[video_player] Seek to live edge

[mx1up]

This PR adds the seekToDefaultPosition method:

fixes flutter/flutter#168097

Before adding tests/changelog/etc, I'd like to know if you agree with this change in general 😉

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[stuartmorgan-g]

Thanks for the contribution!

Please see https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#platform-support. This is exposing new API at the app-facing package layer, but only implementing it on Android, and there's no discussion of what this would look like on other platforms; it would not be landed as-is.

Have you looked at iOS and web to see what the equivalent functionality is?

[mx1up]

Hi @stuartmorgan-g , thanks for the feedback. I have no web or ios programming experience but I gave it a shot. Here are my remarks:

darwin

It seems the current play/pause behavior on macos already is different from android. On Android, playing a live stream automatically jumps to the live edge. When you pause, wait 5 secs, and resume, the video resumes where it left off. On macOS, starting the player on the same url at the same time, I get a whole different picture (and point in time).

My current hypothesis:
when starting to play a livestream on macos it does not automatically seek to the live edge. Instead, it starts playing at the oldest timestamp available. Hence, when you pause the video, wait 5 secs, and resume playing, the player makes a jump in time (5 secs) instead of resuming from where it was paused.
Using the new seekToDefaultPosition functionality, I have the impression it jumps correctly to the live edge. Unfortunately, when pausing and resuming again, it jumps back to the start of the buffer (oldest timestamp).

web

I did not find a sample stream that would decode on web. I kept getting "unsupported format". So I was not able to test the implementation.

conclusion

Unless somebody experienced can jump in and help me out, I think I will have to reduce scope and only add this functionality to android package. If I leave out the changes to the app facing package interface and only add the functionality to video_player_android package, would that get accepted?

BTW, i extended the example to include a HLS live stream.