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<!DOCTYPE html>
<html lang="en">
<head>
<title>
Web MIDI API
</title>
<meta charset="utf-8">
<script src='https://www.w3.org/Tools/respec/respec-w3c' class=
'remove'></script>
<script class='remove'>
var respecConfig = {
shortName: "webmidi",
specStatus: "ED",
copyrightStart: "2015",
editors: [
{ name: "Chris Wilson",
url: "mailto:[email protected]",
w3cid: "3742",
company: "Google",
companyURL: "http://google.com"
},
{ name: "Michael Wilson",
url: "mailto:[email protected]",
w3cid: "136815",
company: "Google",
companyURL: "http://google.com"
},
],
formerEditors: [
{ name: "Jussi Kalliokoski",
url: "http://juss.in",
w3cid: "45545"
},
],
github: "WebAudio/web-midi-api",
group: "wg/audio",
otherLinks: [
{
key: "Implementation report:",
data: [
{
value: "No preliminary interoperability or implementation report exists.",
},
],
},
],
previousPublishDate: "2013-11-26",
previousMaturity: "WD",
testSuiteURI: "https://github.com/web-platform-tests/wpt/tree/master/webmidi",
xref: {
profile: "web-platform",
specs: ["hr-time", "permissions", "permissions-policy"],
}
};
</script>
</head>
<body>
<section id="abstract">
<p>
Some user agents have music devices, such as synthesizers, keyboard and
other controllers, and drum machines connected to their host computer
or device. The widely adopted Musical Instrument Digital Interface
(MIDI) protocol enables electronic musical instruments, controllers and
computers to communicate and synchronize with each other. MIDI does not
transmit audio signals: instead, it sends event messages about musical
notes, controller signals for parameters such as volume, vibrato and
panning, cues and clock signals to set the tempo, and system-specific
MIDI communications (e.g. to remotely store synthesizer-specific patch
data). This same protocol has become a standard for non-musical uses,
such as show control, lighting and special effects control.
</p>
<p>
This specification defines an API supporting the MIDI protocol, enabling
web applications to enumerate and select MIDI input and output devices
on the client system and send and receive [=MIDI messages=]. It is
intended to enable non-music MIDI applications as well as music ones, by
providing low-level access to the [=MIDI devices=] available on the
users' systems. The Web MIDI API is not intended to describe music or
controller inputs semantically; it is designed to expose the mechanics
of MIDI input and output interfaces, and the practical aspects of
sending and receiving [=MIDI messages=], without identifying what those
actions might mean semantically (e.g., in terms of "modulate the vibrato
by 20Hz" or "play a G#7 chord", other than in terms of changing a
controller value or sending a set of note-on messages that happen to
represent a G#7 chord).
</p>
<p>
To some users, "MIDI" has become synonymous with Standard MIDI Files
and General MIDI. That is not the intent of this API; the use case of
simply playing back a .SMF file is not within the purview of this
specification (it could be considered a different format to be
supported by the HTML [^audio^] element, for example). The Web MIDI API
is intended to enable direct access to devices that respond to MIDI -
controllers, external synthesizers or lighting systems, for example.
The Web MIDI API is also explicitly designed to enable a new class of
applications on the web that can respond to MIDI controller inputs -
using external hardware controllers with physical buttons, knobs and
sliders (as well as musical controllers like keyboard, guitar or wind
instrument controllers) to control web applications.
</p>
<p>
The Web MIDI API is also expected to be used in conjunction with other
APIs and elements of the web platform, notably the [=Web Audio
API=]. This API is also intended to be familiar to users of MIDI APIs on
other systems, such as Apple's CoreMIDI and Microsoft's Windows MIDI
API.
