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diff --git a/dom/media/webcodecs/DecoderAgent.h b/dom/media/webcodecs/DecoderAgent.h
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+++ b/dom/media/webcodecs/DecoderAgent.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_DECODERAGENT_H
+#define DOM_MEDIA_WEBCODECS_DECODERAGENT_H
+
+#include "MediaResult.h"
+#include "PlatformDecoderModule.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TaskQueue.h"
+#include "mozilla/UniquePtr.h"
+
+class nsISerialEventTarget;
+
+namespace mozilla {
+
+class PDMFactory;
+class TrackInfo;
+
+namespace layers {
+class ImageContainer;
+}  // namespace layers
+
+// DecoderAgent is a wrapper that contains a MediaDataDecoder. It adapts the
+// MediaDataDecoder 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
+// MediaDataDecoder is shut down. On the other hand, errors are final. A new
+// DecoderAgent must be created when an error is encountered.
+//
+// All the methods need to be called on the DecoderAgent's owner thread. In
+// WebCodecs, it's either on the main thread or worker thread.
+class DecoderAgent final {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(DecoderAgent);
+
+  using Id = uint32_t;
+  DecoderAgent(Id aId, UniquePtr<TrackInfo>&& aInfo);
+
+  // The following APIs are owner thread only.
+
+  using ConfigurePromise = MozPromise<bool, MediaResult, true /* exclusive */>;
+  RefPtr<ConfigurePromise> Configure(bool aPreferSoftwareDecoder,
+                                     bool aLowLatency);
+  RefPtr<ShutdownPromise> Shutdown();
+  using DecodePromise = MediaDataDecoder::DecodePromise;
+  RefPtr<DecodePromise> Decode(MediaRawData* aSample);
+  // WebCodecs's flush() flushes out all the pending decoded data in the
+  // MediaDataDecoder so it is a combination of Drain and Flush. To distinguish
+  // the term from MediaDataDecoder's one, we call it DrainAndFlush() here.
+  RefPtr<DecodePromise> DrainAndFlush();
+
+  const Id mId;  // A unique id.
+  const UniquePtr<TrackInfo> mInfo;
+
+ private:
+  ~DecoderAgent();
+
+  // Push out all the data in the MediaDataDecoder's pipeline.
+  // TODO: MediaDataDecoder should implement this, instead of asking call site
+  // to run `Drain` multiple times.
+  RefPtr<DecodePromise> Dry();
+  void DrainUntilDry();
+
+  enum class State {
+    Unconfigured,
+    Configuring,
+    Configured,
+    Decoding,
+    Flushing,
+    ShuttingDown,
+    Error,
+  };
+  void SetState(State aState);
+
+  const RefPtr<nsISerialEventTarget> mOwnerThread;
+  const RefPtr<PDMFactory> mPDMFactory;
+  const RefPtr<layers::ImageContainer> mImageContainer;
+  RefPtr<MediaDataDecoder> mDecoder;
+  State mState;
+
+  // Configure
+  MozPromiseHolder<ConfigurePromise> mConfigurePromise;
+  using CreateDecoderPromise = PlatformDecoderModule::CreateDecoderPromise;
+  MozPromiseRequestHolder<CreateDecoderPromise> mCreateRequest;
+  using InitPromise = MediaDataDecoder::InitPromise;
+  MozPromiseRequestHolder<InitPromise> mInitRequest;
+
+  // Shutdown
+  MozPromiseHolder<ShutdownPromise> mShutdownWhileCreationPromise;
+
+  // Decode
+  MozPromiseHolder<DecodePromise> mDecodePromise;
+  MozPromiseRequestHolder<DecodePromise> mDecodeRequest;
+
+  // DrainAndFlush
+  MozPromiseHolder<DecodePromise> mDrainAndFlushPromise;
+  MediaDataDecoder::DecodedData mDrainAndFlushData;
+  MozPromiseRequestHolder<DecodePromise> mDryRequest;
+  MozPromiseHolder<DecodePromise> mDryPromise;
+  MediaDataDecoder::DecodedData mDryData;
+  MozPromiseRequestHolder<DecodePromise> mDrainRequest;
+  using FlushPromise = MediaDataDecoder::FlushPromise;
+  MozPromiseRequestHolder<FlushPromise> mFlushRequest;
+};
+
+}  // namespace mozilla
+
+#endif  // DOM_MEDIA_WEBCODECS_DECODERAGENT_H