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>
+class AudioDecoderFactoryT : public AudioDecoderFactory {
+ public:
+  std::vector<AudioCodecSpec> GetSupportedDecoders() override {
+    static const std::vector<AudioCodecSpec> specs = [] {
+      std::vector<AudioCodecSpec> specs;
+      DoAppendSupportedDecoders(Dummy<Ts...>(), &specs);
+      return specs;
+    }();
+    return specs;
+  }
+
+  bool IsSupportedDecoder(const SdpAudioFormat& format) override {
+    return DoIsSupportedDecoder(Dummy<Ts...>(), format);
+  }
+
+  std::unique_ptr<AudioDecoder> MakeAudioDecoder(
+      const SdpAudioFormat& format) override {
+    return DoMakeAudioDecoder(Dummy<Ts...>(), format);
+  }
+
+ private:
+  // Dummy type that's used to ensure we call the proper overloads.
+  template <typename...>

[ossu, 2017/06/16 11:25:18]

As with the encoder factory, this one could do without the extra parameter.

[kwiberg-webrtc, 2017/06/16 13:17:06]

Yes. The decoder factory is very similar to the encoder factory, so I'll
transfer over all the changes once we're done with the encoder factory.

+  struct Dummy {};
+
+  static void DoAppendSupportedDecoders(Dummy<>,
+                                        std::vector<AudioCodecSpec>* specs) {}
+
+  template <typename U, typename... Us>
+  static void DoAppendSupportedDecoders(Dummy<U, Us...>,
+                                        std::vector<AudioCodecSpec>* specs) {
+    U::AppendSupportedDecoders(specs);
+    DoAppendSupportedDecoders(Dummy<Us...>(), specs);
+  }
+
+  bool DoIsSupportedDecoder(Dummy<>, const SdpAudioFormat& format) {
+    return false;
+  }
+
+  template <typename U, typename... Us>
+  bool DoIsSupportedDecoder(Dummy<U, Us...>, const SdpAudioFormat& format) {
+    auto opt_config = U::SdpToConfig(format);
+    return opt_config ? true : DoIsSupportedDecoder(Dummy<Us...>(), format);
+  }
+
+  std::unique_ptr<AudioDecoder> DoMakeAudioDecoder(
+      Dummy<>,
+      const SdpAudioFormat& format) {
+    return nullptr;
+  }
+
+  template <typename U, typename... Us>
+  std::unique_ptr<AudioDecoder> DoMakeAudioDecoder(
+      Dummy<U, Us...>,
+      const SdpAudioFormat& format) {
+    auto opt_config = U::SdpToConfig(format);
+    if (opt_config) {
+      std::unique_ptr<AudioDecoder> dec = U::MakeAudioDecoder(*opt_config);
+      RTC_CHECK(dec);
+      return dec;
+    } else {
+      return DoMakeAudioDecoder(Dummy<Us...>(), 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);

[the sun, 2017/06/16 12:52:21]

Inconsistent with the decoder impls which use by-value semantics so far. Maybe
just stick to const& everywhere?

[kwiberg-webrtc, 2017/06/16 13:17:06]

It's just an implementation detail. const ref is probably the default we want
people to use if they aren't thinking about it, but by-value is a better fit for
passing around empty structs, so I used that in some implementations. If it
bothers you I can change the decoder implementations to const ref like you
suggest, though.

+//
+// ConfigType should be a type that encapsulates all the settings needed to
+// create an AudioDecoder.
+//
+// NOTE: This function is still under development and may change without notice.

[ossu, 2017/06/16 11:25:18]

Should probably mention that the order of the codecs matter here as well.

[kwiberg-webrtc, 2017/06/16 13:17:06]

Yes, that'll be among the things copied over from the encoder factory.

+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_