How does `val answer: String by project` work? `Project` interface doesn’t have `getValue()` and `se...

# gradle

[electrolobzik]

How does val answer: String by project work? Project interface doesn’t have getValue() and setValue(), so how does it work as a delegate then?

[Sam]

It uses this extension function: https://github.com/gradle/gradle/blob/344201c165b776ad8f07a21b21942a9780351b0f/sub[…]-dsl/src/main/kotlin/org/gradle/kotlin/dsl/ProjectExtensions.kt

/**
 * Locates a property on [Project].
 */
operator fun Project.provideDelegate(any: Any?, property: KProperty<*>): PropertyDelegate =
    propertyDelegateFor(this, property)

[Sam]

The mechanism is this: https://kotlinlang.org/docs/delegated-properties.html#providing-a-delegate

[electrolobzik]

@Sam thank you!

[electrolobzik]

@Sam where Project.provideDelegate is called? I don’t follow :)

[Sam]

If the object used on the right-hand side of by defines provideDelegate as a member or extension function, that function will be called to create the property delegate instance.

(https://kotlinlang.org/docs/delegated-properties.html#providing-a-delegate)

[Sam]

It just happens automatically if provideDelegate exists (and is imported)

[Sam]

If getValue exists, then properties using by will automatically call through to getValue(...). If provideDelegate exists, then properties using by will instead call provideDelegate(...).getValue(...). 🧙

[Sam]

The docs give some examples of why you might want to use the latter instead of the former

[electrolobzik]

@Sam I see. provideDelegate() references to propertyDelegateFor() . Where it is located?

[electrolobzik]

I don’t see such method in Project or in ProjectDelegate .

[Sam]

https://github.com/gradle/gradle/blob/344201c165b776ad8f07a21b21942a9780351b0f/sub[…]n-dsl/src/main/kotlin/org/gradle/kotlin/dsl/PropertyDelegate.kt

[electrolobzik]

Much more clear now, thanks!

[Sam]

🐕