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diff --git a/dom/media/webcodecs/EncoderAgent.h b/dom/media/webcodecs/EncoderAgent.h
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+++ b/dom/media/webcodecs/EncoderAgent.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 DOM_MEDIA_WEBCODECS_EncoderAgent_H
+#define DOM_MEDIA_WEBCODECS_EncoderAgent_H
+
+#include "MediaResult.h"
+#include "PlatformEncoderModule.h"
+#include "PEMFactory.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TaskQueue.h"
+#include "WebCodecsUtils.h"
+
+class nsISerialEventTarget;
+
+namespace mozilla {
+
+class PDMFactory;
+class TrackInfo;
+
+namespace layers {
+class ImageContainer;
+}  // namespace layers
+
+// EncoderAgent is a wrapper that contains a MediaDataEncoder. It adapts the
+// MediaDataEncoder APIs for use in WebCodecs.
+//
+// If Configure() is called, Shutdown() must be called to release the resources
+// gracefully. Except Shutdown(), all the methods can't be called concurrently,
+// meaning a method can only be called when the previous API call has completed.
+// The responsability of arranging the method calls is on the caller.
+//
+// When Shutdown() is called, all the operations in flight are canceled and the
+// MediaDataEncoder is shut down. On the other hand, errors are final. A new
+// EncoderAgent must be created when an error is encountered.
+//
+// All the methods need to be called on the EncoderAgent's owner thread. In
+// WebCodecs, it's either on the main thread or worker thread.
+class EncoderAgent final {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(EncoderAgent);
+
+  explicit EncoderAgent(WebCodecsId aId);
+
+  // The following APIs are owner thread only.
+
+  using ConfigurePromise = MozPromise<bool, MediaResult, true /* exclusive */>;
+  using ReconfigurationPromise = MediaDataEncoder::ReconfigurationPromise;
+  RefPtr<ConfigurePromise> Configure(const EncoderConfig& aConfig);
+  RefPtr<ReconfigurationPromise> Reconfigure(
+      const RefPtr<const EncoderConfigurationChangeList>& aConfigChange);
+  RefPtr<ShutdownPromise> Shutdown();
+  using EncodePromise = MediaDataEncoder::EncodePromise;
+  RefPtr<EncodePromise> Encode(MediaData* aInput);
+  // WebCodecs's flush() flushes out all the pending encoded data in the
+  // encoder. It's called Drain internally.
+  RefPtr<EncodePromise> Drain();
+
+  const WebCodecsId mId;
+
+ private:
+  ~EncoderAgent();
+
+  // Push out all the data in the MediaDataEncoder's pipeline.
+  // TODO: MediaDataEncoder should implement this, instead of asking call site
+  // to run `Drain` multiple times.
+  RefPtr<EncodePromise> Dry();
+  void DryUntilDrain();
+
+  enum class State {
+    Unconfigured,
+    Configuring,
+    Configured,
+    Encoding,
+    Flushing,
+    ShuttingDown,
+    Error,
+  };
+  void SetState(State aState);
+
+  const RefPtr<nsISerialEventTarget> mOwnerThread;
+  const RefPtr<PEMFactory> mPEMFactory;
+  RefPtr<MediaDataEncoder> mEncoder;
+  State mState;
+
+  // Configure
+  MozPromiseHolder<ConfigurePromise> mConfigurePromise;
+  using CreateEncoderPromise = PlatformEncoderModule::CreateEncoderPromise;
+  MozPromiseRequestHolder<CreateEncoderPromise> mCreateRequest;
+  using InitPromise = MediaDataEncoder::InitPromise;
+  MozPromiseRequestHolder<InitPromise> mInitRequest;
+
+  // Reconfigure
+  MozPromiseHolder<ReconfigurationPromise> mReconfigurationPromise;
+  using ReconfigureEncoderRequest = ReconfigurationPromise;
+  MozPromiseRequestHolder<ReconfigureEncoderRequest> mReconfigurationRequest;
+
+  // Shutdown
+  MozPromiseHolder<ShutdownPromise> mShutdownWhileCreationPromise;
+
+  // Encoding
+  MozPromiseHolder<EncodePromise> mEncodePromise;
+  MozPromiseRequestHolder<EncodePromise> mEncodeRequest;
+
+  // Drain
+  MozPromiseRequestHolder<EncodePromise> mDrainRequest;
+  MozPromiseHolder<EncodePromise> mDrainPromise;
+  MediaDataEncoder::EncodedData mDrainData;
+};
+
+}  // namespace mozilla
+
+#endif  // DOM_MEDIA_WEBCODECS_EncoderAgent_H