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diff --git a/dom/midi/MIDIPlatformService.h b/dom/midi/MIDIPlatformService.h
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+++ b/dom/midi/MIDIPlatformService.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_MIDIPlatformService_h
+#define mozilla_dom_MIDIPlatformService_h
+
+#include "nsClassHashtable.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/dom/MIDIPortBinding.h"
+#include "nsHashKeys.h"
+
+// XXX Avoid including this here by moving function implementations to the cpp
+// file.
+#include "mozilla/dom/MIDIMessageQueue.h"
+
+namespace mozilla::dom {
+
+class MIDIManagerParent;
+class MIDIPortParent;
+class MIDIMessage;
+class MIDIMessageQueue;
+class MIDIPortInfo;
+
+/**
+ * Base class for platform specific MIDI implementations. Handles aggregation of
+ * IPC service objects, as well as sending/receiving updates about port
+ * connection events and messages.
+ *
+ */
+class MIDIPlatformService {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(MIDIPlatformService);
+  // Adds info about MIDI Port that has been connected.
+  void AddPortInfo(MIDIPortInfo& aPortInfo);
+
+  // Removes info of MIDI Port that has been disconnected.
+  void RemovePortInfo(MIDIPortInfo& aPortInfo);
+
+  // Adds a newly created manager protocol object to manager array.
+  void AddManager(MIDIManagerParent* aManager);
+
+  // Removes a deleted manager protocol object from manager array.
+  void RemoveManager(MIDIManagerParent* aManager);
+
+  // Adds a newly created port protocol object to port array.
+  void AddPort(MIDIPortParent* aPort);
+
+  // Removes a deleted port protocol object from port array.
+  void RemovePort(MIDIPortParent* aPort);
+
+  // Platform specific init function.
+  virtual void Init() = 0;
+
+  // Forces the implementation to refresh the port list.
+  virtual void Refresh() = 0;
+
+  // Platform specific MIDI port opening function.
+  virtual void Open(MIDIPortParent* aPort) = 0;
+
+  // Clears all queued MIDI messages for a port.
+  void Clear(MIDIPortParent* aPort);
+
+  // Puts in a request to destroy the singleton MIDIPlatformService object.
+  // Object will only be destroyed if there are no more MIDIManager and MIDIPort
+  // protocols left to communicate with.
+  void MaybeStop();
+
+  // Initializes statics on startup.
+  static void InitStatics();
+
+  // Returns the MIDI Task Queue.
+  static nsISerialEventTarget* OwnerThread();
+
+  // Asserts that we're on the above task queue.
+  static void AssertThread() {
+    MOZ_DIAGNOSTIC_ASSERT(OwnerThread()->IsOnCurrentThread());
+  }
+
+  // True if service is live.
+  static bool IsRunning();
+
+  // Returns a pointer to the MIDIPlatformService object, creating it and
+  // starting the platform specific service if it is not currently running.
+  static MIDIPlatformService* Get();
+
+  // Sends a list of all currently connected ports in order to populate a new
+  // MIDIAccess object.
+  void SendPortList();
+
+  // Receives a new set of messages from an MIDI Input Port, and checks their
+  // validity.
+  void CheckAndReceive(const nsAString& aPortID,
+                       const nsTArray<MIDIMessage>& aMsgs);
+
+  // Sends connection/disconnect/open/closed/etc status updates about a MIDI
+  // Port to all port listeners.
+  void UpdateStatus(MIDIPortParent* aPort,
+                    const MIDIPortDeviceState& aDeviceState,
+                    const MIDIPortConnectionState& aConnectionState);
+
+  // Adds outgoing messages to the sorted message queue, for sending later.
+  void QueueMessages(const nsAString& aId, nsTArray<MIDIMessage>& aMsgs);
+
+  // Clears all messages later than now, sends all outgoing message scheduled
+  // before/at now, and schedules MIDI Port connection closing.
+  void Close(MIDIPortParent* aPort);
+
+  // Returns whether there are currently any MIDI devices.
+  bool HasDevice() { return !mPortInfo.IsEmpty(); }
+
+ protected:
+  MIDIPlatformService();
+  virtual ~MIDIPlatformService();
+  // Platform specific MIDI service shutdown method.
+  virtual void Stop() = 0;
+
+  // When device state of a MIDI Port changes, broadcast to all IPC port
+  // objects.
+  void BroadcastState(const MIDIPortInfo& aPortInfo,
+                      const MIDIPortDeviceState& aState);
+
+  // Platform specific MIDI port closing function. Named "Schedule" due to the
+  // fact that it needs to happen in the context of the I/O thread for the
+  // platform MIDI implementation, and therefore will happen async.
+  virtual void ScheduleClose(MIDIPortParent* aPort) = 0;
+
+  // Platform specific MIDI message sending function. Named "Schedule" due to
+  // the fact that it needs to happen in the context of the I/O thread for the
+  // platform MIDI implementation, and therefore will happen async.
+  virtual void ScheduleSend(const nsAString& aPortId) = 0;
+
+  // Allows platform specific IO Threads to retrieve all messages to be sent.
+  // Handles mutex locking.
+  void GetMessages(const nsAString& aPortId, nsTArray<MIDIMessage>& aMsgs);
+
+  // Allows platform specific IO Threads to retrieve all messages to be sent
+  // before a certain timestamp. Handles mutex locking.
+  void GetMessagesBefore(const nsAString& aPortId, const TimeStamp& aTimeStamp,
+                         nsTArray<MIDIMessage>& aMsgs);
+
+ private:
+  // When the MIDIPlatformService is created, we need to know whether or not the
+  // corresponding IPC MIDIManager objects have received the MIDIPort list after
+  // it is populated. This is set to True when that is done, so we don't
+  // constantly spam MIDIManagers with port lists.
+  bool mHasSentPortList;
+
+  // Array of MIDIManager IPC objects. This array manages the lifetime of
+  // MIDIManager objects in the parent process, and IPC will call
+  // RemoveManager() end lifetime when IPC channel is destroyed.
+  nsTArray<RefPtr<MIDIManagerParent>> mManagers;
+
+  // Array of information for currently connected Ports
+  nsTArray<MIDIPortInfo> mPortInfo;
+
+  // Array of MIDIPort IPC objects. May contain ports not listed in mPortInfo,
+  // as we can hold port objects even after they are disconnected.
+  //
+  // TODO Split this into input and output ports. Will make life easier.
+  nsTArray<RefPtr<MIDIPortParent>> mPorts;
+
+  // Per-port message queue hashtable. Handles scheduling messages for sending.
+  nsClassHashtable<nsStringHashKey, MIDIMessageQueue> mMessageQueues;
+
+  // Mutex for managing access to message queue objects.
+  Mutex mMessageQueueMutex MOZ_UNANNOTATED;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_MIDIPlatformService_h