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diff --git a/cast/sender/public/cast_app_discovery_service.h b/cast/sender/public/cast_app_discovery_service.h
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+++ b/cast/sender/public/cast_app_discovery_service.h
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+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CAST_SENDER_PUBLIC_CAST_APP_DISCOVERY_SERVICE_H_
+#define CAST_SENDER_PUBLIC_CAST_APP_DISCOVERY_SERVICE_H_
+
+#include <vector>
+
+#include "cast/common/public/service_info.h"
+
+namespace openscreen {
+namespace cast {
+
+class CastMediaSource;
+
+// Interface for app discovery for Cast devices.
+class CastAppDiscoveryService {
+ public:
+  using AvailabilityCallback =
+      std::function<void(const CastMediaSource& source,
+                         const std::vector<ServiceInfo>& devices)>;
+
+  class Subscription {
+   public:
+    Subscription(Subscription&&);
+    ~Subscription();
+    Subscription& operator=(Subscription);
+
+    void Reset();
+
+   private:
+    friend class CastAppDiscoveryService;
+
+    Subscription(CastAppDiscoveryService* discovery_service, uint32_t id);
+
+    void Swap(Subscription& other);
+
+    CastAppDiscoveryService* discovery_service_;
+    uint32_t id_;
+  };
+
+  virtual ~CastAppDiscoveryService() = default;
+
+  // Adds an availability query for |source|. Results will be continuously
+  // returned via |callback| until the returned Subscription is destroyed by the
+  // caller.  If there are cached results available, |callback| will be invoked
+  // before this method returns.  |callback| may be invoked with an empty list
+  // if all devices respond to the respective queries with "unavailable" or
+  // don't respond before a timeout.  |callback| may be invoked successively
+  // with the same list.
+  virtual Subscription StartObservingAvailability(
+      const CastMediaSource& source,
+      AvailabilityCallback callback) = 0;
+
+  // Refreshes the state of app discovery in the service. It is suitable to call
+  // this method when the user initiates a user gesture.
+  virtual void Refresh() = 0;
+
+ protected:
+  Subscription MakeSubscription(CastAppDiscoveryService* discovery_service,
+                                uint32_t id) {
+    return Subscription(discovery_service, id);
+  }
+
+ private:
+  friend class Subscription;
+
+  virtual void RemoveAvailabilityCallback(uint32_t id) = 0;
+};
+
+}  // namespace cast
+}  // namespace openscreen
+
+#endif  // CAST_SENDER_PUBLIC_CAST_APP_DISCOVERY_SERVICE_H_