From fb9f9848bd2c5196a91bbe04fc1d22eacd7abe12 Mon Sep 17 00:00:00 2001
From: Romain Vimont <rom@rom1v.com>
Date: Fri, 19 Feb 2021 22:02:36 +0100
Subject: [PATCH] Use a callback to notify a new frame

Make the decoder independant of the SDL even mechanism, by making the
consumer register a callback on the video_buffer.
---
 app/src/decoder.c      |  6 ------
 app/src/screen.c       | 19 +++++++++++++++++++
 app/src/video_buffer.c | 26 +++++++++++++++++++++++++-
 app/src/video_buffer.h | 15 +++++++++++++++
 4 files changed, 59 insertions(+), 7 deletions(-)

diff --git a/app/src/decoder.c b/app/src/decoder.c
index 87cd0fe7..7b016d21 100644
--- a/app/src/decoder.c
+++ b/app/src/decoder.c
@@ -19,13 +19,7 @@ push_frame(struct decoder *decoder) {
                                       &previous_frame_skipped);
     if (previous_frame_skipped) {
         fps_counter_add_skipped_frame(decoder->fps_counter);
-        // the previous EVENT_NEW_FRAME will consume this frame
-        return;
     }
-    static SDL_Event new_frame_event = {
-        .type = EVENT_NEW_FRAME,
-    };
-    SDL_PushEvent(&new_frame_event);
 }
 
 void
diff --git a/app/src/screen.c b/app/src/screen.c
index 473d2e35..397aa2d1 100644
--- a/app/src/screen.c
+++ b/app/src/screen.c
@@ -191,12 +191,31 @@ screen_update_content_rect(struct screen *screen) {
     }
 }
 
+static void
+on_frame_available(struct video_buffer *vb, void *userdata) {
+    (void) vb;
+    (void) userdata;
+
+    static SDL_Event new_frame_event = {
+        .type = EVENT_NEW_FRAME,
+    };
+
+    // Post the event on the UI thread
+    SDL_PushEvent(&new_frame_event);
+}
+
 void
 screen_init(struct screen *screen, struct video_buffer *vb,
             struct fps_counter *fps_counter) {
     *screen = (struct screen) SCREEN_INITIALIZER;
     screen->vb = vb;
     screen->fps_counter = fps_counter;
+
+    static const struct video_buffer_callbacks cbs = {
+        .on_frame_available = on_frame_available,
+    };
+
+    video_buffer_set_consumer_callbacks(vb, &cbs, NULL);
 }
 
 static inline SDL_Texture *
diff --git a/app/src/video_buffer.c b/app/src/video_buffer.c
index e7e150a2..c145de08 100644
--- a/app/src/video_buffer.c
+++ b/app/src/video_buffer.c
@@ -43,6 +43,10 @@ video_buffer_init(struct video_buffer *vb, bool wait_consumer) {
     // there is initially no frame, so consider it has already been consumed
     vb->pending_frame_consumed = true;
 
+    // The callbacks must be set by the consumer via
+    // video_buffer_set_consumer_callbacks()
+    vb->cbs = NULL;
+
     return true;
 
 error_3:
@@ -82,9 +86,22 @@ video_buffer_swap_consumer_frame(struct video_buffer *vb) {
     vb->pending_frame = tmp;
 }
 
+void
+video_buffer_set_consumer_callbacks(struct video_buffer *vb,
+                                    const struct video_buffer_callbacks *cbs,
+                                    void *cbs_userdata) {
+    assert(!vb->cbs); // must be set only once
+    assert(cbs);
+    assert(cbs->on_frame_available);
+    vb->cbs = cbs;
+    vb->cbs_userdata = cbs_userdata;
+}
+
 void
 video_buffer_producer_offer_frame(struct video_buffer *vb,
                                   bool *previous_frame_skipped) {
+    assert(vb->cbs);
+
     sc_mutex_lock(&vb->mutex);
     if (vb->wait_consumer) {
         // wait for the current (expired) frame to be consumed
@@ -95,10 +112,17 @@ video_buffer_producer_offer_frame(struct video_buffer *vb,
 
     video_buffer_swap_producer_frame(vb);
 
-    *previous_frame_skipped = !vb->pending_frame_consumed;
+    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
+        vb->cbs->on_frame_available(vb, vb->cbs_userdata);
+    }
 }
 
 const AVFrame *
diff --git a/app/src/video_buffer.h b/app/src/video_buffer.h
index 6c5d197c..a41521a3 100644
--- a/app/src/video_buffer.h
+++ b/app/src/video_buffer.h
@@ -39,6 +39,16 @@ struct video_buffer {
 
     sc_cond pending_frame_consumed_cond;
     bool pending_frame_consumed;
+
+    const struct video_buffer_callbacks *cbs;
+    void *cbs_userdata;
+};
+
+struct video_buffer_callbacks {
+    // Called when a new frame can be consumed by
+    // 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);
 };
 
 bool
@@ -47,6 +57,11 @@ video_buffer_init(struct video_buffer *vb, bool wait_consumer);
 void
 video_buffer_destroy(struct video_buffer *vb);
 
+void
+video_buffer_set_consumer_callbacks(struct video_buffer *vb,
+                                    const struct video_buffer_callbacks *cbs,
+                                    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