Writing glyph overview filters

Save Glyph Overview filters as Python files in Counterpunch/Filters. A filter reads one glyph at a time and decides whether it belongs in the Overview result.

Every filter needs literal EVENT_TYPES and classify_glyph(glyph):

EVENT_TYPES = ["glyph.unicode.changed"]

def classify_glyph(glyph):
    return not glyph.codepoints

Return False to hide a glyph and True to show it without groups. Return a mapping with a non-empty groups list only when the filter needs categories. Each group always includes both fields: {"name": "Readable group name", "color": "CSS color"}. The group name is its visible label and identity. The app combines equal names across glyphs. If matching groups use different colors, the most recently classified glyph supplies the displayed color.

Use is_candidate(glyph) only for a cheap early rejection. The app calls it before classify_glyph(glyph).

EVENT_TYPES = ["glyph.anchors.changed"]

def is_candidate(glyph):
    return glyph.name.endswith(".sc")

def classify_glyph(glyph):
    if glyph.anchors:
        return False
    return {
        "groups": [
            {"name": "Unanchored small caps", "color": "orange"}
        ]
    }

is_candidate() does not need to predict the final answer. It only decides whether classify_glyph() is worth calling. A candidate may still return None. For a filter that finds glyphs without anchors, do not use a gate: both anchor-bearing and anchorless glyphs must be classified.

EVENT_TYPES lists only glyph-content changes that can change this filter's answer. The app reclassifies the affected glyph after a matching committed edit. Do not subscribe to glyph.created, glyph.deleted, glyph.renamed, or font.masters.changed. The app always manages those. It also emits glyph.compatibility.changed only when a glyph's compatibility actually changes, so compatibility filters should subscribe to that targeted event.

Do not use filter_glyphs, apply_changes, GROUPS, glyph_name, or cache management. The app owns complete scans, incremental updates, group cleanup, and result cache keys.

The app scans all glyphs when a filter first loads, reloads, or is explicitly refreshed. After that, it updates only changed glyphs. Filters run read-only; they must not edit the font. Keep both functions small. Expensive whole-font searches, I/O, and asynchronous work do not belong here.

When creating or editing a filter with Assistant, turn Assistant editing on, then let it draft into the Script Editor. After every buffer edit it should validate the document. The assistant does not run or save the filter file.

Overview usage is in Code-driven filters.