API for exposing download progress

[perlatus]

A feature I need is exposing download progress to applications. While for smaller files this isn't an issue, my use-case includes absurdly large images, animated GIFs, and videos. I'd like to be able to show download progress in my UI without sidestepping the cache, since not having to download a big file twice is kinda the whole point.

What should the API look like? It should be accessible from flutter_cache_manager as well as flutter_cached_network_image.

I'm thinking usage in applications could look like:

double _progress;

@override
Widget build(BuildContext ctx) {
    return new CachedNetworkImage(
        imageUrl: post.sampleUrl,
        errorWidget: const Icon(Icons.error),
        placeholder: new CircularProgressIndicator(value: _progress),
        onCumulativeBytesReceived: (int bytes) {
            setState(() {
                _progress = bytes / post.fileSize;
            });
        },
    );
}

AFAICT with a StreamedResponse you aren't guaranteed to know the size of the file, so returning a percent complete in the callback isn't always possible. CacheManager.getFile could have a similar callback added to it that gets linked up with the CachedNetworkImage.

I've started changing flutter_cache_manager so it'll be possible to count the bytes as they come (StreamedResponse and await for).

[perlatus]

Actually while this would still be nice to have, I think an extension of getFileIfCached could be more immediately useful so I can use Android's DownloadManager API for showing download progress. Will open issue/PR for that once it's ready