AudioCodingModule: Add methods for injecting external encoder stacks

webrtc/modules/audio_coding/include/audio_coding_module.h

Index: webrtc/modules/audio_coding/include/audio_coding_module.h
diff --git a/webrtc/modules/audio_coding/include/audio_coding_module.h b/webrtc/modules/audio_coding/include/audio_coding_module.h
index 9e7991f22f37930ea613c59ea702a9eb22210eba..55571f29d505197b764ee3b26f992e97edeb785e 100644
--- a/webrtc/modules/audio_coding/include/audio_coding_module.h
+++ b/webrtc/modules/audio_coding/include/audio_coding_module.h
@@ -207,6 +207,37 @@ class AudioCodingModule {
   virtual void RegisterExternalSendCodec(
       AudioEncoder* external_speech_encoder) = 0;
 
+  // Class for doing type erasure on a functor with signature
+  // AudioEncoder*(AudioEncoder*). Any functor with this signature will
+  // implicitly convert to EncoderVisitor.
+  class EncoderVisitor final {
+   public:
+    template <typename F>
+    EncoderVisitor(const F& f) : f_(&f), call_(Call<F>) {}
+    AudioEncoder* operator()(AudioEncoder* enc) const { return call_(f_, enc); }
+
+   private:
+    template <typename F>
+    static AudioEncoder* Call(const void* f, AudioEncoder* enc) {
+      return (*static_cast<const F*>(f))(enc);
+    }
+    const void* const f_;
+    AudioEncoder* (*const call_)(const void* f, AudioEncoder* enc);
+  };
+
+  // |ev| is called exactly once with one argument: the current encoder (or
+  // nullptr if there is no current encoder). For the duration of the call,
+  // |ev| has exclusive access to its argument. |ev|'s return value (which may
+  // be its argument, another AudioEncoder*, or nullptr) is the new current
+  // encoder. The encoder will be used as-is, without applying VAD/DTX or RED
+  // or anything.
+  virtual void VisitEncoder(const EncoderVisitor& ev) = 0;
+
+  // Utility method for simply replacing the existing encoder with a new one.
+  void SetEncoder(AudioEncoder* new_encoder) {
+    VisitEncoder([&](AudioEncoder* old_encoder) { return new_encoder; });
+  }
+
   ///////////////////////////////////////////////////////////////////////////
   // int32_t SendCodec()
   // Get parameters for the codec currently registered as send codec.
@@ -472,6 +503,29 @@ class AudioCodingModule {
   //
   virtual int RegisterReceiveCodec(const CodecInst& receive_codec) = 0;
 
+  // Class for doing type erasure on a functor with signature AudioDecoder*().
+  // Any functor with this signature will implicitly convert to DecoderFactory.
+  class DecoderFactory final {
+   public:
+    template <typename F>
+    DecoderFactory(const F& f) : f_(&f), call_(Call<F>) {}
+    AudioDecoder* operator()() const { return call_(f_); }
+
+   private:
+    template <typename F>
+    static AudioDecoder* Call(const void* f) {
+      return (*static_cast<const F*>(f))();
+    }
+    const void* const f_;
+    AudioDecoder* (*const call_)(const void* f);
+  };
+
+  // Register a decoder; call repeatedly to register multiple decoders. |df| is
+  // a decoder factory that returns an iSAC decoder; it will be called once if
+  // the decoder being registered is iSAC.
+  virtual int RegisterReceiveCodec(const CodecInst& receive_codec,
+                                   const DecoderFactory& df) = 0;
+
   // Registers an external decoder. The name is only used to provide information
   // back to the caller about the decoder. Hence, the name is arbitrary, and may
   // be empty.