How do I set the selected state of an image button with xml

A newer solution to the problem would be to use DataBinding.

1. Create a @BindingAdapter (somewhere in your code):

@BindingAdapter("is_selected")
fun setSelected(view: View, selected: Boolean) {
    view.isSelected = selected
}

2. Use DataBinding in your xml

<layout xmlns:android="http://schemas.android.com/apk/res/android">

    <data>
    </data>

    <ImageButton
        android:id="@+id/player_ctrl_btn"
        is_selected="@{true}"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:background="@drawable/opaque"
        android:gravity="center_horizontal"
        android:padding="0px"
        android:src="@drawable/playpause"
        android:text="Play" />
</layout>

---

IMPORTANT: remember that you need to use DataBinding here. So you cannot simply call add is_selected="true" (because it will fail not telling you about it), you need to add is_selected="@{true}"

No need to create BindingAdapter method, just enable DataBidning and type:

<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">

<data>
</data>

<ImageButton
    android:id="@+id/player_ctrl_btn"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:background="@drawable/opaque"
    android:gravity="center_horizontal"
    android:padding="0px"
    android:src="@drawable/playpause"
    android:text="Play"
    app:selected="@{true}" />
</layout>

It doesn't look like you can -- sorry!