/* inside gst_app_sink_class_init */
static void
gst_app_sink_class_init (GstAppSinkClass * klass)
{

  /* ... */

  gst_app_sink_signals[SIGNAL_TRY_PULL_SAMPLE] =
      g_signal_new ("try-pull-sample", G_TYPE_FROM_CLASS (klass),
      G_SIGNAL_RUN_LAST | G_SIGNAL_ACTION, G_STRUCT_OFFSET (GstAppSinkClass,
          try_pull_sample), NULL, NULL, NULL, GST_TYPE_SAMPLE, 1, /* WE WANT GstClockTime HERE */);

  /* ... */

}

/**
 * gst_app_sink_try_pull_sample:
 * @appsink: a #GstAppSink
 * @timeout: the maximum amount of time to wait for a sample
 *
 * This function blocks until a sample or EOS becomes available or the appsink
 * element is set to the READY/NULL state or the timeout expires.
 *
 * This function will only return samples when the appsink is in the PLAYING
 * state. All rendered buffers will be put in a queue so that the application
 * can pull samples at its own rate. Note that when the application does not
 * pull samples fast enough, the queued buffers could consume a lot of memory,
 * especially when dealing with raw video frames.
 *
 * If an EOS event was received before any buffers or the timeout expires,
 * this function returns %NULL. Use gst_app_sink_is_eos () to check for the EOS
 * condition.
 *
 * Returns: (transfer full): a #GstSample or NULL when the appsink is stopped or EOS or the timeout expires.
 * Call gst_sample_unref() after usage.
 */
GstSample *
gst_app_sink_try_pull_sample (GstAppSink * appsink, GstClockTime timeout)
{
  GstSample *sample = NULL;
  GstBuffer *buffer;
  GstAppSinkPrivate *priv;
  gint64 end_time;
  const gboolean timeout_valid = timeout != GST_CLOCK_TIME_NONE;

  g_return_val_if_fail (GST_IS_APP_SINK (appsink), NULL);

  if (timeout_valid)
    end_time =
        g_get_monotonic_time () + timeout / (GST_SECOND / G_TIME_SPAN_SECOND);

  priv = appsink->priv;

  g_mutex_lock (&priv->mutex);

  while (TRUE) {
    GST_DEBUG_OBJECT (appsink, "trying to grab a buffer");
    if (!priv->started)
      goto not_started;

    if (priv->num_buffers > 0)
      break;

    if (priv->is_eos)
      goto eos;

    /* nothing to return, wait */
    GST_DEBUG_OBJECT (appsink, "waiting for a buffer");
    if (timeout_valid) {
      if (!g_cond_wait_until (&priv->cond, &priv->mutex, end_time))
        goto expired;
    } else {
      g_cond_wait (&priv->cond, &priv->mutex);
    }
  }
  buffer = dequeue_buffer (appsink);
  GST_DEBUG_OBJECT (appsink, "we have a buffer %p", buffer);
  sample = gst_sample_new (buffer, priv->last_caps, &priv->last_segment, NULL);
  gst_buffer_unref (buffer);

  g_cond_signal (&priv->cond);
  g_mutex_unlock (&priv->mutex);

  return sample;

  /* special conditions */
expired:
  {
    GST_DEBUG_OBJECT (appsink, "timeout expired, return NULL");
    g_mutex_unlock (&priv->mutex);
    return NULL;
  }
eos:
  {
    GST_DEBUG_OBJECT (appsink, "we are EOS, return NULL");
    g_mutex_unlock (&priv->mutex);
    return NULL;
  }
not_started:
  {
    GST_DEBUG_OBJECT (appsink, "we are stopped, return NULL");
    g_mutex_unlock (&priv->mutex);
    return NULL;
  }
}