Name conflicts when using Method extension and Inheritance

Question

I have an extension

fun Fragment.showToast(message: String, toastLength: Int = Toast.LENGTH_SHORT) {
    Toast.makeText(context, message, toastLength).show()
}

In project we're using MVP:

interface MyContract {
    interface View {
        fun showToast(message: String)
    }
}

class MyFragment : Fragment(), MyContract.View {
    override fun showToast(message: String) {
        showToast(message)
    }
}

So in MyFragment.showToast(message) I expect to call extension function instead of StackOverflowException.

Is it possible to call extension function directly? Something like:

Fragment.showToast(this, message)

or I just need to give another name?

Try calling `this.showToast(message)` in overridden method in your fragment. — Jeel Vankhede, Feb 18 '19 at 12:04
Still doesn't work. `this`, `this@MyFragment` are both references to `MyFragment` — Taras Parshenko, Feb 18 '19 at 12:11

score 2 · Accepted Answer · answered Feb 18 '19 at 13:21

You could cast your MyFragment class to Fragment and do:

class MyFragment : Fragment(), MyContract.View {
    override fun showToast(message: String) {
        val fragment = this as Fragment
        fragment.showToast(message)
    }
}

This way you will refer to the extension function of the Fragment class

score 2 · Answer 2 · edited Feb 18 '19 at 19:35

Instance methods have precedence over extension functions, in case of signature conflict, so the behaviour you're experiencing is the expected one.

You can refer directly to shadowed extension function importing it with an alias: pretend you have a file FragmentExt.kt containing the extension method in a package named mypkg:

package mypkg

fun Fragment.showToast(message: String, toastLength: Int = Toast.LENGTH_SHORT) {
    Toast.makeText(context, message, toastLength).show()
}

So, you can invoke extension function (avoiding to call instance method) as follows:

import mypkg.showToast as extShowToast
val fragment : Fragment = ...
fragment.extShowToast(message)

This approach works both in the MyFragment subclass and in any client class.

In Java code, you can do something similar by accessing Kotlin extension method as a static method on a class names by the file name: if your extension function is in FragmentExt.kt, Java code can point to extension method using FragmentExtKt.showToats("Message here").

score 1 · Answer 3 · answered Feb 18 '19 at 13:19

1

You've got two methods that you can call on a Fragment instance with the exact same name and the parameters.

There's no way to distinguish which one you want to call.

I'd change the name of one.

E.g.

fun Fragment.toast(message: String, toastLength: Int = Toast.LENGTH_SHORT) {
    Toast.makeText(context, message, toastLength).show()
}

answered Feb 18 '19 at 13:19

TomH

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you can distinguish between instance and extension method using an import with alias, as I showed in my response... I think this can solve the problem without renaming the extension function, as in your proposal... – Pietro Martinelli Feb 18 '19 at 13:40

Name conflicts when using Method extension and Inheritance

3 Answers3