QTextDocument: add setLayoutEnabled()

This allows to set up everything first - without paying for the layout
calculation at every step - and only then trigger the layout once.

Results:
 0.065 msecs to create a QGraphicsTextItem with some text (layouted)
 0.036 msecs to set everything up in a QGraphicsTextItem with 0 width

Change-Id: I138bd1d58941d029bc0a36d2730216778f1fbd97
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/text/qtextdocument.cpp

@@ -634,6 +634,40 @@ bool QTextDocument::useDesignMetrics() const
     return d->defaultTextOption.useDesignMetrics();
 }
 
+/*!
+    \property QTextDocument::layoutEnabled
+    \since 6.4
+    \brief whether QTextDocument should recalculate the layout after every change
+
+    If this property is set to true, any change to the document triggers a layout,
+    which makes everything work as expected but takes time.
+
+    Temporarily disabling the layout can save time when making multiple changes
+    (not just text content, but also default font, default text option....)
+    so that the document is only laid out once at the end. This can be useful when
+    the text width or page size isn't yet known, for instance.
+
+    By default, this property is \c true.
+
+    \sa setTextWidth
+*/
+
+void QTextDocument::setLayoutEnabled(bool b)
+{
+    Q_D(QTextDocument);
+    if (d->layoutEnabled == b)
+        return;
+    d->layoutEnabled = b;
+    if (b && d->lout)
+        d->lout->documentChanged(0, 0, d->length());
+}
+
+bool QTextDocument::isLayoutEnabled() const
+{
+    Q_D(const QTextDocument);
+    return d->layoutEnabled;
+}
+
 /*!
     \since 4.2
 

src/gui/text/qtextdocument.h

@@ -62,6 +62,7 @@ class Q_GUI_EXPORT QTextDocument : public QObject
     Q_PROPERTY(QSizeF pageSize READ pageSize WRITE setPageSize)
     Q_PROPERTY(QFont defaultFont READ defaultFont WRITE setDefaultFont)
     Q_PROPERTY(bool useDesignMetrics READ useDesignMetrics WRITE setUseDesignMetrics)
+    Q_PROPERTY(bool layoutEnabled READ isLayoutEnabled WRITE setLayoutEnabled)
     Q_PROPERTY(QSizeF size READ size)
     Q_PROPERTY(qreal textWidth READ textWidth WRITE setTextWidth)
     Q_PROPERTY(int blockCount READ blockCount)
@@ -218,6 +219,9 @@ public:
     void setUseDesignMetrics(bool b);
     bool useDesignMetrics() const;
 
+    void setLayoutEnabled(bool b);
+    bool isLayoutEnabled() const;
+
     void drawContents(QPainter *painter, const QRectF &rect = QRectF());
 
     void setTextWidth(qreal width);

src/gui/text/qtextdocument_p.h

@@ -291,6 +291,8 @@ public:
         return get(object->document());
     }
 
+    bool canLayout() const { return layoutEnabled && !pageSize.isNull(); }
+
 private:
     QTextDocumentPrivate(const QTextDocumentPrivate& m);
     QTextDocumentPrivate& operator= (const QTextDocumentPrivate& m);
@@ -337,6 +339,7 @@ private:
 
 public:
     bool inContentsChange;
+    bool layoutEnabled = true;
     QTextOption defaultTextOption;
     Qt::CursorMoveStyle defaultCursorMoveStyle;
 #ifndef QT_NO_CSSPARSER

src/gui/text/qtextdocumentlayout.cpp

@@ -3754,7 +3754,7 @@ void QTextDocumentLayout::documentChanged(int from, int oldLength, int length)
      for (; blockIt.isValid() && blockIt != endIt; blockIt = blockIt.next())
          blockIt.clearLayout();
 
-    if (d->docPrivate->pageSize.isNull())
+    if (!d->docPrivate->canLayout())
         return;
 
