[pitivi] viewer: Fix preview flicker when trimming clip

[Alexandru Băluț <alexbalut src gnome org>]

commit 6d49ee544c28d666a38be2b12a302f4f93492ca4
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Thu Jan 17 00:49:01 2019 +0100

    viewer: Fix preview flicker when trimming clip
    
    When we show the video widget of the asset backing the trimmed clip,
    the first frame of the asset is shown for a brief moment until the
    initial seek is performed and the desired frame is shown. This causes an
    unwanted flicker.
    
    We have to prepare the trim preview video widget and switch the project
    video widget with it only when it's ready.

 pitivi/viewer/viewer.py | 41 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 37 insertions(+), 4 deletions(-)
---
diff --git a/pitivi/viewer/viewer.py b/pitivi/viewer/viewer.py
index 86f6ef17..73b34909 100644
--- a/pitivi/viewer/viewer.py
+++ b/pitivi/viewer/viewer.py
@@ -294,6 +294,14 @@ class ViewerContainer(Gtk.Box, Loggable):
             _("Detach the viewer\nYou can re-attach it by closing the newly created window."))
         bbox.pack_start(self.undock_button, False, False, 0)
 
+        self.show_all()
+
+        # Create a hidden container for the clip trim preview video widget.
+        self.hidden_chest = Gtk.Frame()
+        # It has to be added to the window, otherwise when we add
+        # a video widget to it, it will create a new window!
+        self.pack_end(self.hidden_chest, False, False, 0)
+
         self._haveUI = True
 
         # Identify widgets for AT-SPI, making our test suite easier to develop
@@ -307,7 +315,6 @@ class ViewerContainer(Gtk.Box, Loggable):
         self.undock_button.get_accessible().set_name("undock_button")
 
         self.buttons_container = bbox
-        self.show_all()
         self.external_vbox.show_all()
 
     def __corner_draw_cb(self, unused_widget, cr, lines, space, margin):
@@ -513,12 +520,39 @@ class ViewerContainer(Gtk.Box, Loggable):
         if not self.trim_pipeline:
             self.debug("Creating temporary pipeline for clip %s", clip.props.uri)
             self.trim_pipeline = AssetPipeline(clip)
-            video_sink, sink_widget = self.trim_pipeline.create_sink()
-            self.target.switch_widget(sink_widget)
+            unused_video_sink, sink_widget = self.trim_pipeline.create_sink()
+            # Add the widget to a hidden container and make it appear later
+            # when it's ready. If we show it before the initial seek completion,
+            # there is a flicker when the first frame of the asset is shown for
+            # a brief moment until the initial seek to the frame we actually
+            # want to show is performed.
+            # First make sure the container itself is ready.
+            widget = self.hidden_chest.get_child()
+            if widget:
+                self.warning("The previous trim preview video widget should have been removed already")
+                self.hidden_chest.remove(widget)
+            self.hidden_chest.add(sink_widget)
+            sink_widget.show()
+            self.trim_pipeline.connect("state-change", self._state_change_cb)
             self.trim_pipeline.setState(Gst.State.PAUSED)
+            self._last_trim_ns = 0
 
         self.trim_pipeline.simple_seek(position)
 
+    def _state_change_cb(self, trim_pipeline, state, prev_state):
+        if self.trim_pipeline is not trim_pipeline:
+            self.warning("State change reported for previous trim preview pipeline")
+            trim_pipeline.disconnect_by_func(self._state_change_cb)
+            return
+        # First the pipeline goes from READY to PAUSED, and then it goes
+        # from PAUSED to PAUSED, and this is a good moment.
+        if prev_state == Gst.State.PAUSED and state == Gst.State.PAUSED:
+            sink_widget = self.hidden_chest.get_child()
+            if sink_widget:
+                self.hidden_chest.remove(sink_widget)
+                self.target.switch_widget(sink_widget)
+            trim_pipeline.disconnect_by_func(self._state_change_cb)
+
     def clipTrimPreviewFinished(self):
         """Switches back to the project pipeline following a clip trimming."""
         if not self.trim_pipeline:
@@ -581,7 +615,6 @@ class ViewerWidget(Gtk.AspectFrame, Loggable):
         child = self.get_child()
         if child:
             self.remove(child)
-        widget.show_all()
         self.add(widget)
 
     def update_aspect_ratio(self, project):