Another Delegates function is vetoable(). This also accepts a lambda expression, one that also takes a KProperty along with old and new values. This time, the return value of the lambda expression needs to be a Boolean, where true means "accept this change" and false means "reject this change". If false is returned, the property value will not be changed. So, in this example, we only accept even values, so setting the property to 5 quietly fails.

• the documentation
• Elements of Kotlin