</p>
</section>
<section id="sotd"></section>
<section class="informative">
<h2>
Introduction
</h2>
<p>
The Web MIDI API specification defines a means for web developers to
enumerate, manipulate and access [=MIDI devices=] - for example,
interfaces that may provide hardware MIDI ports with other devices
plugged in to them and USB devices that support the USB-MIDI
specification. Having a Web API for MIDI enables web applications that
use existing software and hardware synthesizers, hardware music
controllers and light systems and other mechanical apparatus controlled
by MIDI. This API has been defined with this wide variety of use cases
in mind.
</p>
<p>
The approaches taken by this API are similar to those taken in Apple's
CoreMIDI API and Microsoft's Windows MIDI API; that is, the API is
designed to represent the low-level software protocol of MIDI, in order
to enable developers to build powerful MIDI software on top. The API
enables the developer to enumerate input and output interfaces, and send
and receive [=MIDI messages=], but (similar to the aforementioned APIs)
it does not attempt to semantically define or interpret [=MIDI
messages=] beyond what is necessary to robustly support current devices.
</p>
<p>
The Web MIDI API is not intended to directly implement high-level
concepts such as sequencing; it does not directly support Standard MIDI
Files, for example, although a Standard MIDI File player can be built
on top of the Web MIDI API. It is also not intended to semantically
capture patches or controller assignments, as General MIDI does; such
interpretation is outside the scope of the Web MIDI API (though again,
General MIDI can easily be utilized through the Web MIDI API).
</p>
</section>
<section id="conformance">
<p>
This specification defines conformance criteria that apply to a single
product: the [=user agent=] that implements the interfaces that
it contains.
</p>
<p>
Implementations that use ECMAScript to implement the APIs defined in
this specification MUST implement them in a manner consistent with the
ECMAScript Bindings defined in the Web IDL specification [[WEBIDL]], as
this specification uses that specification and terminology.
</p>
</section>
<section>
<h2>
Terminology
</h2>
<p>
The <dfn id="WebAudio">Web Audio API</dfn> and its associated
interfaces and concepts are defined in [[webaudio]].
</p>
<p>
The terms <dfn>MIDI</dfn>, <dfn>MIDI device</dfn>, <dfn>MIDI input
port</dfn>, <dfn>MIDI output port</dfn>, <dfn>MIDI interface</dfn>,
<dfn>MIDI message</dfn>, <dfn>System Real Time</dfn> and
<dfn>System Exclusive</dfn> are defined in [[MIDI]].
</p>
<div class="note">
<dfn>Valid MIDI message</dfn> is defined in [[MIDI]]. The
following may be used as a non-normative guide:
<ul>
<li>The first byte (the status byte) should have the high bit
set, any following bytes should not unless they are part of a
[=System Exclusive=] message</li>
<li>If the high nibble of the status byte in hex is `8`, `9`,
`A`, `B`, or `E` then the total message length should be 3
bytes
<li>If the high nibble of the status byte is `C` or `D` then
the total message length should be 2 bytes</li>
<li>If the status byte is `F1` or `F3` then the total message
length should be 2 bytes</li>
<li>If the status byte is `F2` then the total message length
should be 3 bytes</li>
<li>If the status byte is `F6`, `F8`, `FA`, `FB`, `FC`, `FE`,
or `FF` then the total message length should be 1 byte (only
the status byte)</li>
<li>If the status byte is `F0` then this is a [=System
Exclusive=] message with no length restriction, and the last
byte should be `F7`</li>
<li>If the status byte is `F4`, `F5`, `F7`, `F9`, or `FD` then
the message is not valid</li>
</ul>
</div>
</section>
<section>
<h2>
Obtaining Access to [=MIDI Devices=]
</h2>
<section data-link-for="Navigator">
<h2>
Permissions Integration
</h2>
<p>
The Web Midi API is a [=powerful feature=] that is identified
by the [=powerful feature/name=] <a>"midi"</a>. It integrates with
[[[Permissions]]] by defining the following permission-related flags:
</p>
<dl>
<dt>
[=powerful feature/permission descriptor type=]
</dt>
<dd>
<pre class="idl">
dictionary MidiPermissionDescriptor : PermissionDescriptor {
boolean sysex = false;
};
</pre>
<p>
`{name: "midi", sysex: true}` is [=PermissionDescriptor/stronger than=] `{name:
"midi", sysex: false}`.
