Attempt to workaround problem reported about empty plc.xml after unrelated crash. Now, if etree model is corrupted in a way tostring() would generate an empty string, exception prevents saving.

SVGHMI: JsonTable can now have clickable elements, that trigger a request with extra argument whose content is taken from Json data.

Merged

SVGHMI: cosmetic fix in test

All widgets reworked to use widget class and animate function if needed

SVGHMI: Moved JsonTable/Alarm related test in a separate page.

SVGHMI: JsonTable now ignores elements and groups starting with #

SVGHMI: Fix keypad : non-number input was always considered invalid because of missing type checking.

SVGHMI: JsonTable now updates range and position variables so that slider widget can update.

SVGHMI: Fix multiple variable support in widget

SVGHMI: Add TODOs in Button widgets.

SVGHMI: Alarm test not using Button widget anymore, too many problems. Use Input widget instead to increment value on each click and execute python code on change.

SVGHMI: little fix in HMI:Button

SVGHMI: JsonTable now use intermediate variables again to address JSON data without duplicating code or referencing. Using intermediate variables also alows to check for availability of data and stop evaluating early if data is missing. Finally added complete roundtrip example to illustrate use of JSonTable to display "alarms" collected in python from changes on PLC boolean variables.

SVGHMI: removed debug code forgotten here and there.