Error running property change script

Ignition
1

I have a property change script running on a view custom property as follows:

if previousValue.value > 0 and currentValue.value == 0:
	system.perspective.closeDock('alarmBanner')

The intention is, when the number of alarms shown in the alarm dock goes from >0 to 0, I close the alarm dock. It works fine, but in the designer, every time I open the view, or every time the value changes, or every time I look at the change script, I get an error:

AttributeError: 'NoneType' object has no attribute 'value'

…which I would normally take to mean that either previousValue or currentValue are null - but these are arguments automatically generated by the change script, so I don’t see how they can be.

As I say - it works at runtime, I’m just wanting to understand why the designer repeatedly throws error messages at me.

2

previousValue is always None when the change event runs the first time (when initialChange is True).

1 Like
3

Ah, so the value of that custom property isn’t maintained when I close and reopen the display in the designer then. And when I edit the change script, it’s presumably reset as well. That makes sense. So this presumably I just need to preface the entire thing with if not initialChange:

4

It appears that initialChange is not one of the available parameters on a custom property change script, only on a tag change script. But if previousValue is not None: works just as well.