</p>
</dd>
</dl>
</section>
<section data-link-for="Navigator">
<h2>
Permissions Policy Integration
</h2>
<p>
The Web Midi API defines a [=policy-controlled feature=] named
<dfn class="permission">"midi"</dfn> which has a
[=policy-controlled feature/default allowlist=] of `'self'`.
</p>
</section>
<section data-dfn-for="Navigator">
<h2>
Extensions to the <dfn>Navigator</dfn> interface
</h2>
<pre class="idl">
partial interface Navigator {
[SecureContext]
Promise <MIDIAccess> requestMIDIAccess(optional MIDIOptions options = {});
};
</pre>
<dl>
<dt>
<dfn>requestMIDIAccess()</dfn> method
</dt>
<dd>
<p>
When invoked, returns a Promise object representing a request for
access to [=MIDI devices=] on the user's system.
</p>
<p>
Requesting MIDI access SHOULD prompt the user for access to MIDI
devices, particularly if [=System Exclusive=] access is
requested. In some scenarios, this permission may have already
been implicitly or explicitly granted, in which case this prompt
may not appear. If the user gives express permission or the call
is otherwise approved, the vended Promise is resolved. The
underlying system may choose to allow the user to select specific
[=MIDI interfaces=] to expose to this API (i.e. pick and choose
interfaces on an individual basis), although this is not
required. The system may also choose to prompt (or not) based on
whether [=System Exclusive=] support is requested, as [=System
Exclusive=] access has greater privacy and security implications.
</p>
<p>
If the user declines or the call is denied for any other reason,
the Promise is rejected with a {{DOMException}} parameter.
</p>
<p>
Multiple calls to <code>requestMIDIAccess()</code> are
allowed even if not all vended Promises have settled.
</p>
<p data-link-for="Navigator">
When the {{requestMIDIAccess()}} method is called, the user agent
MUST run the following steps:
</p>
<ol>
<li>
<p>
Let <var>promise</var> be a new Promise object and
<var>resolver</var> be its associated resolver.
</p>
</li>
<li>
<p>
Return <var>promise</var> and run the following steps
asynchronously.
</p>
</li>
<li>
<p>
Let <var>document</var> be the calling context's
[=Document=].
</p>
</li>
<li>
<p>
If <var>document</var> is not [=allowed to use=] the
[=policy-controlled feature=] named <a>midi</a>, jump to
the step labeled <em>failure</em> below.
</p>
</li>
<li>
<p>
Optionally, e.g. based on a previously-established user
preference, for security reasons, or due to platform
limitations, jump to the step labeled <em>failure</em> below.
</p>
</li>
<li>
<p>
Optionally, e.g. based on a previously-established user
preference, jump to the step labeled <em>success</em> below.
</p>
</li>
<li>
<p>
Prompt the user in a user-agent-specific manner for permission
to provide the entry script's origin with a {{MIDIAccess}}
object representing control over user's [=MIDI devices=]. This
prompt may be contingent upon whether [=System Exclusive=]
support was requested, and may allow the user to enable or
disable that access.
</p>
<p>
If permission is denied, jump to the step labeled
<em>failure</em> below. If the user never responds, this
algorithm will never progress beyond this step. If permission
is granted, continue the following steps.
</p>
</li>
<li>
<p>
<em><b>success</b></em>: Let <var>access</var> be a new
{{MIDIAccess}} object. (It is possible to call
requestMIDIAccess() multiple times; this may prompt the user
multiple times, so it may not be best practice, and the same
instance of MIDIAccess will not be returned each time.)
</p>
</li>
<li>
<p>
Call <var>resolver</var>'s <code>accept(value)</code> method
with <var>access</var> as value argument.
</p>
</li>
<li>
<p>
Terminate these steps.
