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Add sendQueueSize and ondequeue to MIDIOutput for backpressure #287

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Add sendQueueSize and ondequeue to MIDIOutput for backpressure #287

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43 changes: 42 additions & 1 deletion index.html
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Original file line number Diff line number Diff line change
Expand Up @@ -1029,11 +1029,41 @@ <h3 id="MIDIOutput">
</h3>
<pre class="idl">
[SecureContext, Exposed=(Window,Worker)] interface MIDIOutput : MIDIPort {
readonly attribute unsigned long sendQueueSize;
attribute EventHandler ondequeue;
undefined send(sequence&lt;octet&gt; data, optional DOMHighResTimeStamp timestamp = 0);
undefined clear();
};
</pre>
<dl>
<dt>
<dfn>sendQueueSize</dfn>
</dt>
<dd>
<p>
The number of bytes remaining in the {{MIDIOutput}}'s queue
waiting to be sent by the underlying system. While the
underlying system is saturated (i.e. active send requests have
reached an implementation-specific maximum and the system is
temporarily not able to accept more work), additional calls to
{{MIDIOutput/send()}} are buffered in the queue and increase
{{MIDIOutput/sendQueueSize}} by the number of bytes in
<code>data</code>. Once the underlying system makes sufficient
progress on the current workload, it becomes unsaturated and
processes queued data, decreasing {{MIDIOutput/sendQueueSize}}
by the number of bytes sent.
</p>
</dd>
<dt>
<dfn>ondequeue</dfn>
</dt>
<dd>
<p>
This [=event handler=], of type "dequeue", MUST be supported by
all objects implementing the {{MIDIOutput}} interface. It is
called to signal a decrease in {{MIDIOutput/sendQueueSize}}.
</p>
</dd>
<dt>
<dfn>send</dfn>
</dt>
Expand Down Expand Up @@ -1074,6 +1104,14 @@ <h3 id="MIDIOutput">
"MIDIPortConnectionState.closed">"closed"</a>, asynchronously
try to [=open the port=].
</p>
<p>
Increase {{MIDIOutput/sendQueueSize}} by the number of bytes in
<var>data</var> when the message is enqueued. When the
underlying system finishes sending the enqueued <var>data</var>
to the port, decrease {{MIDIOutput/sendQueueSize}} by the number
of bytes in <var>data</var> and [=fire an event=] named
"dequeue" at the {{MIDIOutput}}.
</p>
<dl class='parameters'>
<dt>
sequence&lt;octet&gt; data
Expand Down Expand Up @@ -1106,7 +1144,10 @@ <h3 id="MIDIOutput">
the {{MIDIOutput}}'s queue. The implementation will
need to ensure the MIDI stream is left in a good state, so if
the output port is in the middle of a sysex message, a sysex
termination byte (0xf7) should be sent.
termination byte (0xf7) should be sent. If
{{MIDIOutput/sendQueueSize}} is greater than 0, set
{{MIDIOutput/sendQueueSize}} to 0 and [=fire an event=] named
"dequeue" at the {{MIDIOutput}}.
</p>
</dd>
</dl>
Expand Down
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