Android Development with Kotlin

The vetoable delegate

The vetoable function is a standard library property delegate that works in a similar way to the observable delegate, but with two main differences:

• The lambda from an argument is called before a new value is set
• It allows the lambda function from a declaration to decide if a new value should be accepted or rejected

For example, if we have an assumption that the list must always contain a larger number of items than the old one, then we will define the following vetoable delegate:

var list: List<String> by Delegates.vetoable(emptyList()) { _, old, new ->  
   new.size > old.size 
} 

If a new list does not contain a larger number of items than the old one, then the value will not change. So, we can treat vetoable like observable ...