Implement "Link Metrics..." custom parameters

Lib/glyphsLib/builder/glyph.py

@@ -123,7 +123,22 @@ def to_ufo_glyph(self, ufo_glyph, layer, glyph):  # noqa: C901
         ufo_glyph.lib[GLYPHLIB_PREFIX + "subCategory"] = subCategory
 
     # load width before background, which is loaded with lib data
-    width = layer.width
+
+    # The width may be taken from another master via the customParameters
+    # 'Link Metrics With Master' or 'Link Metrics With First Master'.
+    master = self.font.masters[layer.associatedMasterId or layer.layerId]
+    metric_source = master.metricsSource.id
+    metric_layer = self.font.glyphs[glyph.name].layers[metric_source]
+    if metric_layer:
+        width = metric_layer.width
+        if layer.width != width:
+            logger.debug(
+                f"{layer.parent.name}: Applying width from master "
+                f"'{metric_source}': {layer.width} -> {width}"
+            )
+    else:
+        width = None
+
     if width is None:
         pass
     elif category == "Mark" and subCategory == "Nonspacing" and width > 0:

Lib/glyphsLib/builder/kerning.py

@@ -21,8 +21,12 @@
 
 
 def to_ufo_kerning(self):
-    for master_id, kerning in self.font.kerning.items():
-        _to_ufo_kerning(self, self._sources[master_id].font, kerning)
+    for master in self.font.masters:
+        master_id = master.id
+        kerning_source = master.metricsSource.id  # Maybe be a linked master
+        if kerning_source in self.font.kerning:
+            kerning = self.font.kerning[kerning_source]
+            _to_ufo_kerning(self, self._sources[master_id].font, kerning)
 
 
 def _to_ufo_kerning(self, ufo, kerning_data):

Lib/glyphsLib/classes.py

@@ -1531,6 +1531,32 @@ def shouldWriteValueForKey(self, key):
             return self._name != self.name
         return super().shouldWriteValueForKey(key)
 
+    @property
+    def metricsSource(self):
+        """Returns the source master to be used for glyph and kerning metrics.
+
+        Normally this is the current master itself, but when linked metrics
+        custom parameters are being used, the master referred to in the custom
+        parameter is returned instead."""
+        if self.customParameters["Link Metrics With First Master"]:
+            return self.font.masters[0]
+        source_master_id = self.customParameters["Link Metrics With Master"]
+
+        # No custom parameters apply, go home
+        if not source_master_id:
+            return self
+
+        if source_master_id in self.font.masters:
+            return self.font.masters[source_master_id]
+
+        # Try by name
+        for source_master in self.font.masters:
+            if source_master.name == source_master_id:
+                return source_master
+
+        logger.warning(f"Source master for metrics not found: '{source_master_id}'")
+        return self
+
     @property
     def name(self):
         name = self.customParameters["Master Name"]
@@ -3269,7 +3295,7 @@ def bounds(self):
             return Rect(Point(left, bottom), Point(right - left, top - bottom))
 
     def _find_node_by_indices(self, point):
-        """"Find the GSNode that is refered to by the given indices.
+        """ "Find the GSNode that is refered to by the given indices.
 
         See GSNode::_indices()
         """