How to reference implementation callback inside "run" or "apply" in Kotlin

Question

How to have the implementation reference inside a "run" or "apply"

class MyClass : MyCallback {

    ...
    fun myMethod() {
        val myObject = MyObject()
        myObject.run {
            setCallback(this) // <-- How to have the MyCallback's reference here?
        }
    }

    override fun onMyCallback() {
        // Do something
    }
}

Expressions with labels generally increase complexity and worsen the maintainability of the code. It will be blocked by detekt with LabeledExpression error. — Bruno Martins
– Bruno Martins, Commented Apr 11, 2019 at 13:59

recke96 · Accepted Answer · 2019-04-11 15:01:17Z

4

As run is a function that takes a lambda with a receiver (kotlin reference) in the block that is given to run this refers to the receiver, which is in your example myObject.

Therefore you need to use a qualified this-expression to refer to the enclosing MyClass-instance. As stated in the comments you need to change your code like this:

class MyClass : MyCallback {

    ...
    fun myMethod() {
        val myObject = MyObject()
        myObject.run {
            setCallback(this@MyClass) // <-- qualified this
        }
    }

    override fun onMyCallback() {
        // Do something
    }
}

If you want to avoid labels you have to replace the run function. A possible alternative is also which instead of using a lambda with receiver gets the object as parameter:

class MyClass : MyCallback {

    ...
    fun myMethod() {
        val myObject = MyObject()
        myObject.also { obj ->
            //lambda without receiver, this refers to the enclosing instance of MyClass
            setCallback(this)
        }
    }

    override fun onMyCallback() {
        // Do something
    }
}

edited Apr 11, 2019 at 15:01

answered Apr 11, 2019 at 13:35

recke96

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2 Comments

Bruno Martins Over a year ago

Expressions with labels generally increase complexity and worsen the maintainability of the code. It will be blocked by detekt with LabeledExpression error.

recke96 Over a year ago

It is still the only way to achieve what you want. The other possibility is to avoid run because it uses a lambda with receiver. A possible alternative is also.

Willi Mentzel · Accepted Answer · 2019-04-11 16:57:12Z

0

recke96 already pointed out two ways to get around that problem, but here is another. Just give this a name outside of the lambda.

class MyClass  {

    fun myMethod() {
        val myObject = MyObject()
        val callback = this // <--
        myObject.run {
            setCallback(callback)
        }
    }
    // ...
}

Or, using let

class MyClass  {

    fun myMethod() {
        val myObject = MyObject()

        let { callback ->
            myObject.run {
                setCallback(callback)
            }
        }
    }
    // ...
}

edited Apr 11, 2019 at 16:57

answered Apr 11, 2019 at 16:30

Willi Mentzel

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