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diff --git a/toolkit/components/perfmonitoring/PerformanceMetricsCollector.h b/toolkit/components/perfmonitoring/PerformanceMetricsCollector.h
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+++ b/toolkit/components/perfmonitoring/PerformanceMetricsCollector.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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 PerformanceMetricsCollector_h
+#define PerformanceMetricsCollector_h
+
+#include "nsIObserver.h"
+#include "nsITimer.h"
+#include "nsID.h"
+#include "nsTHashMap.h"
+#include "mozilla/dom/ChromeUtilsBinding.h"  // defines PerformanceInfoDictionary
+#include "mozilla/dom/DOMTypes.h"            // defines PerformanceInfo
+#include "mozilla/PerformanceTypes.h"
+
+namespace mozilla {
+
+namespace dom {
+class Promise;
+}
+
+class PerformanceMetricsCollector;
+class AggregatedResults;
+
+class IPCTimeout final : public nsITimerCallback, public nsINamed {
+ public:
+  NS_DECL_NSITIMERCALLBACK
+  NS_DECL_NSINAMED
+  NS_DECL_ISUPPORTS
+  static IPCTimeout* CreateInstance(AggregatedResults* aResults);
+  void Cancel();
+
+ private:
+  IPCTimeout(AggregatedResults* aResults, uint32_t aDelay);
+  ~IPCTimeout();
+
+  nsCOMPtr<nsITimer> mTimer;
+  AggregatedResults* mResults;
+};
+
+// AggregatedResults receives PerformanceInfo results that are collected
+// asynchronously via IPDL from all content processes.
+// They are converted into an array of
+// PerformanceInfoDictionary dictionaries (webidl)
+//
+// Once every process have sent back its results, AggregatedResults will
+// resolve the MozPromise returned by GetPromise()
+// with all the collected data.
+//
+// See ChromeUtils::RequestPerformanceMetrics.
+class AggregatedResults final {
+ public:
+  AggregatedResults(nsID aUUID, PerformanceMetricsCollector* aCollector);
+  ~AggregatedResults() = default;
+  void AppendResult(const nsTArray<dom::PerformanceInfo>& aMetrics);
+  void SetNumResultsRequired(uint32_t aNumResultsRequired);
+  void Abort(nsresult aReason);
+  void ResolveNow();
+  RefPtr<RequestMetricsPromise> GetPromise();
+
+ private:
+  RefPtr<IPCTimeout> mIPCTimeout;
+  MozPromiseHolder<RequestMetricsPromise> mHolder;
+  uint32_t mPendingResults;
+  FallibleTArray<dom::PerformanceInfoDictionary> mData;
+
+  // AggregatedResults keeps a reference on the collector
+  // so it gets destructed when all pending AggregatedResults
+  // are themselves destructed when removed from
+  // PerformanceMetricsCollector::mAggregatedResults.
+  //
+  // This lifecycle ensures that everything is released once
+  // all pending results are sent.
+  RefPtr<PerformanceMetricsCollector> mCollector;
+  nsID mUUID;
+};
+
+//
+// PerformanceMetricsCollector is instanciated as a singleton, and creates
+// one AggregatedResults instance everytime metrics are requested.
+//
+// Each AggregatedResults has a unique identifier (UUID) that is used
+// to send metrics requests via IPDL. When metrics are back in an
+// asynchronous fashion, the UUID is used to append the data to the
+// right AggregatedResults instance and eventually let it resolve the
+// linked promise.
+//
+class PerformanceMetricsCollector final {
+ public:
+  NS_INLINE_DECL_REFCOUNTING(PerformanceMetricsCollector)
+  static RefPtr<RequestMetricsPromise> RequestMetrics();
+  static nsresult DataReceived(const nsID& aUUID,
+                               const nsTArray<dom::PerformanceInfo>& aMetrics);
+  void ForgetAggregatedResults(const nsID& aUUID);
+
+ private:
+  ~PerformanceMetricsCollector();
+  RefPtr<RequestMetricsPromise> RequestMetricsInternal();
+  nsresult DataReceivedInternal(const nsID& aUUID,
+                                const nsTArray<dom::PerformanceInfo>& aMetrics);
+  nsTHashMap<nsID, UniquePtr<AggregatedResults>> mAggregatedResults;
+};
+
+}  // namespace mozilla
+#endif  // PerformanceMetricsCollector_h