Observable property delegation in Kotlin - Step-by-Step Execution

Concept Flow - Observable property delegation

Declare observable property

↓

Set initial value

↓

Change property value

↓

Trigger observer lambda

↓

Observer reacts to change

↓

Property updated with new value

The observable property starts with an initial value. When you change it, a special observer function runs automatically, reacting to the change.

Execution Sample

Kotlin

import kotlin.properties.Delegates

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

name = "Jane"

This code creates an observable property 'name' starting as "John". When 'name' changes to "Jane", it prints the change.

Execution Table

Step	Action	Property Value	Observer Triggered	Output
1	Declare 'name' with initial value "John"	John	No
2	Assign 'name' = "Jane"	Jane	Yes	Property name changed from John to Jane
3	End of code	Jane	No

💡 No more code to run, program ends.

Variable Tracker

Variable	Start	After Step 2	Final
name	John	Jane	Jane

Key Moments - 2 Insights

Why does the observer lambda run only when the property value changes?

Because the observable delegate calls the observer lambda every time the property is assigned a value, as shown in step 2 of the execution_table.

What do the parameters 'prop', 'old', and 'new' represent in the observer lambda?

'prop' is the property metadata, 'old' is the previous value, and 'new' is the new value assigned. This is shown in the output message in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'name' after step 1?

A "John"

B "Jane"

C null

D Undefined

💡 Hint

Check the 'Property Value' column in row for step 1.

At which step does the observer lambda print the change message?

A Step 3

B Step 1

C Step 2

D Never

💡 Hint

Look at the 'Observer Triggered' and 'Output' columns in the execution_table.

If we assign 'name' = "Jane" again after step 2, what happens?

A Observer lambda triggers and prints change

B Observer lambda does not trigger

C Observer lambda triggers but no print

D Program crashes

💡 Hint

The observable delegate calls the observer lambda only when the property value actually changes. Assigning the same value does not trigger it.

Concept Snapshot

Observable property delegation in Kotlin:
- Use Delegates.observable(initialValue) { prop, old, new -> ... }
- The lambda runs every time the property is assigned a value
- 'prop' is property info, 'old' is old value, 'new' is new value
- Useful to react automatically when a property changes

Full Transcript

Observable property delegation lets you watch a property for changes. You declare a property with Delegates.observable and give it an initial value. When you assign a new value, the observer lambda runs automatically. It receives the property metadata, the old value, and the new value. This lets you react to changes, like printing a message. This is shown in the example where 'name' changes from "John" to "Jane" and the observer prints the change.