Templated AudioDecoderFactory

webrtc/api/audio_codecs/audio_decoder_factory_template.h

+/*
+ *  Copyright (c) 2017 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 WEBRTC_API_AUDIO_CODECS_AUDIO_DECODER_FACTORY_TEMPLATE_H_
+#define WEBRTC_API_AUDIO_CODECS_AUDIO_DECODER_FACTORY_TEMPLATE_H_
+
+#include <memory>
+#include <vector>
+
+#include "webrtc/api/audio_codecs/audio_decoder_factory.h"
+#include "webrtc/base/scoped_ref_ptr.h"
+
+namespace webrtc {
+
+namespace audio_decoder_factory_template_impl {
+
+template <typename... Ts>
+struct Helper;
+
+// Base case: 0 template parameters.
+template <>
+struct Helper<> {
+  static void AppendSupportedDecoders(std::vector<AudioCodecSpec>* specs) {}
+  static bool IsSupportedDecoder(const SdpAudioFormat& format) { return false; }
+  static std::unique_ptr<AudioDecoder> MakeAudioDecoder(
+      const SdpAudioFormat& format) {
+    return nullptr;
+  }
+};
+
+// Inductive case: Called with n + 1 template parameters; calls subroutines
+// with n template parameters.
+template <typename T, typename... Ts>
+struct Helper<T, Ts...> {
+  static void AppendSupportedDecoders(std::vector<AudioCodecSpec>* specs) {
+    T::AppendSupportedDecoders(specs);
+    Helper<Ts...>::AppendSupportedDecoders(specs);
+  }
+  static bool IsSupportedDecoder(const SdpAudioFormat& format) {
+    auto opt_config = T::SdpToConfig(format);
+    return opt_config ? true : Helper<Ts...>::IsSupportedDecoder(format);
+  }
+  static std::unique_ptr<AudioDecoder> MakeAudioDecoder(
+      const SdpAudioFormat& format) {
+    auto opt_config = T::SdpToConfig(format);
+    return opt_config ? T::MakeAudioDecoder(*opt_config)
+                      : Helper<Ts...>::MakeAudioDecoder(format);
+  }
+};
+
+template <typename... Ts>
+class AudioDecoderFactoryT : public AudioDecoderFactory {
+ public:
+  std::vector<AudioCodecSpec> GetSupportedDecoders() override {
+    std::vector<AudioCodecSpec> specs;
+    Helper<Ts...>::AppendSupportedDecoders(&specs);
+    return specs;
+  }
+
+  bool IsSupportedDecoder(const SdpAudioFormat& format) override {
+    return Helper<Ts...>::IsSupportedDecoder(format);
+  }
+
+  std::unique_ptr<AudioDecoder> MakeAudioDecoder(
+      const SdpAudioFormat& format) override {
+    return Helper<Ts...>::MakeAudioDecoder(format);
+  }
+};
+
+}  // namespace audio_decoder_factory_template_impl
+
+// Make an AudioDecoderFactory that can create instances of the given decoders.
+//
+// Each decoder type is given as a template argument to the function; it should
+// be a struct with the following static member functions:
+//
+//   // Converts |audio_format| to a ConfigType instance. Returns an empty
+//   // optional if |audio_format| doesn't correctly specify an decoder of our
+//   // type.
+//   rtc::Optional<ConfigType> SdpToConfig(const SdpAudioFormat& audio_format);
+//
+//   // Appends zero or more AudioCodecSpecs to the list that will be returned
+//   // by AudioDecoderFactory::GetSupportedDecoders().
+//   void AppendSupportedDecoders(std::vector<AudioCodecSpec>* specs);
+//
+//   // Creates an AudioDecoder for the specified format. Used to implement
+//   // AudioDecoderFactory::MakeAudioDecoder().
+//   std::unique_ptr<AudioDecoder> MakeAudioDecoder(const ConfigType& config);
+//
+// ConfigType should be a type that encapsulates all the settings needed to
+// create an AudioDecoder.
+//
+// Whenever it tries to do something, the new factory will try each of the
+// decoder types in the order they were specified in the template argument
+// list, stopping at the first one that claims to be able to do the job.
+//
+// NOTE: This function is still under development and may change without notice.
+//
+// TODO(kwiberg): Point at CreateBuiltinAudioDecoderFactory() for an example of
+// how it is used.
+template <typename... Ts>
+rtc::scoped_refptr<AudioDecoderFactory> CreateAudioDecoderFactory() {
+  // There's no technical reason we couldn't allow zero template parameters,
+  // but such a factory couldn't create any decoders, and callers can do this
+  // by mistake by simply forgetting the <> altogether. So we forbid it in
+  // order to prevent caller foot-shooting.
+  static_assert(sizeof...(Ts) >= 1,
+                "Caller must give at least one template parameter");
+
+  return rtc::scoped_refptr<AudioDecoderFactory>(
+      new rtc::RefCountedObject<
+          audio_decoder_factory_template_impl::AudioDecoderFactoryT<Ts...>>());
+}
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_API_AUDIO_CODECS_AUDIO_DECODER_FACTORY_TEMPLATE_H_