Use a callback to notify frame skip

A skipped frame is detected when the producer offers a frame while the
current pending frame has not been consumed.

However, the producer (in practice the decoder) is not interested in the
fact that a frame has been skipped, only the consumer (the renderer) is.

Therefore, notify frame skip via a consumer callback. This allows to
manage the skipped and rendered frames count at the same place, and
remove fps_counter from decoder.

app/src/decoder.c

@@ -11,22 +11,9 @@
 #include "util/buffer_util.h"
 #include "util/log.h"
 
-// set the decoded frame as ready for rendering, and notify
-static void
-push_frame(struct decoder *decoder) {
-    bool previous_frame_skipped;
-    video_buffer_producer_offer_frame(decoder->video_buffer,
-                                      &previous_frame_skipped);
-    if (previous_frame_skipped) {
-        fps_counter_add_skipped_frame(decoder->fps_counter);
-    }
-}
-
 void
-decoder_init(struct decoder *decoder, struct video_buffer *vb,
-             struct fps_counter *fps_counter) {
+decoder_init(struct decoder *decoder, struct video_buffer *vb) {
     decoder->video_buffer = vb;
-    decoder->fps_counter = fps_counter;
 }
 
 bool
@@ -66,7 +53,7 @@ decoder_push(struct decoder *decoder, const AVPacket *packet) {
                                 decoder->video_buffer->producer_frame);
     if (!ret) {
         // a frame was received
-        push_frame(decoder);
+        video_buffer_producer_offer_frame(decoder->video_buffer);
     } else if (ret != AVERROR(EAGAIN)) {
         LOGE("Could not receive video frame: %d", ret);
         return false;

app/src/decoder.h

@@ -10,14 +10,12 @@ struct video_buffer;
 
 struct decoder {
     struct video_buffer *video_buffer;
-    struct fps_counter *fps_counter;
 
     AVCodecContext *codec_ctx;
 };
 
 void
-decoder_init(struct decoder *decoder, struct video_buffer *vb,
-             struct fps_counter *fps_counter);
+decoder_init(struct decoder *decoder, struct video_buffer *vb);
 
 bool
 decoder_open(struct decoder *decoder, const AVCodec *codec);

app/src/scrcpy.c

@@ -346,7 +346,7 @@ scrcpy(const struct scrcpy_options *options) {
             file_handler_initialized = true;
         }
 
-        decoder_init(&decoder, &video_buffer, &fps_counter);
+        decoder_init(&decoder, &video_buffer);
         dec = &decoder;
     }
 

app/src/screen.c

@@ -204,6 +204,14 @@ on_frame_available(struct video_buffer *vb, void *userdata) {
     SDL_PushEvent(&new_frame_event);
 }
 
+static void
+on_frame_skipped(struct video_buffer *vb, void *userdata) {
+    (void) vb;
+
+    struct screen *screen = userdata;
+    fps_counter_add_skipped_frame(screen->fps_counter);
+}
+
 void
 screen_init(struct screen *screen, struct video_buffer *vb,
             struct fps_counter *fps_counter) {
@@ -213,9 +221,10 @@ screen_init(struct screen *screen, struct video_buffer *vb,
 
     static const struct video_buffer_callbacks cbs = {
         .on_frame_available = on_frame_available,
+        .on_frame_skipped = on_frame_skipped,
     };
 
-    video_buffer_set_consumer_callbacks(vb, &cbs, NULL);
+    video_buffer_set_consumer_callbacks(vb, &cbs, screen);
 }
 
 static inline SDL_Texture *

app/src/video_buffer.c

@@ -98,8 +98,7 @@ video_buffer_set_consumer_callbacks(struct video_buffer *vb,
 }
 
 void
-video_buffer_producer_offer_frame(struct video_buffer *vb,
-                                  bool *previous_frame_skipped) {
+video_buffer_producer_offer_frame(struct video_buffer *vb) {
     assert(vb->cbs);
 
     sc_mutex_lock(&vb->mutex);
@@ -113,14 +112,14 @@ video_buffer_producer_offer_frame(struct video_buffer *vb,
     video_buffer_swap_producer_frame(vb);
 
     bool skipped = !vb->pending_frame_consumed;
-    *previous_frame_skipped = skipped;
     vb->pending_frame_consumed = false;
 
     sc_mutex_unlock(&vb->mutex);
 
-    if (!skipped) {
-        // If skipped, then the previous call will consume this frame, the
-        // callback must not be called
+    if (skipped) {
+        if (vb->cbs->on_frame_skipped)
+            vb->cbs->on_frame_skipped(vb, vb->cbs_userdata);
+    } else {
         vb->cbs->on_frame_available(vb, vb->cbs_userdata);
     }
 }

app/src/video_buffer.h

@@ -49,6 +49,10 @@ struct video_buffer_callbacks {
     // video_buffer_consumer_take_frame(vb)
     // This callback is mandatory (it must not be NULL).
     void (*on_frame_available)(struct video_buffer *vb, void *userdata);
+
+    // Called when a pending frame has been overwritten by the producer
+    // This callback is optional (it may be NULL).
+    void (*on_frame_skipped)(struct video_buffer *vb, void *userdata);
 };
 
 bool
@@ -63,10 +67,8 @@ video_buffer_set_consumer_callbacks(struct video_buffer *vb,
                                     void *cbs_userdata);
 
 // set the producer frame as ready for consuming
-// the output flag is set to report whether the previous frame has been skipped
 void
-video_buffer_producer_offer_frame(struct video_buffer *vb,
-                                  bool *previous_frame_skipped);
+video_buffer_producer_offer_frame(struct video_buffer *vb);
 
 // mark the consumer frame as consumed and return it
 // the frame is valid until the next call to this function