I am trying to do some validations using two fields scheduled date and assignee.
Sheduled date should be mandatory field if the user add any name in assignee field.
If there is no name in assignee field the system should allow to continue without scheduled date.
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Every User Interface input component has a "required" attribute. You can set this using a rule, and you can make the rule examine the value of the assignee field. Your rule can simply check if the assignee's value is null:
additional info: that this "required" parameter of an UI component is working properly you need to have a buttonwidget with the parameter "submit : true" or at least if the button parameter "submit" is false or not filled (default value is false) the paramater "validate" needs to be "true".