</p>
</li>
<li>
<p>
<em><b>failure</b></em>: Let <var>error</var> be a new
{{DOMException}}. This exception's .name should be
{{"NotAllowedError"}} if the user or their security settings
denied the application from creating a MIDIAccess instance
with the requested options, or if the error is the result of
<var>document</var> not being [=allowed to use=] the feature,
{{"AbortError"}} if the page is going to be closed for a user
navigation, {{"InvalidStateError"}} if the underlying systems
raise any errors, or otherwise it should be
{{"NotSupportedError"}}.
</p>
</li>
<li>
<p>
Call <var>resolver</var>'s <code>reject(value)</code> method
with <var>error</var> as value argument.
</p>
</li>
</ol>
</dd>
</dl>
<section data-dfn-for="MIDIOptions" data-link-for="Navigator">
<h2 id="MIDIOptions">
<dfn>MIDIOptions</dfn> Dictionary
</h2>
<p>
This dictionary contains optional settings that may be provided to
the {{requestMIDIAccess()}} request.
</p>
<pre class="idl">
dictionary MIDIOptions {
boolean sysex;
boolean software;
};
</pre>
<dl>
<dt>
<dfn>sysex</dfn>
</dt>
<dd>
<p>
This member informs the system whether the ability to send and
receive [=System Exclusive=] messages is requested or allowed on
a given {{MIDIAccess}} object. On the option passed to
{{requestMIDIAccess()}}, if this member is set to true, but
[=System Exclusive=] support is denied (either by policy or by
user action), the access request will fail with a
{{"NotAllowedError"}} error. If this support is not requested (and
allowed), the system will throw exceptions if the user tries to
send [=System Exclusive=] messages, and will silently mask out
any [=System Exclusive=] messages received on the port.
</p>
</dd>
<dt>
<dfn>software</dfn>
</dt>
<dd>
<p>
This member informs the system whether the ability to utilize any
software synthesizers installed in the host system is requested
or allowed on a given {{MIDIAccess}} object. On
{{requestMIDIAccess()}}, if this member is set to true, but
software synthesizer support is denied (either by policy or by
user action), the access request will fail with a
{{"NotAllowedError"}} error. If this support is not requested, the
system should not include any software synthesizers in the
{{MIDIAccess}} exposure of available ports.
</p>
<p>
Note that may result in a two-step request procedure if software
synthesizer support is desired but not required - software
synthesizers may be disabled when MIDI hardware device access is
allowed.
</p>
</dd>
</dl>
</section>
</section>
</section>
<section>
<h2>
The MIDI API
</h2>
<section data-dfn-for="MIDIInputMap">
<h3 id="MIDIInputMap">
<dfn>MIDIInputMap</dfn> Interface
</h3>
<pre class="idl">
[SecureContext, Exposed=(Window,Worker)] interface MIDIInputMap {
readonly maplike <DOMString, MIDIInput>;
};
</pre>
<p>
The {{MIDIInputMap}} is a maplike interface whose value is a
{{MIDIInput}} instance and key is its ID.
</p>
<p>
This type is used to represent all the currently available [=MIDI input
ports=].
</p>
</section>
<section data-dfn-for="MIDIOutputMap">
<h3 id="MIDIOutputMap">
<dfn>MIDIOutputMap</dfn> Interface
</h3>
<pre class="idl">
[SecureContext, Exposed=(Window,Worker)] interface MIDIOutputMap {
readonly maplike <DOMString, MIDIOutput>;
};
</pre>
<p>
The {{MIDIOutputMap}} is a maplike interface whose value is a
{{MIDIOutput}} instance and key is its ID.
</p>
<p>
This type is used to represent all the currently available [=MIDI
output ports=].
</p>
</section>
<section data-dfn-for="MIDIAccess" data-link-for="MIDIAccess">
<h2 id="MIDIAccess">
<dfn>MIDIAccess</dfn> Interface
</h2>
<p>
This interface provides the methods to list MIDI input and output
devices, and obtain access to an individual device.
