Add method to QgsTextRenderer to retreive scaled QFontMetricsF

from a text format in a specified render context

python/core/qgstextrenderer.sip.in

@@ -1627,6 +1627,15 @@ with the text or background parts)
 :param drawAsOutlines: set to false to render text as text. This allows outputs to
 formats like SVG to maintain text as text objects, but at the cost of degraded
 rendering and may result in side effects like misaligned text buffers.
+%End
+
+    static QFontMetricsF fontMetrics( const QgsRenderContext &context, const QgsTextFormat &format );
+%Docstring
+Returns the font metrics for the given text ``format``, when rendered
+in the specified render ``context``. The font metrics will take into account
+all scaling required by the render context.
+
+.. versionadded:: 3.2
 %End
 
     static double textWidth( const QgsRenderContext &context, const QgsTextFormat &format, const QStringList &textLines,

src/core/qgstextrenderer.cpp

@@ -1873,6 +1873,11 @@ void QgsTextRenderer::drawPart( QPointF origin, double rotation, QgsTextRenderer
   }
 }
 
+QFontMetricsF QgsTextRenderer::fontMetrics( const QgsRenderContext &context, const QgsTextFormat &format )
+{
+  return QFontMetricsF( format.scaledFont( context ) );
+}
+
 void QgsTextRenderer::drawBuffer( QgsRenderContext &context, const QgsTextRenderer::Component &component, const QgsTextFormat &format )
 {
   QPainter *p = context.painter();

src/core/qgstextrenderer.h

@@ -1402,6 +1402,14 @@ class CORE_EXPORT QgsTextRenderer
                           QgsRenderContext &context, const QgsTextFormat &format,
                           TextPart part, bool drawAsOutlines = true );
 
+    /**
+     * Returns the font metrics for the given text \a format, when rendered
+     * in the specified render \a context. The font metrics will take into account
+     * all scaling required by the render context.
+     * \since QGIS 3.2
+     */
+    static QFontMetricsF fontMetrics( const QgsRenderContext &context, const QgsTextFormat &format );
+
     /**
      * Returns the width of a text based on a given format.
      * \param context render context

tests/src/python/test_qgstextrenderer.py

@@ -29,7 +29,7 @@
                        QgsRectangle,
                        QgsRenderChecker,
                        QgsBlurEffect)
-from qgis.PyQt.QtGui import (QColor, QPainter, QFont, QImage, QBrush, QPen)
+from qgis.PyQt.QtGui import (QColor, QPainter, QFont, QImage, QBrush, QPen, QFontMetricsF)
 from qgis.PyQt.QtCore import (Qt, QSizeF, QPointF, QRectF, QDir)
 from qgis.PyQt.QtXml import QDomDocument
 from qgis.testing import unittest, start_app
@@ -436,6 +436,31 @@ def testToQFont(self):
         qfont = s.toQFont()
         self.assertAlmostEqual(qfont.pointSizeF(), 360.0, 2)
 
+    def testFontMetrics(self):
+        """
+        Test calculating font metrics from scaled text formats
+        """
+        s = QgsTextFormat()
+        f = getTestFont()
+        s.setFont(f)
+        s.setSize(12)
+        s.setSizeUnit(QgsUnitTypes.RenderPoints)
+
+        string = 'xxxxxxxxxxxxxxxxxxxxxx'
+
+        # calculated expected width
+        f = s.toQFont()
+        expected = QFontMetricsF(f).width(string)
+        scale = expected / 416.625
+
+        context = QgsRenderContext()
+        context.setScaleFactor(1)
+        metrics = QgsTextRenderer.fontMetrics(context, s)
+        self.assertAlmostEqual(metrics.width(string), 51.9 * scale, -1)
+        context.setScaleFactor(2)
+        metrics = QgsTextRenderer.fontMetrics(context, s)
+        self.assertAlmostEqual(metrics.width(string), 104.15 * scale, -1)
+
     def imageCheck(self, name, reference_image, image):
         self.report += "<h2>Render {}</h2>\n".format(name)
         temp_dir = QDir.tempPath() + '/'