Material Button Toggle Group single selection

UPDATE :

app:selectionRequired="true" attribute is available as of version 1.2.0

Override the toggle() method of the MaterialButton class and use it instead of MaterialButton

import android.content.Context
import android.util.AttributeSet
import com.google.android.material.button.MaterialButton

class CustomMaterialToggleButton : MaterialButton {

    constructor(context: Context) : super(context)

    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)

    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(context, attrs, defStyleAttr)

    override fun toggle() {
        if (!isChecked) {
            super.toggle()
        }
    }
}

This will make sure that already checked button is not unchecked on single selection.

As of 1.2.0-alpha03 you can simply use the selectionRequired option:

<com.google.android.material.button.MaterialButtonToggleGroup
    android:id="@+id/toggle_button_group"
    app:singleSelection="true"
    app:selectionRequired="true">

</com.google.android.material.button.MaterialButtonToggleGroup>