Fire CanPlayThrough immediately for local and streaming media files

media/base/download_rate_monitor.cc

Index: media/base/download_rate_monitor.cc
diff --git a/media/base/download_rate_monitor.cc b/media/base/download_rate_monitor.cc
index 88916e4b57a4b3f27f69c71a57b756e85d7869c6..b561fa59b62d21a89a0db7ac93ef3642dfbadcd8 100644
--- a/media/base/download_rate_monitor.cc
+++ b/media/base/download_rate_monitor.cc
@@ -142,6 +142,8 @@ void DownloadRateMonitor::Reset() {
   loaded_ = false;
   bitrate_ = 0;
   stopped_ = true;
+  media_stream_ = false;
+  streaming_ = false;
 }
 
 DownloadRateMonitor::~DownloadRateMonitor() { }
@@ -198,14 +200,25 @@ bool DownloadRateMonitor::ShouldNotifyCanPlayThrough() {
   if (has_notified_can_play_through_)
     return false;
 
-  // If the media is from a local file (|loaded_|) or if all bytes are
-  // buffered, fire CanPlayThrough.
-  if (loaded_ || buffered_bytes_ == total_bytes_)
+  // Fire CanPlayThrough immediately if the source is loaded or streaming.
+  //
+  // NOTE: It is a requirement for CanPlayThrough to fire immediately if the
+  // source is |loaded_|, but the choice to fire the event for any streaming
+  // element is an optimistic design decision that may need to be tweaked. The
+  // only streaming element that *must* fire CanPlayThrough immediately is a
+  // media_stream_ whose source is local, such as from a webcam.
+  if (loaded_ || streaming_ || media_stream_)

[wjia(left Chromium), 2011/11/22 23:01:17]

should it be: if (loaded_ || !streaming_ || media_stream_) ?

For streaming except media_stream, download control is needed, right?

[vrk (LEFT CHROMIUM), 2011/11/23 21:29:45]

So to be clear, the DownloadRateMonitor does not do any download "controlling."
Its purpose is to decide when CanPlayThrough should be fired. To make this
decision, it needs a variety of input, such as some information about the media
itself (loaded_, total_bytes_, etc) and it does some estimation of the rate of
download by watching calls to SetBufferedBytes.

With that: For streaming outside of media_stream, I intentionally changed the
code to fire CanPlayThrough immediately as a design decision. With a stream,
you're either going to have a fast enough connection to keep up, or you won't.
Instead of trying to estimate whether you can keep up (which can be inaccurate
and has time overhead), might as well just fire the event and try playback. (As
I mention in the comment, this may need tweaking.)

+    return true;
+
+  // If all bytes are buffered, fire CanPlayThrough.
+  if (buffered_bytes_ == total_bytes_)
     return true;
 
-  // Cannot approximate when the media can play through if bitrate is unknown.
+  // If bitrate is unknown, optimistically fire CanPlayThrough immediately.
+  // This is so a video with an unknown bitrate with the "autoplay" attribute
+  // will not wait until the entire file is downloaded before playback begins.
   if (bitrate_ <= 0)
-    return false;
+    return true;
 
   float bytes_needed_per_second = bitrate_ / 8;
   float download_rate = ApproximateDownloadByteRate();