Where to place android BindingAdapter method?

You place it in your model class.

Example:

XML:

 <data>

    <variable
        name="item"
        type="com.yourpackage.Model"/>
      </data>
         ......

           <ImageView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:src="@{item.resId}"/>

Model:

public class Model {

    @DrawableRes
    private final int resId;

    public Model(int resId) {
        this.resId = resId;
    }

    public int getResId() {
        return resId;
    }

    @BindingAdapter ("android:src")
    public static void setImageResource(ImageView imageView, int resource){
        imageView.setImageResource(resource);
    }
}

After navigating through the internet I've finally found some info from one of the developers themselves. I wish they would have been more clear on the basics in the documentation.

Quote:

Binding adapters are annotated methods in any class that are used to do just this. Typically, you’d organize your adapters into [-a] classes based on the target View type.

This obviously means that at compile time all methods in any class with the annotation BindingAdapter will generate the BindingAdapter.