</p>
<pre class="idl">
[SecureContext, Exposed=(Window,Worker), Transferable] interface MIDIAccess: EventTarget {
readonly attribute MIDIInputMap inputs;
readonly attribute MIDIOutputMap outputs;
attribute EventHandler onstatechange;
readonly attribute boolean sysexEnabled;
};
</pre>
<dl>
<dt>
<dfn>inputs</dfn>
</dt>
<dd>
The [=MIDI input ports=] available to the system.
</dd>
<dt>
<dfn>outputs</dfn>
</dt>
<dd>
The [=MIDI output ports=] available to the system.
</dd>
<dt>
<dfn>onstatechange</dfn>
</dt>
<dd>
<p>
The handler called when a new port is connected or an existing
port changes the state attribute.
</p>
<p>
This [=event handler=], of type {{MIDIConnectionEvent}}, MUST be
supported by all objects implementing the {{MIDIAccess}}
interface.
</p>
<p class="note">
It is important to understand that leaving an {{EventHandler}}
attached to this object will prevent it from being
garbage-collected; when finished using the {{MIDIAccess}}, you
should remove any {{onstatechange}} listeners.
</p>
<p id="event-midiaccess-statechange">
Whenever a previously unavailable MIDI port becomes available for
use, or an existing port changes the state attribute, the user
agent SHOULD run the following steps:
</p>
<ol>
<li>Let |port:MIDIPort| be the {{MIDIPort}} corresponding to the
newly-available, or the existing port.
</li>
<li>
[=Fire an event=] named "statechange" at the {{MIDIAccess}},
using {{MIDIConnectionEvent}} with {{MIDIConnectionEvent/port}} set to |port|.
</li>
</ol>
</dd>
<dt>
<dfn>sysexEnabled</dfn>
</dt>
<dd>
This attribute informs the user whether [=System Exclusive=]
support is enabled on this MIDIAccess.
</dd>
</dl>
</section>
<section data-dfn-for="MIDIPort">
<h2 id="MIDIPort">
<dfn>MIDIPort</dfn> Interface
</h2>
<p>
This interface represents a MIDI input or output port.
</p>
<pre class="idl">
[SecureContext, Exposed=(Window,Worker)] interface MIDIPort: EventTarget {
readonly attribute DOMString id;
readonly attribute DOMString? manufacturer;
readonly attribute DOMString? name;
readonly attribute MIDIPortType type;
readonly attribute DOMString? version;
readonly attribute MIDIPortDeviceState state;
readonly attribute MIDIPortConnectionState connection;
attribute EventHandler onstatechange;
Promise <MIDIPort> open();
Promise <MIDIPort> close();
};
</pre>
<dl>
<dt>
<dfn>id</dfn>
</dt>
<dd>
<p>
A unique ID of the port. This can be used by developers to
remember ports the user has chosen for their application. The
User Agent MUST ensure that the {{MIDIPort/id}} is unique to only
that port. The User Agent SHOULD ensure that the id is maintained
across instances of the application - e.g., when the system is
rebooted - and when a device is removed from the system.
Applications may want to cache these ids locally to re-create a
MIDI setup. Some systems may not support completely unique
persistent identifiers; in such cases, it will be more
challenging to maintain identifiers when another interface is
added or removed from the system. (This might throw off the index
of the requested port.) It is expected that the system will do
the best it can to match a port across instances of the MIDI API:
for example, an implementation may opaquely use some form of hash
of the port interface manufacturer, name and index as the id, so
that a reference to that port id is likely to match the port when
plugged in. Applications may use the comparison of id of
MIDIPorts to test for equality.
</p>
</dd>
<dt>
<dfn>manufacturer</dfn>
</dt>
<dd>
<p>
The manufacturer of the port.
</p>
</dd>
<dt>
<dfn>name</dfn>
</dt>
<dd>
<p>
The system name of the port.
