registerOnTouched abstract method
Set the function to be called when the control is touched.
A control's touched state is most commonly used to determine whether or not to show a validation error (for example, many forms would not want to show a required field as having an error before it's been interacted with).
Idiomatically, the control is considered touched once it has received a blur event. Certain controls, such as a dropdown select, may want to consider the control as touched if the user has opened and then closed the list of options.
See TouchHandler for the default idiomatic implementation.
Implementation
void registerOnTouched(TouchFunction f);