cached_video_thumbnail 0.1.0-dev.1

High-performance video thumbnail engine: cached, scheduled, cancellable thumbnail generation from asset, file, and network videos.

cached_video_thumbnail

A high-performance video thumbnail engine for Flutter. Generates thumbnails from bundle assets, local files, and network URLs through a cached, scheduled, cancellable pipeline built for one goal: in an infinite, fast-flinging feed on a low-end device, thumbnail generation is never the reason the UI janks.

Why another thumbnail package?

Existing packages give you a one-shot video -> image call. That is the easy 20%. The hard 80% is what happens when a feed asks for 40 thumbnails during a fling: unbounded native decoders, no caching, no deduplication, no cancellation, bytes shipped over the platform channel, and slow exact-frame seeks. This package is an engine, not a converter:

Capability	cached_video_thumbnail
Disk cache (deterministic keys, LRU, atomic writes)	yes
Cache hits without decoding (dims stored in the entry)	yes
Bounded native concurrency (default 2)	yes
Newest-first priority scheduling (visible > normal > prefetch)	yes
Identical requests coalesce into one extraction	yes
Cancellation (free pre-dispatch, native abort on iOS)	yes
Negative caching of recent failures	yes
Keyframe-fast seeks by default, exact frames opt-in	yes
Direct-to-file native encoding (no byte arrays over the channel)	yes
Typed errors, metrics, event hook	yes

Native side: Kotlin MediaMetadataRetriever with OPTION_CLOSEST_SYNC and getScaledFrameAtTime (decode-time downscaling); Swift AVAssetImageGenerator with infinite tolerances by default, a precomputed fit box, and per-request cancellation. No FFmpeg, no extra native dependencies.

Quickstart

import 'package:cached_video_thumbnail/cached_video_thumbnail.dart';

// The simplest path: drop the provider into any Image widget.
Image(
  image: VideoThumbnailImage(
    VideoSource.network(Uri.parse('https://cdn.example.com/v.mp4')),
    spec: const ThumbnailSpec(maxWidth: 480, maxHeight: 480),
  ),
  fit: BoxFit.cover,
)

Or drive the engine directly:

final engine = ThumbnailEngine.instance;

final thumb = await engine.getThumbnail(
  VideoSource.asset('assets/videos/intro.mp4'),
  spec: const ThumbnailSpec(maxWidth: 320, maxHeight: 320),
);
// thumb.filePath, thumb.width, thumb.height, thumb.wasCached

Sources

VideoSource.asset('assets/videos/intro.mp4', package: 'my_pkg'); // bundle asset
VideoSource.file('/path/to/video.mp4');                          // absolute path
VideoSource.network(url, headers: {'Authorization': 'Bearer t'}); // http(s)

• File sources embed mtime + size in the cache identity, so edited files self-invalidate.
• Network headers are passed to the platform media stack but are not part of the cache identity (tokens rotate; content does not). Call evict if a URL's content genuinely changed.

Specs

const ThumbnailSpec(
  maxWidth: 480,   // fit-within box, physical px; 0 = unconstrained
  maxHeight: 480,  // aspect preserved; frames are never upscaled
  position: Duration.zero,
  exact: false,    // false = nearest keyframe (fast); true = exact frame
  format: ThumbnailFormat.jpeg, // or png
  quality: 80,
);

Keep exact: false for feeds. Exact seeking can force the decoder to chew through every frame since the previous keyframe.

Requests, priorities, cancellation

final request = engine.thumbnail(
  source,
  spec: spec,
  priority: ThumbnailPriority.visible, // visible > normal > prefetch
  timeout: const Duration(seconds: 15), // counted from native dispatch
);
final thumb = await request.result;

request.cancel();                    // detach; queued work is dequeued free
request.bumpPriority(ThumbnailPriority.visible);

engine.prefetch(source, spec: spec); // fire-and-forget warmup

Within a band the newest request dispatches first: during a fling, the cells the user is looking at now beat the ones from two flings ago. Identical in-flight requests share one extraction; cancelling one joiner never affects the others.

