examples: Add an example about using keyframes in python

diff --git a/examples/python/keyframes.py b/examples/python/keyframes.py
new file mode 100755 (executable)
index 0000000..05a4d03
--- /dev/null
+++ b/examples/python/keyframes.py
@@ -0,0 +1,87 @@
+#!/usr/bin/env python3
+#
+# GStreamer
+#
+# Copyright (C) 2019 Thibault Saunier <tsaunier@igalia.com>
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Library General Public
+# License as published by the Free Software Foundation; either
+# version 2 of the License, or (at your option) any later version.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Library General Public License for more details.
+#
+# You should have received a copy of the GNU Library General Public
+# License along with this library; if not, write to the
+# Free Software Foundation, Inc., 51 Franklin Street, Suite 500,
+# Boston, MA 02110-1335, USA.
+
+import gi
+import sys
+
+gi.require_version('Gst', '1.0')
+gi.require_version('GES', '1.0')
+gi.require_version('GstController', '1.0')
+
+from gi.repository import Gst, GES, GLib, GstController # noqa
+
+Gst.init(None)
+GES.init()
+
+
+def play_timeline(timeline):
+    pipeline = GES.Pipeline()
+    pipeline.set_timeline(timeline)
+    bus = pipeline.get_bus()
+    bus.add_signal_watch()
+    loop = GLib.MainLoop()
+    bus.connect("message", bus_message_cb, loop, pipeline)
+    pipeline.set_state(Gst.State.PLAYING)
+
+    loop.run()
+
+def bus_message_cb(unused_bus, message, loop, pipeline):
+    if message.type == Gst.MessageType.EOS:
+        print("eos")
+        pipeline.set_state(Gst.State.NULL)
+        loop.quit()
+    elif message.type == Gst.MessageType.ERROR:
+        error = message.parse_error()
+        pipeline.set_state(Gst.State.NULL)
+        print("error %s" % error[1])
+        loop.quit()
+
+if __name__ == "__main__":
+    if len(sys.argv) != 2:
+        print("You must specify a file path")
+        exit(-1)
+
+    timeline = GES.Timeline.new_audio_video()
+
+    layer = timeline.append_layer()
+    clip = GES.UriClip.new(Gst.filename_to_uri(sys.argv[1]))
+
+    # Adding clip to the layer so the TrackElements are created
+    layer.add_clip(clip)
+
+    # Create an InterpolationControlSource and make sure it interpolates linearly
+    control_source = GstController.InterpolationControlSource.new()
+    control_source.props.mode = GstController.InterpolationMode.LINEAR
+
+    # Set the keyframes
+    control_source.set(0, 0.0)  # Fully transparent at 0 second
+    control_source.set(Gst.SECOND, 1.0)  # Fully opaque at 1 second
+
+    # Get the video source
+    video_source = clip.find_track_element(None, GES.VideoSource)
+    assert(video_source)
+
+    # And set the control source on the "alpha" property of the video source
+    # Using a "direct" binding but "direct-absolute" would work the exact
+    # same way as the alpha property range is [0.0 - 1.0] anyway.
+    video_source.set_control_source(control_source, "alpha", "direct")
+
+    play_timeline(timeline)
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