QTextDocument: add setLayoutEnabled()

This allows to set up everything first - without paying for layouting
at every step - and only then trigger layouting.

Same performance behavior as setTextWidth(0), but this is a more
explicit/readable API.

Change-Id: I044dbd8b1301b1c97a92f9a29ccde2baf27a7665
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/text/qtextdocument.cpp

@@ -634,6 +634,41 @@ 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 triggers a laying out of the
+    document, which makes everything work as expected, but takes time.
+
+    Temporarily disabling the layout process 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 in 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

@@ -294,7 +294,7 @@ public:
     // Only test the width for 0:
     // * setTextWidth(x) leads to height -1, which is valid
     // * the default page size of (-1, -1) means size determined from contents, this is valid too
-    bool canLayout() const { return !qIsNull(pageSize.width()); }
+    bool canLayout() const { return layoutEnabled && !qIsNull(pageSize.width()); }
 
 private:
     QTextDocumentPrivate(const QTextDocumentPrivate& m);
@@ -342,6 +342,7 @@ private:
 
 public:
     bool inContentsChange;
+    bool layoutEnabled = true;
     QTextOption defaultTextOption;
     Qt::CursorMoveStyle defaultCursorMoveStyle;
 #ifndef QT_NO_CSSPARSER

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

@@ -35,6 +35,7 @@ private slots:
     void translate();
     void createTextItem();
     void createTextItemZeroWidth();
+    void createTextItemNoLayouting();
 };
 
 tst_QGraphicsItem::tst_QGraphicsItem()
@@ -235,5 +236,22 @@ void tst_QGraphicsItem::createTextItemZeroWidth()
     }
 }
 
+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"