title: how to use viewmodel singleton for activities?
body: <p>MyApp need hold a User object in whole context，A,B,C activities'xml use this User object,when A edit User,I want B and C notifyChange,how to deal this problem with databinding,livedata and viewModel?</p>
  
  <p>Formerly I make User.class extend BaseObservable,but POJO will be very troublesome and must not be a null,sometimes User maybe null such as not login.
  Now I Change to use LiveData, make Pojo simple and not extend BaseObservable,but when A edit,B and C not work,I think i need ABC use same viewModel instance in memory,but this will cause viewModel's onClear() trigger manytimes.</p>