+            inViewport = true;
+            engine.setCurrentLine(line);
+            engine.addGlyphsForRanges(colorChanges, start, end, selectionStart, selectionEnd);
+        } else if (inViewport) {
+            Q_ASSERT(!viewport.isNull());
+            qCDebug(lcSgText) << "first omitted line past bottom of viewport" << i << "@" << line.y();
+            break; // went past the bottom of the viewport, so we're done
+        }
     }
 
     engine.addToSceneGraph(this, style, styleColor);