Ufo::Layer

A single layer in a UFO source. A Layer holds a set of glyphs keyed by name. The default layer is public.default per UFO 3.

layer = Fontisan::Ufo::Layer.new                       # => "public.default"
layer.add(Fontisan::Ufo::Glyph.new(name: "A"))
layer["A"]                                             # => the glyph
layer.size                                             # => 1

Naming contract

Glyph names are the layer's primary key. Two distinct glyphs with the same name cannot coexist — adding the second would silently destroy the first, a class of bug that has historically cost real data (e.g. the CJK Ext G cmap loss when CBDT placeholders collided with outline glyphs sharing "gid{N}" names).

To make that contract unbreakable, the API exposes two methods with named semantics:

Method	Behavior on conflict
Layer#add(glyph)	Raises Ufo::GlyphExistsError (the safe default)
Layer#put(glyph)	Overwrites the existing glyph (explicit replace)

Callers that need auto-renaming — insert without giving up on a collision — call Stitcher::UniqueGlyphName.in(target, base) first to allocate a free name, then Layer#add the glyph under that name.

API

Layer.new(name = "public.default")

Initialize an empty layer.

#add(glyph) -> Glyph

Insert glyph. Raises Ufo::GlyphExistsError if a glyph with the same name is already present.

layer = Fontisan::Ufo::Layer.new
layer.add(Fontisan::Ufo::Glyph.new(name: "A"))

layer.add(Fontisan::Ufo::Glyph.new(name: "A"))
# => raises Fontisan::Ufo::GlyphExistsError: a glyph named "A" already exists

The original glyph is preserved when the second add raises — the layer is never partially mutated.

#put(glyph) -> Glyph

Insert glyph, replacing any existing glyph with the same name. Use this when the caller has positively decided the previous glyph (if any) should be discarded.

layer.put(Fontisan::Ufo::Glyph.new(name: "A"))  # overwrites the prior "A"

#[](glyph_name) -> Glyph, nil

Look up a glyph by name.

#each { |glyph| ... }

Iterate over glyphs (no ordering guarantee).

#size -> Integer

Number of glyphs in the layer.

Ufo::GlyphExistsError

Raised by Layer#add on a name conflict. Carries the colliding name so callers can branch programmatically:

begin
  layer.add(glyph)
rescue Fontisan::Ufo::GlyphExistsError => e
  puts "name #{e.name.inspect} already taken"
  # Decide: layer.put(glyph) to overwrite, or
  # unique = Fontisan::Stitcher::UniqueGlyphName.in(target, e.name)
  # layer.add(glyph.dup_as(name: unique))
end

The error class is a sibling of Layer (not nested inside it) so the namespace stays flat — both Ufo::Layer and Ufo::GlyphExistsError live directly under Fontisan::Ufo.

See also

• docs/STITCHER_GUIDE.adoc — UniqueGlyphName, GlyphCopier, and CbdtPropagator all build on the Layer naming contract.
• docs/UFO_COMPILATION.adoc — how layers feed the TTF/OTF/CFF2 compilers.