Can not use BindingAdapter in Kotlin

Question

I used to create DataBindingAdapter for creating custom xml attributes in data binding.

object DataBindingAdapter {
    @BindingAdapter("android:src")
    fun setImageByRes(imageView: ImageView, @DrawableRes resId: Int) {
        imageView.setImageResource(resId)
    }
}

It was working well in Java. But not working in kotlin.

As I understand object in kotlin are similer to static method of Java. But its not working in data binding.

java.lang.IllegalStateException: Required DataBindingComponent is null in class FragmentBottomBarBinding. A BindingAdapter in acr.browser.lightning.utils.DataBindingAdapter is not static and requires an object to use, retrieved from the DataBindingComponent. If you don't use an inflation method taking a DataBindingComponent, use DataBindingUtil.setDefaultComponent or make all BindingAdapter methods static.

Answer 1

Just add the @Jvmstatic annotation on setImageByRes method.

object DataBindingAdapter {
    @BindingAdapter("android:src")
    @JvmStatic
    fun setImageByRes(imageView: ImageView, @DrawableRes resId: Int) {
        imageView.setImageResource(resId)
    }
}

as per the @Jvmstatic doc

Specifies that an additional static method needs to be generated from this element if it's a function. If this element is a property, additional static getter/setter methods should be generated.

In short method declared in one place and used from multiple languages of JVM. If you're calling a method from Java, then you should declare it as @JvmStatic, because adding the @JvmStatic annotation in one place will allow you to leave out multiple .Companion references in multiple places.

Answer 2

No. Object in kotlin is same like singleton. I think u dont need put it in Object. Just make new file lets say BindingAdapters.kt and u dont need write any class or object keywords.

It should look like this. Nothing else. If u need more functions just add it below this one. Again no class keyword or brackets are needed. It will be global function. Maybe u should also use ContextCompat for getting resource properly with context from imageView. And i would rather name it differently than android:src. What about imageResBinder

@BindingAdapter("imageResBinder")
fun setImageByRes(imageView: ImageView, @DrawableRes resId: Int) {
    imageView.setImageResource(resId)
}

and after that in your .xml file

<android.support.v7.widget.AppCompatImageView
    style="@style/Image.SomeImageStyle"
    app:imageResBinder="@{viewModel.getImageRes()}" />

Answer 3

in my case the reason was that I should have put the DataBinding setter method outside of the class