VideoThumbnailImage wires cancellation automatically: when the last listener detaches before completion, the request is cancelled.

Configuration

await ThumbnailEngine.instance.configure(const ThumbnailEngineConfig(
  maxConcurrentExtractions: 2, // 1..8; use 1 for very low-end fleets
  defaultTimeout: Duration(seconds: 15),
  negativeCacheTtl: Duration(seconds: 60),
  maxCacheBytes: 256 << 20,
  maxCacheEntries: 4000,
  defaultSpec: ThumbnailSpec(maxWidth: 480, maxHeight: 480, quality: 80),
));

Cache sizing must be configured before the first request; everything else applies live.

Cache behavior

• Location: the app cache directory (<cache>/thumbnail/v1/). The OS may purge it; the engine re-extracts on demand.
• Hits are answered from an in-memory index: no platform channel, no disk stat, and no decoding (entry dimensions are stored in the file name).
• Writes are atomic (temp file + rename); crash leftovers are swept on init.
• LRU eviction targets 90% of the caps and runs off the request path.
• evict(source) removes every spec and content version of one source; clearCache() removes everything.

Errors

Every failure is a ThumbnailException with a ThumbnailErrorCode:

Code	Meaning
invalidSource	Structurally invalid input (bad URL, ...)
assetNotFound / fileNotFound	Source does not exist
network	Connectivity/HTTP failure
unsupportedMedia	No decodable video track / unsupported codec
extractionFailed	Decoder failed for another reason
encodingFailed	Frame could not be encoded
io	Local write/rename failure
timeout	Exceeded the request timeout
cancelled	Cancelled by the caller

Recent failures are negative-cached for negativeCacheTtl, so a broken URL fast-fails during scroll instead of re-hammering the network. Cancellations are never negative-cached.

Metrics

final m = ThumbnailEngine.instance.metrics;
// m.requests, m.cacheHits, m.coalescedJoins, m.extractions,
// m.extractP50/P95, m.queueWaitP50/P95, m.failures, m.queueDepth ...

ThumbnailEngine.instance.onEvent = (event) => log(event.toString());

The example app ships a benchmark page (cold vs warm runs, percentiles, a 200-cell scroll test) so you can measure on your own fleet.

Feed checklist

1. Use VideoThumbnailImage inside cells; give every cell an explicit ThumbnailSpec box matching its physical pixel size.
2. Prefetch ahead of the viewport with engine.prefetch (prefetch band never blocks visible work).
3. Keep maxConcurrentExtractions at 2 (or 1 for very low-end devices).
4. Remote videos: on-device extraction downloads enough of the file to decode a frame (cheap for faststart MP4s, worse for badly muxed files). If you control the backend, server-generated thumbnails are still the fastest option for users; this engine is for when you do not.

Platform notes

• Android: minSdk 24. Assets are read straight from the APK via file descriptors (a one-time copy fallback handles compressed assets). Cancellation of in-flight native work is not supported by MediaMetadataRetriever; the engine detaches the request and banks the finished frame in the cache instead. HTTP-error URLs (e.g. a 404) may classify as timeout rather than network on some devices: the platform retriever retries internally without surfacing the status code, so the engine's deadline is what ends the attempt (bounded and negative-cached either way).
• iOS: deployment target 13.0. Uses the async generator API on iOS 16+, with a continuation fallback below. Network headers use the de-facto AVURLAssetHTTPHeaderFieldsKey option.

Not in v1 (by design)

• content:// URIs (planned; cheap to add).
• WebP output (would drag a native dependency and its bug class along).
• Desktop/web backends and an FFmpeg fallback for exotic codecs: the extractor is a small Dart interface (ThumbnailExtractor), so backends can be added without touching the engine.

License

MIT (c) 2026 Omar Hanafy