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+/*
+ *  Copyright 2021 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef API_ASYNC_DNS_RESOLVER_H_
+#define API_ASYNC_DNS_RESOLVER_H_
+
+#include <functional>
+#include <memory>
+
+#include "rtc_base/checks.h"
+#include "rtc_base/socket_address.h"
+#include "rtc_base/system/rtc_export.h"
+
+namespace webrtc {
+
+// This interface defines the methods to resolve a hostname asynchronously.
+// The AsyncDnsResolverInterface class encapsulates a single name query.
+//
+// Usage:
+//   std::unique_ptr<AsyncDnsResolverInterface> resolver =
+//        factory->Create(address-to-be-resolved, [r = resolver.get()]() {
+//     if (r->result.GetResolvedAddress(AF_INET, &addr) {
+//       // success
+//     } else {
+//       // failure
+//       error = r->result().GetError();
+//     }
+//     // Release resolver.
+//     resolver_list.erase(std::remove_if(resolver_list.begin(),
+//     resolver_list.end(),
+//                         [](refptr) { refptr.get() == r; });
+//   });
+//   resolver_list.push_back(std::move(resolver));
+
+class AsyncDnsResolverResult {
+ public:
+  virtual ~AsyncDnsResolverResult() = default;
+  // Returns true iff the address from `Start` was successfully resolved.
+  // If the address was successfully resolved, sets `addr` to a copy of the
+  // address from `Start` with the IP address set to the top most resolved
+  // address of `family` (`addr` will have both hostname and the resolved ip).
+  virtual bool GetResolvedAddress(int family,
+                                  rtc::SocketAddress* addr) const = 0;
+  // Returns error from resolver.
+  virtual int GetError() const = 0;
+};
+
+// The API for a single name query.
+// The constructor, destructor and all functions must be called from
+// the same sequence, and the callback will also be called on that sequence.
+// The class guarantees that the callback will not be called if the
+// resolver's destructor has been called.
+class RTC_EXPORT AsyncDnsResolverInterface {
+ public:
+  virtual ~AsyncDnsResolverInterface() = default;
+
+  // Start address resolution of the hostname in `addr`.
+  virtual void Start(const rtc::SocketAddress& addr,
+                     std::function<void()> callback) = 0;
+  // Start address resolution of the hostname in `addr` matching `family`.
+  virtual void Start(const rtc::SocketAddress& addr,
+                     int family,
+                     std::function<void()> callback) = 0;
+  virtual const AsyncDnsResolverResult& result() const = 0;
+};
+
+// An abstract factory for creating AsyncDnsResolverInterfaces. This allows
+// client applications to provide WebRTC with their own mechanism for
+// performing DNS resolution.
+class AsyncDnsResolverFactoryInterface {
+ public:
+  virtual ~AsyncDnsResolverFactoryInterface() = default;
+
+  // Creates an AsyncDnsResolver and starts resolving the name. The callback
+  // will be called when resolution is finished.
+  // The callback will be called on the sequence that the caller runs on.
+  virtual std::unique_ptr<webrtc::AsyncDnsResolverInterface> CreateAndResolve(
+      const rtc::SocketAddress& addr,
+      std::function<void()> callback) = 0;
+  // Creates an AsyncDnsResolver and starts resolving the name to an address
+  // matching the specified family. The callback will be called when resolution
+  // is finished. The callback will be called on the sequence that the caller
+  // runs on.
+  virtual std::unique_ptr<webrtc::AsyncDnsResolverInterface> CreateAndResolve(
+      const rtc::SocketAddress& addr,
+      int family,
+      std::function<void()> callback) = 0;
+  // Creates an AsyncDnsResolver and does not start it.
+  // For backwards compatibility, will be deprecated and removed.
+  // One has to do a separate Start() call on the
+  // resolver to start name resolution.
+  virtual std::unique_ptr<webrtc::AsyncDnsResolverInterface> Create() = 0;
+};
+
+}  // namespace webrtc
+
+#endif  // API_ASYNC_DNS_RESOLVER_H_