     QRectF updateRect;
@@ -4032,7 +4032,7 @@ QRectF QTextDocumentLayout::tableCellBoundingRect(QTextTable *table, const QText
 QRectF QTextDocumentLayout::tableBoundingRect(QTextTable *table) const
 {
     Q_D(const QTextDocumentLayout);
-    if (d->docPrivate->pageSize.isNull())
+    if (!d->docPrivate->canLayout())
         return QRectF();
     d->ensureLayoutFinished();
 
@@ -4059,7 +4059,7 @@ QRectF QTextDocumentLayout::tableBoundingRect(QTextTable *table) const
 QRectF QTextDocumentLayout::frameBoundingRect(QTextFrame *frame) const
 {
     Q_D(const QTextDocumentLayout);
-    if (d->docPrivate->pageSize.isNull())
+    if (!d->docPrivate->canLayout())
         return QRectF();
     d->ensureLayoutFinished();
     return d->frameBoundingRectInternal(frame);
@@ -4088,7 +4088,7 @@ QRectF QTextDocumentLayoutPrivate::frameBoundingRectInternal(QTextFrame *frame)
 QRectF QTextDocumentLayout::blockBoundingRect(const QTextBlock &block) const
 {
     Q_D(const QTextDocumentLayout);
-    if (d->docPrivate->pageSize.isNull() || !block.isValid() || !block.isVisible())
+    if (!d->docPrivate->canLayout() || !block.isValid() || !block.isVisible())
         return QRectF();
     d->ensureLayoutedByPosition(block.position() + block.length());
     QTextFrame *frame = d->document->frameAt(block.position());

tests/auto/gui/text/qtextdocument/tst_qtextdocument.cpp

@@ -180,6 +180,8 @@ private slots:
     void insertHtmlWithComments_data();
     void insertHtmlWithComments();
 
+    void delayedLayout();
+
 private:
     void backgroundImage_checkExpectedHtml(const QTextDocument &doc);
     void buildRegExpData();
@@ -3931,5 +3933,24 @@ void tst_QTextDocument::insertHtmlWithComments()
     QCOMPARE(blockContent, expectedBlocks);
 }
 
+void tst_QTextDocument::delayedLayout()
+{
+    QTextDocument doc;
+    doc.setHtml("<html>Foobar</html>");
+    QCOMPARE(doc.blockCount(), 1);
+
+    doc.setLayoutEnabled(false);
+
+    // Force creation of a layout
+    QVERIFY(doc.documentLayout());
+
+    QTextBlock block = doc.begin();
+    QTextLayout *layout = block.layout();
+    QCOMPARE(layout->lineCount(), 0); // layout didn't happen yet
+
+    doc.setLayoutEnabled(true);
+    QCOMPARE(layout->lineCount(), 1); // layout happened
+}
+
 QTEST_MAIN(tst_QTextDocument)
 #include "tst_qtextdocument.moc"

tests/benchmarks/widgets/graphicsview/qgraphicsitem/tst_qgraphicsitem.cpp

@@ -5,6 +5,7 @@
 #include <QGraphicsItem>
 #include <QGraphicsScene>
 #include <QGraphicsView>
+#include <QTextDocument>
 
 class tst_QGraphicsItem : public QObject
 {
@@ -33,6 +34,7 @@ private slots:
     void shear();
     void translate();
     void createTextItem();
+    void createTextItemNoLayouting();
 };
 
 tst_QGraphicsItem::tst_QGraphicsItem()
@@ -216,5 +218,22 @@ void tst_QGraphicsItem::createTextItem()
     }
 }
 
+void tst_QGraphicsItem::createTextItemNoLayouting()
+{
+    // Ensure QFontDatabase loaded the font beforehand
+    QFontInfo(qApp->font()).family();
+    const QString text = "This is some text";
+    QBENCHMARK {
+        QGraphicsTextItem item;
+        item.document()->setLayoutEnabled(false);
+        // Prepare everything
+        item.setPlainText(text);
+        QTextOption option = item.document()->defaultTextOption();
+        option.setAlignment(Qt::AlignHCenter);
+        item.document()->setDefaultTextOption(option);
+        // And (in a real app) enable layouting here
+    }
+}
+
 QTEST_MAIN(tst_QGraphicsItem)
 #include "tst_qgraphicsitem.moc"