Observable property delegation in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an observable property with initial value 10.

Kotlin

var score: Int by [1](10) { prop, old, new ->
    println("Score changed from $old to $new")
}

Drag options to blanks, or click blank then click option'

ADelegates.observable

Blazy

Cval

Dlateinit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'lazy' instead of 'observable' delegate.

Trying to declare the property as 'val' which is immutable.

Using 'lateinit' which is not a delegate.

The Delegates.observable function creates an observable property that calls the lambda whenever the value changes.

2fill in blank

medium

Complete the code to print the old and new values when the property changes.

Kotlin

var name: String by Delegates.observable("John") { [1], old, new ->
    println("Name changed from $old to $new")
}

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Aproperty

Bprop

Cthis

Dobj

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' which refers to the enclosing class, not the property.

Using 'obj' which is not a standard parameter name here.

The lambda receives three parameters: the property, the old value, and the new value. The property parameter is commonly named prop.

3fill in blank

hard

Fix the error in the observable delegate usage.

Kotlin

var age: Int by Delegates.observable([1]) { prop, old, new ->
    println("Age changed from $old to $new")
}

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Anull

B"25"

C25

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string "25" instead of integer 25.

Using null which is not allowed here.

The initial value must be an Int, so 25 without quotes is correct. Quotes make it a String, causing a type error.

4fill in blank

hard

Fill both blanks to create an observable property that prints a message only when the new value is greater than the old value.

Kotlin

var level: Int by Delegates.observable(1) { prop, old, new ->
    if (new [1] old) {
        println("Level increased to $new")
    }
}

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A>

B<

C!=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' which triggers on decrease.

Using '==' which triggers only on equality.

The condition new > old checks if the new value is greater than the old one, triggering the print only on increase.

5fill in blank

hard

Fill the two blanks to create an observable property that prints the change only when the new value is different from the old value.

Kotlin

var status: String by Delegates.observable("idle") { [1], old, new ->
    if (new [2] old) {
        println("Status changed from $old to $new")
    }
}

Drag options to blanks, or click blank then click option'

Aprop

B!=

C==

Dproperty

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '!=' causing print on no change.

Using wrong parameter names causing errors.

The lambda receives three parameters: the property (named prop), the old value, and the new value. The condition new != old ensures printing only on actual changes.