</p>
</dd>
<dt>
<dfn>type</dfn>
</dt>
<dd>
<p>
A descriptor property to distinguish whether the port is an input
or an output port. For {{MIDIOutput}}, this MUST be
<code>"output"</code>. For {{MIDIInput}}, this MUST be
<code>"input"</code>.
</p>
</dd>
<dt>
<dfn>version</dfn>
</dt>
<dd>
<p>
The version of the port.
</p>
</dd>
<dt>
<dfn>state</dfn>
</dt>
<dd>
The state of the device.
</dd>
<dt>
<dfn>connection</dfn>
</dt>
<dd>
The state of the connection to the device.
</dd>
<dt>
<dfn>onstatechange</dfn>
</dt>
<dd>
<p>
The handler called when an existing port changes its state or
connection attributes.
</p>
<p>
This [=event handler=], of type "statechange", MUST be
supported by all objects implementing {{MIDIPort}} interface.
</p>
<p class="note">
It is important to understand that leaving an {{EventHandler}}
attached to this object will prevent it from being
garbage-collected; when finished using the {{MIDIPort}}, you
should remove any {{MIDIPort/onstatechange}} listeners.
</p>
</dd>
<dt>
<dfn>open</dfn>
</dt>
<dd>
<p>
Makes the [=MIDI device=] corresponding to the {{MIDIPort}}
explicitly available. Note that this call is NOT required in order
to use the {{MIDIPort}}- calling <code>send()</code> on a
{{MIDIOutput}}, attaching a MIDIMessageEvent {{EventHandler}} on a
{{MIDIInput}}, or adding a MIDIMessageEvent {{EventListener}} on a
{{MIDIInput}} will cause an implicit open(). The underlying
implementation may not need to do anything in response to this
call. However, some underlying implementations may not be able to
support shared access to [=MIDI devices=], so using explicit
open() and close() calls will enable MIDI applications to
predictably control this exclusive access to devices.
</p>
<p>
When invoked, this method returns a Promise object representing a
request for access to the given MIDI port on the user's system.
</p>
<p>
If the port device has a state of <a data-lt=
"MIDIPortDeviceState.connected">"connected"</a>, when access to
the port has been obtained (and the port is ready for input or
output), the vended Promise is resolved.
</p>
<p>
If access to a connected port is not available (for example, the
port is already in use in an exclusive-access-only platform), the
Promise is rejected (if any) is invoked.
</p>
<p>
If <code>open()</code> is called on a port that is <a data-lt=
"MIDIPortDeviceState.disconnected">"disconnected"</a>, the port's
<a data-lt="MIDIPort.connection">.connection</a> will transition
to <a data-lt="MIDIPortConnectionState.pending">"pending"</a>,
until the port becomes <a data-lt=
"MIDIPortDeviceState.connected">"connected"</a> or all references
to it are dropped.
</p>
<p>
Multiple calls to <code>open()</code> are allowed even
if not all vended Promises have settled.
</p>
<p>
When this method is called, the user agent MUST run the
<dfn data-lt="open the port">algorithm to open a MIDIPort</dfn>:
</p>
<ol>
<li>
<p>
Let <var>promise</var> be a new Promise object and
<var>resolver</var> be its associated resolver.
</p>
</li>
<li>
<p>
Return <var>promise</var> and run the following steps
asynchronously.
</p>
</li>
<li>
<p>
Let <var>port</var> be the given {{MIDIPort}} object.
</p>
</li>
<li>
<p>
If the device's connection is already <a data-lt=
"MIDIPortConnectionState.open">"open"</a> (e.g. open() has
already been called on this {{MIDIPort}}, or the port has been
implicitly opened), jump to the step labeled <em>success</em>
below.
</p>
</li>
<li>
<p>
If the device's connection is <a data-lt=
"MIDIPortConnectionState.pending">"pending"</a> (i.e. the
connection had been opened and the device was subsequently
disconnected), jump to the step labeled <em>success</em>
below.
