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 view binders java.lang.StackOverflowError when building with no other useful info in the log.

Here is my binding adapter:

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

XML:

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

and method in my ViewModel:

fun testFunction() = Unit

I have been struggling with this for a bit now and nothing I've tried works, so any help is appreciated.

Try looking at the generated databinding code. I don't think what you are trying to achieve will work because you can't use lambdas — Chisko, Apr 11 '18 at 21:06
@Chisko are you sure that you can't use lambdas ? Then how works the default onClick? — Dex Sebas, Apr 18 '18 at 18:28
You absolutely can use lambdas and Kotlin function parameters, it's just the return type of `Unit` that causes an issue. — Eric Bachhuber, Apr 18 '18 at 19:39

score 15 · Accepted Answer · answered Apr 20 '18 at 17:20

15

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.

answered Apr 20 '18 at 17:20

Vibin

843
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18

4

And if we want to pass a parameter to that function? – Saman Sattari Feb 13 '19 at 14:51
if you need to pass parameters use SAM interfaces. See https://stackoverflow.com/a/54564239/6305235 – David Miguel Sep 30 '20 at 16:37

score 1 · Answer 2 · answered Dec 26 '18 at 19:41

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
}

score 0 · Answer 3 · answered Apr 20 '18 at 07:02

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!!

score -1 · Answer 4 · answered Apr 15 '18 at 07:58

-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() = {}

answered Apr 15 '18 at 07:58

Sir Codesalot

7,045
2
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The databinding compiler throws the same exception (cannot generate view binders java.lang.StackOverflowError) if I change the `Unit` return to `{}`. Just so you are aware, returning Unit is exactly the same as returning an empty block, you can check the Kotlin bytecode yourself to see that this is the case. I had just written it like that to keep the example as simple as possible. – Eric Bachhuber Apr 15 '18 at 22:06
@EricBachhuber Did you find a fix ? Because I saw other post saying that maybe a possible bug of kotlin / databinding. – Dex Sebas Apr 18 '18 at 18:29
@Dex Sebas no fix that I know of, as far as I know it's a bug in the databinding compiler. The workaround I'm using is to use a different return type in the parameter (i.e. `function: () -> Boolean` instead of `function: () -> Unit`), but it's hacky and requires changing the return type of the function that is being called which is not always possible (and poor design, imo). – Eric Bachhuber Apr 18 '18 at 19:43
And in your layout / kt file class you're binding like this ? "@{viewModel.itemClick()}" or "@{() -> viewModel.itemClick()}" /// class fun itemClick() : Boolean { Log.e("test", "test") return true } – Dex Sebas Apr 18 '18 at 19:45
I've tried both, but neither work. I opened an issue https://issuetracker.google.com/issues/78197871 – Eric Bachhuber Apr 18 '18 at 19:51
@Dex Sebas sorry, I misread your comment. For the workaround, parameter `function: () -> Boolean` `@{() -> viewModel.itemClick()}` and then `fun itemClick(): Boolean { return true }` would work fine (compiles without StackOverflowError and functions properly in the app) – Eric Bachhuber Apr 18 '18 at 20:02
Yes, I did fun itemClick() : Boolean { Log.e("test", "test") return true } but its not compiling. I added the code in the issueTracker of google. – Dex Sebas Apr 18 '18 at 20:04
Okay, I have not used multiple binding parameters (i.e. @BindingAdapter("adapter", "action") so cannot speak to that. But with just one parameter (i.e. `function () -> Boolean`) it works fine. Do you get a StackOverflowException as well? – Eric Bachhuber Apr 18 '18 at 20:08
Yes, seems like a bug maybe? Because it not works with multiple binding parameter. I just saw after a invalidate cache a error 'Cannot find the setter for attribute 'app:action' with parameter type lambda' but with only one parameter works fine (using Boolean and not Unit). Sad, it breaks my idea of making a masteradapter for recyclerView – Dex Sebas Apr 18 '18 at 20:10

Issue with higher order function as a binding adapter

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