Am I right that implementing interfaces by delegation is restricted to constructor fields? I'd quit...

# getting-started

[Rob Elliot]

Am I right that implementing interfaces by delegation is restricted to constructor fields?

I'd quite like to delegate to a method that returns an instance of the interface, as I can in Groovy.

[Rob Elliot]

Groovy:

class ThreadLocalDslContext implements DSLContext {

    private final ThreadLocal<DSLContext> threadLocalDslContext = new ThreadLocal<>()
    private final Database database

    ThreadLocalDslContext(Database database) {
        this.database = database
    }

    @Delegate
    private DSLContext getContext() {
        return threadLocalDslContext.get() ?: database.dsl
    }
}

Here ThreadLocalDslContext delegates to the result of getContext(). But I don't think I can convert this class to Kotlin without manually delegating all the methods on DSLContext to the result of getContext()?

[Rob Elliot]

Kotlin - does not compile:

class ThreadLocalDslContext(
    private val database: Database,
) : DSLContext by this::getDslContext {

    private val threadLocalDslContext: ThreadLocal<DSLContext?> = ThreadLocal<DSLContext?>()

    fun getDslContext()  = threadLocalDslContext.get() ?: database.dsl
}

[Rob Elliot]

(It's really irritating that IntelliJ can't even do Delegate Methods as it would for a Java class. And it takes about 3 GB of memory and 20 seconds for it to create the stub methods with TODO as their implementation!)

[ephemient]

https://youtrack.jetbrains.com/issue/KT-2747/Delegation-specifier-fails-to-refer-to-val-property-which-is-not-constructor-parameter