</p>
</li>
<li>
<p>
If the device's state is <a data-lt=
"MIDIPortDeviceState.disconnected">"disconnected"</a>, change
the <a data-link-for="MIDIPort">connection</a> attribute of
the {{MIDIPort}} to <a data-lt=
"MIDIPortConnectionState.pending">"pending"</a>, and enqueue
a new <a data-lt=
"MIDIConnectionEvent">MIDIConnectionEvent</a> to the
<a data-lt="MIDIAccess.onstatechange">statechange</a> handler
of the {{MIDIAccess}} and to the <a data-lt=
"MIDIPort.onstatechange">statechange</a> handler of the
{{MIDIPort}} and jump to the step labeled <em>success</em>
below.
</p>
</li>
<li>
<p>
Attempt to obtain access to the given [=MIDI device=] in the
system. If the device is unavailable (e.g. is already in use
by another process and cannot be opened, or is disconnected),
jump to the step labeled <em>failure</em> below. If the device
is available and access is obtained, continue the following
steps.
</p>
</li>
<li>
<p>
Change the <code>connection</code> attribute of the MIDIPort
to <code>"open"</code>, and enqueue a new
{{MIDIConnectionEvent}} to the <a data-lt=
"MIDIAccess.onstatechange">statechange</a> handler of the
{{MIDIAccess}} and to the <a data-lt=
"MIDIPort.onstatechange">statechange</a> handler of the
{{MIDIPort}}.
</p>
</li>
<li>
<p>
If this port is an output port and has any enqueued send data
with timestamps in the future, asynchronously begin sending
that data.
</p>
</li>
<li>
<p>
<em><b>success</b></em>: Call <var>resolver</var>'s
<code>accept(value)</code> method with <var>port</var> as
value argument.
</p>
</li>
<li>
<p>
Terminate these steps.
</p>
</li>
<li>
<p>
<em><b>failure</b></em>: Let <var>error</var> be a new
{{DOMException}}. This exception's .name should be
<code>"InvalidAccessError"</code> if the port is unavailable.
</p>
</li>
<li>
<p>
Call <var>resolver</var>'s <code>reject(value)</code> method
with <var>error</var> as value argument.
</p>
</li>
</ol>
</dd>
<dt>
<dfn>close</dfn>
</dt>
<dd>
<p>
Makes the [=MIDI device=] corresponding to the {{MIDIPort}}
explicitly unavailable (subsequently changing the state from
"open" to "closed"). Note that successful invocation of this
method will result in [=MIDI messages=] no longer being delivered
to MIDIMessageEvent handlers on a {{MIDIInput}} (although setting
a new handler will cause an implicit open()).
</p>
<p>
The underlying implementation may not need to do anything in
response to this call. However, some underlying implementations
may not be able to support shared access to [=MIDI devices=], and
the explicit close() call enables MIDI applications to ensure
other applications can gain access to devices.
</p>
<p>
When invoked, this method returns a Promise object representing a
request for access to the given MIDI port on the user's system.
When the port has been closed (and therefore, in exclusive access
systems, the port is available to other applications), the vended
Promise is resolved. If the port is disconnected, the Promise is
rejected.
</p>
<p>
Multiple calls to <code>close()</code> are allowed even
if not all vended Promises have settled.
</p>
<p>
When the <code>close()</code> method is called, the user agent
MUST run the following steps:
</p>
<ol>
<li>
<p>
Let <var>promise</var> be a new Promise object and
<var>resolver</var> be its associated resolver.
</p>
</li>
<li>
<p>
Return <var>promise</var> and run the following steps
asynchronously.
</p>
</li>
<li>
<p>
Let <var>port</var> be the given {{MIDIPort}} object.
