The observable variable begin with “is” can`t react the BR value#Android #DataBinding

hi,i am new to kotlin. i just add the databindng support. and the kotlin version is 1.2.30.

kapt 'com.android.databinding:compiler:3.1.2'

then i find out where time i define a observable variable, just like:

@get:Bindable
 var isA = true

it can`t react the BR value.