takeTags

The tags that the bytes of the last read brought, such as the song an internet radio station names in a title block between its audio bytes, or null, the default, when that read brought none (#168).

It is asked after every read that returned bytes, on the same thread, and only of the source given to MediaSource.open. A source a MediaByteOpener returns is never asked, because FFmpeg reads no tags from one: the HLS reader reads each segment through an input of its own. On a page's main thread, where the web backend reads a source whole during the open, it is asked after each of those reads, and each answer reaches FFmpeg when FFmpeg reads the byte it belongs at, every time FFmpeg reads that byte.

The tags belong at the first byte of the read that reported them, and FFmpeg merges them into the container's tags, exactly as it does with the titles its own http reads from a station. They reach a caller as Packet.newContainerTags on the first packet a reader hands out after FFmpeg read past that byte, and in MediaSource.metadata from then on. Tags reported during the open's own reads are in MediaSource.metadata when the open returns. A source that stops each read at the byte where its next tags belong, as FFmpeg's http stops at every title block, places them on the packet that holds the bytes after them, and one that reads past that byte places them early by what it read past.

Report each change once. Keys go through as given: FFmpeg's http names a station's fields StreamTitle and StreamUrl, and a source that wants a song to read as the title reports title. A key or a value ends at its first NUL character, as the C string it crosses as does, and a pair whose key is then empty is ignored. Finding the titles is the source's work, since asking a station for them and finding its blocks needs the HTTP headers: send Icy-MetaData: 1, read the block interval from icy-metaint, and take every block out of the bytes before read hands them over. An exception thrown here fails the read that came before it, as one thrown by read does.