[pango/cache-context-metrics] context: Cache metrics for the current font

[Matthias Clasen <matthiasc src gnome org>]

commit 2214326a9669af950593da08c98505c62e27974a
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Mar 18 00:09:58 2021 -0400

    context: Cache metrics for the current font
    
    GTK calls pango_context_get_metrics() frequently to
    determine character widths and alignments, and does not
    expect this to involve expensive layout operations.

 pango/pango-context.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
---
diff --git a/pango/pango-context.c b/pango/pango-context.c
index 1db11a1c..ccaa1bac 100644
--- a/pango/pango-context.c
+++ b/pango/pango-context.c
@@ -63,6 +63,8 @@ struct _PangoContext
 
   PangoFontMap *font_map;
 
+  PangoFontMetrics *metrics;
+
   gboolean round_glyph_positions;
 };
 
@@ -121,6 +123,9 @@ pango_context_finalize (GObject *object)
   if (context->matrix)
     pango_matrix_free (context->matrix);
 
+  if (context->metrics)
+    pango_font_metrics_unref (context->metrics);
+
   G_OBJECT_CLASS (pango_context_parent_class)->finalize (object);
 }
 
@@ -1783,6 +1788,11 @@ pango_context_get_metrics (PangoContext               *context,
   if (!language)
     language = context->language;
 
+  if (desc == context->font_desc &&
+      language == context->language &&
+      context->metrics != NULL)
+    return pango_font_metrics_ref (context->metrics);
+
   current_fonts = pango_font_map_load_fontset (context->font_map, context, desc, language);
   metrics = get_base_metrics (current_fonts);
 
@@ -1797,6 +1807,10 @@ pango_context_get_metrics (PangoContext               *context,
 
   g_object_unref (current_fonts);
 
+  if (desc == context->font_desc &&
+      language == context->language)
+    context->metrics = pango_font_metrics_ref (metrics);
+
   return metrics;
 }
 
@@ -1806,6 +1820,8 @@ context_changed (PangoContext *context)
   context->serial++;
   if (context->serial == 0)
     context->serial++;
+
+  g_clear_pointer (&context->metrics, pango_font_metrics_unref);
 }
 
 /**