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diff --git a/toolkit/components/glean/bindings/private/CustomDistribution.h b/toolkit/components/glean/bindings/private/CustomDistribution.h
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+++ b/toolkit/components/glean/bindings/private/CustomDistribution.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_glean_GleanCustomDistribution_h
+#define mozilla_glean_GleanCustomDistribution_h
+
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/glean/bindings/GleanMetric.h"
+#include "mozilla/glean/bindings/DistributionData.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Result.h"
+#include "nsTArray.h"
+
+namespace mozilla::dom {
+struct GleanDistributionData;
+}  // namespace mozilla::dom
+
+namespace mozilla::glean {
+
+namespace impl {
+
+class CustomDistributionMetric {
+ public:
+  constexpr explicit CustomDistributionMetric(uint32_t aId) : mId(aId) {}
+
+  /**
+   * Accumulates the provided samples in the metric.
+   *
+   * @param aSamples The vector holding the samples to be recorded by the
+   *                 metric.
+   */
+  void AccumulateSamples(const nsTArray<uint64_t>& aSamples) const;
+
+  /**
+   * Accumulates the provided samples in the metric.
+   *
+   * @param aSamples The vector holding the samples to be recorded by the
+   *                 metric.
+   *
+   * Notes: Discards any negative value in `samples`
+   * and reports an `InvalidValue` error for each of them.
+   */
+  void AccumulateSamplesSigned(const nsTArray<int64_t>& aSamples) const;
+
+  /**
+   * **Test-only API**
+   *
+   * Gets the currently stored value as a DistributionData.
+   *
+   * This function will attempt to await the last parent-process task (if any)
+   * writing to the the metric's storage engine before returning a value.
+   * This function will not wait for data from child processes.
+   *
+   * This doesn't clear the stored value.
+   * Parent process only. Panics in child processes.
+   *
+   * @param aPingName The (optional) name of the ping to retrieve the metric
+   *        for. Defaults to the first value in `send_in_pings`.
+   *
+   * @return value of the stored metric, or Nothing() if there is no value.
+   */
+  Result<Maybe<DistributionData>, nsCString> TestGetValue(
+      const nsACString& aPingName = nsCString()) const;
+
+ private:
+  const uint32_t mId;
+};
+}  // namespace impl
+
+class GleanCustomDistribution final : public GleanMetric {
+ public:
+  explicit GleanCustomDistribution(uint64_t aId, nsISupports* aParent)
+      : GleanMetric(aParent), mCustomDist(aId) {}
+
+  virtual JSObject* WrapObject(
+      JSContext* aCx, JS::Handle<JSObject*> aGivenProto) override final;
+
+  void AccumulateSamples(const dom::Sequence<int64_t>& aSamples);
+
+  void TestGetValue(const nsACString& aPingName,
+                    dom::Nullable<dom::GleanDistributionData>& aRetval,
+                    ErrorResult& aRv);
+
+ private:
+  virtual ~GleanCustomDistribution() = default;
+
+  const impl::CustomDistributionMetric mCustomDist;
+};
+
+}  // namespace mozilla::glean
+
+#endif /* mozilla_glean_GleanCustomDistribution_h */