Issue with higher order function as a binding adapter

Question

I am running into issues trying to take a function as a parameter in a binding adapter using Kotlin/Android databinding. This example code throws e: error: cannot generate

Answer 1

The declaration () -> Unit suggests a function which takes no input and returns nothing (Unit is the return type in this statement). Your function should look like this:

fun testFunction() = {}

Answer 2

In the Event Handling section I came across this line:

In Listener Bindings, only your return value must match the expected return value of the listener (unless it is expecting void)

For more about error :

cannot generate view binders java.lang.StackOverflowError

read this article. hope it will help you!!

Answer 3

Put apply plugin: 'kotlin-kapt' in build.gradle

Then you can create Binding Adapter like

@JvmStatic
@BindingAdapter("onDelayedClick")
fun onDelayedClick(view: View, function: () -> Unit) {
    // TODO: Do something
}

And XML Like

<View
   android:id="@+id/test_view"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"
   app:onDelayedClick="@{viewModel.testFunction}"/>

And VM Like

val testFunction =  {
    // TODO: Do something
}

Answer 4

Use function: Runnable instead of function: () -> Unit.

Android's data-binding compiler generates java code, to which, your kotlin function's signature looks like void testFunction(), as kotlin adapts Unit as void when calling from java.

On the other hand, () -> Unit looks like kotlin.jvm.functions.Function0 which is a function which takes 0 inputs and returns Unit.INSTANCE.

As you can see these two function signatures don't match, and that's why the compilation fails.