</p>
</li>
<li>
<p>
If the port is already closed (its <a data-lt=
"MIDIPort.connection">.connection</a> is <a data-lt=
"MIDIPortConnectionState.closed">"closed"</a> - e.g. the port
has not yet been implicitly or explicitly opened, or
<a data-lt="MIDIPort.close">close()</a> has already been
called on this {{MIDIPort}}), jump to the step labeled
<em><b>closed</b></em> below.
</p>
</li>
<li>
<p>
If the port is an input port, skip to the next step. If the
output port's <a data-lt="MIDIPort.state">.state</a> is not
<a data-lt="MIDIPortDeviceState.connected">"connected"</a>
or if its .connection is "pending", clear all enqueued send
data and skip to the next step. Clear any enqueued send data
in the system with timestamps in the future, then finish
sending any send messages with no timestamp or with a
timestamp in the past or present, prior to proceeding to
the next step.
</p>
</li>
<li>
<p>
Close access to the port in the underlying system if open,
and release any blocking resources in the underlying system.
</p>
</li>
<li>
<p>
Change the <code>connection</code> attribute of the MIDIPort
to <code>"closed"</code>, and enqueue a new
{{MIDIConnectionEvent}} to the <a data-lt=
"MIDIAccess.onstatechange">statechange</a> handler of the
{{MIDIAccess}} and to the <a data-lt=
"MIDIPort.onstatechange">statechange</a> handler of the
{{MIDIPort}}.
</p>
</li>
<li>
<p>
<em><b>closed</b></em>: Call <var>resolver</var>'s
<code>accept(value)</code> method with <var>port</var> as
value argument.
</p>
</li>
</ol>
</dd>
</dl>
<p id="event-midiport-statechange">
Whenever the MIDI port corresponding to the {{MIDIPort}} changes the
state attribute, the user agent SHOULD run the following steps:
</p>
<ol>
<li>
<p>
Let |port:MIDIPort| be the {{MIDIPort}}.
</p>
</li>
<li>
<p>
[=Fire an event=] named <a data-lt=
"MIDIPort.onstatechange">statechange</a> at the {{MIDIPort}}, and
<a data-lt="MIDIAccess.onstatechange">statechange</a> at the
{{MIDIAccess}}, using {{MIDIConnectionEvent}} with the {{MIDIConnectionEvent/port}} attribute set
to |port|.
</p>
</li>
</ol>
<section data-dfn-for="MIDIInput">
<h3 id="MIDIInput">
<dfn>MIDIInput</dfn> Interface
</h3>
<pre class="idl">
[SecureContext, Exposed=(Window,Worker)] interface MIDIInput: MIDIPort {
attribute EventHandler onmidimessage;
};
</pre>
<dl>
<dt>
<dfn>onmidimessage</dfn>
</dt>
<dd>
<p>
This [=event handler=], of type "midimessage", MUST be
supported by all objects implementing {{MIDIInput}} interface.
</p>
<p>
If the handler is set and the state attribute is not
<code>"opened"</code>, underlying implementation tries to make
the port available, and change the state attribute to
<code>"opened"</code>. If succeeded, {{MIDIConnectionEvent}} is
delivered to the corresponding <code>MIDIPort</code> and
{{MIDIAccess}}.
</p>
</dd>
</dl>
<p id="event-midiinput-message">
Whenever the MIDI port corresponding to the {{MIDIInput}} finishes
receiving one or more [=MIDI messages=], the user agent MUST run the
following steps:
</p>
<ol>
<li>
<p>
Let |port:MIDIInput| be the {{MIDIInput}}.
</p>
</li>
<li>
<p>
If the {{MIDIAccess}} did not enable [=System Exclusive=]
access, and the message is a [=System Exclusive=] message, abort
this process.
</p>
</li>
<li>
<p>
[=Fire an event=] named "midimessage" at |port|, using
{{MIDIMessageEvent}} with the {{Event/timeStamp}} attribute set
to the time the message was received by the system, and with the
{{MIDIMessageEvent/data}} attribute set to a {{Uint8Array}} of
MIDI data bytes representing a single [=MIDI message=].
</p>
</li>
</ol>