vetoable

Learn vetoable properties with vetoable.

vetoable

vetoable lets you reject an assignment to a property based on a condition. Example: var age by Delegates.vetoable(0) { _, _, new -> new >= 0 }. If the lambda returns true the assignment is accepted; if it returns false the assignment is rejected and the value stays unchanged.

This is useful for protecting invariants: age cannot be negative, email cannot be empty and so on. observable accepts every assignment; vetoable validates first. Both come from the Delegates object and use a similar lambda signature.

How vetoable works

The vetoable lambda takes three parameters: the property reference, the old value and the new value. The lambda returns a Boolean. If true the new value is assigned; if false the old value is kept.

Example: var age by Delegates.vetoable(0) { _, _, new -> new >= 0 }. age = 25 is accepted (25 >= 0). age = -5 is rejected (-5 < 0); age stays 25. This is a clean way to protect a business rule in one line. Compare with observable: observable always accepts, vetoable is selective.

Exercise Validate age with vetoable

Add import kotlin.properties.Delegates. Define var age by Delegates.vetoable(0) { _, _, new -> new >= 0 }. In main assign age = 25 and call println(age). Then assign age = -5 and call println(age) again.

Show example solution

import kotlin.properties.Delegates

var age by Delegates.vetoable(0) { _, _, new -> new >= 0 }

fun main() {
    age = 25
    println(age)
    age = -5
    println(age)
}

Test yourself

What does vetoable do?
- Can reject by condition
- Never changes
- Always accepts
- Only reads
Show answer

Can reject by condition — vetoable rejects the assignment when the lambda returns false.

Complete the vetoable declaration

Delegates.____(0) { _, _, new -> new >= 0 }

Show answer

Delegates.vetoable(0) { _, _, new -> new >= 0 }

Match the vetoable concepts

vetoable

Can reject
observable

Tracks
true

Accept
false

Reject

Next lesson: Custom delegates

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