[camera] Emit imageExposed signal at the right time. JB#45747

Droidmedia supports a callback to indicate the exact moment when the shutter fires, and this is passed on as a GstMessage by Gst-droid. There is already an imageExposed signal being emitted from QGstreamerCaptureSession, but it's only sent when the image is finished, right before imageCaptured. This class already parses GstMessages, so we can move the imageExposed signal to be sent when the shutter actually fires. This can trigger the shutter sound and animation in camera apps as intended.

src/plugins/gstreamer/camerabin/camerabinsession.cpp

@@ -960,11 +960,6 @@ bool CameraBinSession::processSyncMessage(const QGstreamerMessage &message)
             gst_buffer_unref(buffer);
 #endif
             if (!image.isNull()) {
-                static QMetaMethod exposedSignal = QMetaMethod::fromSignal(&CameraBinSession::imageExposed);
-                exposedSignal.invoke(this,
-                                     Qt::QueuedConnection,
-                                     Q_ARG(int,m_requestId));
-
                 static QMetaMethod capturedSignal = QMetaMethod::fromSignal(&CameraBinSession::imageCaptured);
                 capturedSignal.invoke(this,
                                       Qt::QueuedConnection,
@@ -987,6 +982,15 @@ bool CameraBinSession::processBusMessage(const QGstreamerMessage &message)
     GstMessage* gm = message.rawMessage();
 
     if (gm) {
+        if (GST_MESSAGE_TYPE(gm) == GST_MESSAGE_ELEMENT &&
+                        gst_structure_has_name(gst_message_get_structure(gm), "photo-capture-start")) {
+            static QMetaMethod exposedSignal =
+                            QMetaMethod::fromSignal(&CameraBinSession::imageExposed);
+            exposedSignal.invoke(this,
+                                 Qt::QueuedConnection,
+                                 Q_ARG(int,m_requestId));
+        }
+
         if (GST_MESSAGE_TYPE(gm) == GST_MESSAGE_ERROR) {
             GError *err;
             gchar *debug;