captions fetcher architecture.
1. ManifestlessCaptionsMetadata
Other captions fetchers must first fetch a manifest (HLS or manifest) to
discover captions tracks. This process does not exist for manifestless. All
we need to do is scan the FormatStream's for the right itag, so this is an
all-static class.
2. ManifestlessSubtitleWindowProvider
Once a captions track is selected, a subtitles provider is instantiated. This
is the main interface used by the player to retrieve captions according to
playback position. This class stores fetched captions in a tree index by time
for efficient lookups. Background captions fetches are used to populate
the tree.
3. ManifestlessCaptionsFetch
Captions are fetched one segment at a time. One instance of this object
is required per fetch. It performs a blocking fetch on call(), and is
intended to be submitted to a background-thread executor.
4. ManifestlessCaptionsFetch.CaptionSegment
This is the result of the caption fetch. These values are used to populate
the captions tree.
Manifestlessness
The initial request is always a headm request. There is a separate tree
of every segment indexed by start time. This tree is used to improve
manifestless sequence number calculation. Once we have data for the current
timestamp, we walk forward through the tree to find the next unfetched
sequence number, and fetch that.
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Created by MOE: https://github.com/google/moe
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