safeUnbox() cannot be inverted

I haven't worked with Android Architecture Components or with the Data Binding libraries in this particular way, but I think I can still help.

Within your XML, you've got this:

android:checked="@={viewModel.value}"

The system is giving you a warning because it wants you to know that in the case where viewModel.value is null, it's going to do something special (behave as though it were false instead, presumably). It does this via the safeUnbox() method.

To solve the warning, it's suggesting making the safeUnbox() call explicit. You can't do that because there's no "inverse" of safeUnbox() to go back from boolean to Boolean.

But it doesn't sound like you have to use safeUnbox(); you could create your own method that converts Boolean to boolean, and then you could use the suggested annotation to declare which method will convert back from boolean to Boolean.

public class MyConversions {

    @InverseMethod("myBox")
    public static boolean myUnbox(Boolean b) {
        return (b != null) && b.booleanValue();
    }

    public static Boolean myBox(boolean b) {
        return b ? Boolean.TRUE : Boolean.FALSE;
    }
}

Now you can change your XML to:

android:checked="@={com.example.stackoverflow.MyConversions.myUnbox(viewModel.value)}"

I hope this helps. If it turns out that I'm way off-base, let me know; I'd love to learn more about this topic.

Most of what I have in this answer I learned from https://medium.com/google-developers/android-data-binding-inverse-functions-95aab4b11873

Came across this issue and found an easier solution. You can avoid this warning by creating a custom BindingAdapter for the boxed type like this:

@BindingAdapter("android:checked")
public static void setChecked(CompoundButton checkableView, Boolean isChecked) {
    checkableView.setChecked(isChecked != null ? isChecked : false);
}

This solution can be replicated to any property like visibility, enabled etc. and to any boxed primitive like Integer, Float etc.

You can also provide the value to be used in case your LiveData value is null like this:

@BindingAdapter(value={"android:checked", "nullValue"}, requireAll=false)
public static void setChecked(CompoundButton checkableView, Boolean isChecked, boolean nullValue) {
    checkableView.setChecked(isChecked != null ? isChecked : nullValue);
}

And call it like this:

<CheckBox
    ...
    android:checked='@{viewModel.value}'
    app:nullValue="@{false}"
    />

do like this in xml. Last line of code is the most important.

<layout>

  <data>
    <import type="android.view.View"/>
    <variable name="entryViewModel"
        type="com.craiovadata.groupmap.viewmodel.EntryViewModel" />
  </data>

  <Button 
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:visibility="@{safeUnbox(entryViewModel.showBtnMyGroups) ? View.VISIBLE : View.GONE, default = gone}" />

I find a code ,this this safeUnbox implementation。

/** @hide */
protected static int safeUnbox(java.lang.Integer boxed) {
    return boxed == null ? 0 : (int)boxed;
}

/** @hide */
protected static long safeUnbox(java.lang.Long boxed) {
    return boxed == null ? 0L : (long)boxed;
}

/** @hide */
protected static short safeUnbox(java.lang.Short boxed) {
    return boxed == null ? 0 : (short)boxed;
}

/** @hide */
protected static byte safeUnbox(java.lang.Byte boxed) {
    return boxed == null ? 0 : (byte)boxed;
}

/** @hide */
protected static char safeUnbox(java.lang.Character boxed) {
    return boxed == null ? '\u0000' : (char)boxed;
}

/** @hide */
protected static double safeUnbox(java.lang.Double boxed) {
    return boxed == null ? 0.0 : (double)boxed;
}

/** @hide */
protected static float safeUnbox(java.lang.Float boxed) {
    return boxed == null ? 0f : (float)boxed;
}

/** @hide */
protected static boolean safeUnbox(java.lang.Boolean boxed) {
    return boxed == null ? false : (boolean)boxed;
}

so ,if you value is not base type,you can not direct use safeUnbox,you should define a static function to safe unbox by youself.