doc: add documentation about creating custom GUI applications embedding BEC Widgets

docs/user/customisation.md

@@ -1,8 +1,118 @@
 (user.customisation)=
 # Customisation
 
-BEC Widgets are designed to be used with QtDesigner to quicly design GUI.
+## Leveraging BEC Widgets in custom GUI applications
 
+BEC Widgets can be used to compose a complete Qt graphical application, along with
+other QWidgets. The only requirement is to connect to BEC servers in order to get
+data, or to interact with BEC components. This role is devoted to the BECDispatcher,
+a singleton object which has to be instantiated **after the QApplication is created**.
+
+A typical BEC Widgets custom application "main" entry point should follow the template
+below: 
+
+```
+import argparse
+import sys
+
+from bec_widgets.utils.bec_dispatcher import BECDispatcher
+from qtpy.QtWidgets import QApplication
+
+# optional command line arguments processing
+parser = argparse.ArgumentParser(description="...")
+parser.add_argument( ...)
+...
+args = parser.parse_args()
+
+# creation of the Qt application
+app = QApplication([])
+
+# creation of BEC Dispatcher
+# /!\ important: after the QApplication has been instantiated
+bec_dispatcher = BECDispatcher()
+client = bec_dispatcher.client
+client.start()
+
+# (optional) processing of command line args,
+# creation of a main window depending on the command line arguments (or not)
+if args.xxx == "...":
+    window = ...
+ 
+# display of the main window and start of Qt event loop
+window.show()
+sys.exit(app.exec())
+```
+
+The main "window" object presents the layout of widgets to the user and allows
+users to interact. BEC Widgets must be placed in the window:
+
+```
+from qtpy.QWidgets import QMainWindow
+from bec_widgets.widgets import BECFigure
+
+window = QMainWindow()
+bec_figure = BECFigure(gui_id="my_gui_app_id")
+window.setCentralWidget(bec_figure)
+
+# prepare to plot samx motor vs bpm4i value
+bec_figure.plot(x_name="samx", y_name="bpm4i")
+```
+
+In the example just above, the resulting application will show a plot of samx
+positions on the horizontal axis, and beam intensity on the vertical axis
+(when the next scan will be started).
+
+It is important to ensure proper cleanup of the resources is done when application
+quits:
+
+```
+def final_cleanup():
+    bec_figure.clear_all()
+    bec_figure.client.shutdown()
+
+window.aboutToQuit.connect(final_cleanup)
+```
+
+Final example:
+
+```
+import sys
+from qtpy.QtWidgets import QMainWindow, QApplication
+from bec_widgets.widgets import BECFigure
+from bec_widgets.utils.bec_dispatcher import BECDispatcher
+
+# creation of the Qt application
+app = QApplication([])
+
+# creation of BEC Dispatcher
+bec_dispatcher = BECDispatcher()
+client = bec_dispatcher.client
+client.start()
+
+# creation of main window
+window = QMainWindow()
+
+# inserting BEC Widgets
+bec_figure = BECFigure(parent=window, gui_id="my_gui_app_id")
+window.setCentralWidget(bec_figure)
+
+bec_figure.plot(x_name="samx", y_name="bpm4i")
+
+# ensuring proper cleanup
+def final_cleanup():
+    bec_figure.clear_all()
+    bec_figure.client.shutdown()
+
+app.aboutToQuit.connect(final_cleanup)
+
+# execution
+window.show()
+sys.exit(app.exec())
+```
+
+## Writing applications using Qt Designer
+
+BEC Widgets are designed to be used with QtDesigner to quickly design GUI.
 
 ## Example of promoting widgets in Qt Designer