ArtPlayer/packages/artplayer-plugin-vtt-thumbnail/ARCHITECTURE.md

VTT thumbnail maintenance

The factory remains artplayerPluginVttThumbnail(option). Its registration function is asynchronous and resolves to { name: 'artplayerPluginVttThumbnail' }. Existing public declarations still describe a synchronous result; task PKG-VTT-THUMB-04 owns that conflict and the public declaration/consumer migration. The seven internal TS files are strict. Do not infer the return shape from those declarations.

Modules and ownership

• src/index.ts composes registration, captures the VTT URL before awaiting, reads style afterwards, and installs the vtt-thumbnail control with its existing CSS class/index.
• src/lifetime.ts owns one registration's destroy listener, event listeners and cleanup callbacks. Close is idempotent. Cleanup continues after a callback fails. An independent cancellation Promise settles pending work even if fetch does not honor AbortSignal.
• src/request.ts owns a native AbortController when available and reads the response body. Destroy cancels both phases; late rejections are observed. Failed HTTP responses reject before parsing. The controller cleanup is released when the request ends.
• src/parseVtt.ts parses thumbnail metadata and exports the pure findThumbnail lookup. It preserves timestamp flooring, first inclusive matches, decimal xywh strings and lexical URL joining. It handles cue IDs, NOTE/STYLE/REGION blocks, BOM/line endings and timing settings; invalid input raises TypeError with the original line before any UI is installed.
• src/preview.ts owns the mobile 500ms timer and creates the setBar callback. Timer generations reject old callbacks. Every style write checks lifetime state, including callbacks captured before destroy. Geometry, mobile/desktop edges and cue selection retain the latest published behavior.
• src/getVttArray.ts retains the old source helper signature as a fetch/parse wrapper. The player registration uses the owned request module instead. Distribution/deep-import review belongs to PKG-VTT-THUMB-06.

The registration owns only the control element passed to its mounted callback. Before removing it, cleanup compares the live art.controls['vtt-thumbnail'] alias with that exact element. It must not remove a later replacement. Partial listener/control installation is rolled back if initialization throws, preserving the original rejection.

Runtime types

src/types.ts defines options, result, rectangle/cue data, owned events, cleanup and preview inputs. Request cancellation is typed as a void branch; actual registration is Promise. The lifetime accepts only destroy/setBar and uses the core event tuples. The preview receives only its required DOM/style/duration inputs.

Assertions are limited to the real Artplayer constructor boundary, validated regex groups and loop indexes, the completed four-key rectangle, and native responses after the closed check. The latter is valid because native fetch returns Response and cancellation only resolves void after closing; malformed custom responses still throw at runtime. Numeric rectangle strings are explicitly converted for arithmetic while the zero style write stays numeric. No any, unchecked JS or skipLibCheck exception is introduced.

The package tsconfig also checks test/types/vtt-thumbnail-runtime.ts. It tests the async result, required options, CSS/URL types, event tuples, cancellation, readonly state and raw rectangle strings, including expected errors. It separately proves the old public result is synchronous in declarations; those declarations are not silently corrected in this runtime checkpoint. New/old declaration consumers and export aliases remain task 04.

Compatibility and intentional fixes

Option/style remain live at their historical read points. The return stays a Promise; destroy during loading now settles it with the normal name result, without installing UI. Existing core plugin managers can complete their own destroyed-instance handling. This does not introduce a public destroy method or a new registration result field.

Registration still downloads VTT once. A video restart does not reload it or reinterpret later option.vtt changes. Empty cues remain valid and hide the preview. HTTP failures now reject explicitly; before this fix, a 404 with a parseable body incorrectly displayed cues. The historical failure tests retain the former outcome rather than claiming it was valid API.

This is a thumbnail metadata profile, not a complete subtitle renderer or WebVTT validator. Caption style/region blocks and timing settings do not style the player's DOM. Preserve legacy compact arrows, seconds-only timestamps, numeric overflow between minute/second fields, zero-length cues, unsorted/overlapping cues and dense pairs. Lookup does not sort: the first inclusive match in input order wins. Fractions still floor only after conversion. Three-or-more-digit hours now use the entire field rather than silently truncating it.

Malformed headers/timing, reversed ranges and missing/duplicate/unknown rectangle keys reject. Coordinates must be finite non-negative decimal strings, and width/height positive; coordinate whitespace is trimmed before forming CSS. Broken input no longer creates invalid CSS or partially mounted controls. Each cue has one image/rectangle payload line; multiline subtitle text, timestamp-map offsets and full caption semantics are outside this plugin. Empty or comment-only input remains a valid empty preview.

The old 1.0.x export objects, earlier thumbnails control name and incorrect declarations are separately frozen and remain compatibility work for tasks 04-06. Current runtime work does not claim those consumer migrations or the full package refactor are finished.

Verification and future changes

Use Yarn 1.22.22 and the repository's pinned Node version:

yarn test:vtt-thumbnail
yarn test:browser test/browser/vtt-thumbnail-lifecycle.spec.js
yarn build artplayer-plugin-vtt-thumbnail

The unit helper can load actual main/legacy files with ARTPLAYER_VTT_THUMBNAIL_ARTIFACT or the frozen former implementation with ARTPLAYER_VTT_THUMBNAIL_BASELINE=1. Historical tests are distinct from candidate expectations. Browser tests use real published/candidate cores, native fetch/abort, mouse hover, sprite decoding and cleanup. Their scope is not full old core/device coverage, installed tarballs, mobile Safari or the online editor.

Modify parsing in parseVtt, requests in request, layout/timers in preview, and ownership in lifetime. Preserve the normal/failure lifecycle tests alongside any parser improvements. Pure parser comparison can use ARTPLAYER_VTT_PARSER_BASELINE=1, which loads the exact parser from the 524ddf78 resource checkpoint. Bundle-mode parser tests also run malformed payloads and extended registration through the actual factory; pure parsing assertions still target source and must not be described as tests of an installed tarball. See ../../refactor/baselines/vtt-thumbnail-contract.md and the task 03 change record for historical evidence and outstanding acceptance work.