Shared ViewModel to help communication between fragments and parent activity

问题

While Navigation component of JetPack looks pretty promising I got to a place where I could not find a way to implement something I wanted.

Let's take a look at a sample app screen:

The app has one main activity, a top toolbar, a bottom toolbar with fab attached. There are 2 challenges that I am facing and I want to make them the right way.

1. I need to implement fragment transactions in order to allow replacing the fragment on the screen, based on the user interaction. There are three ways I can think of and have this implemented:

• the callbacks way. Having a interface onFragmentAction callback in fragment and have activity implement it. So basically when user presses a button in FragmentA I can call onFragmentAction with params so the activity will trigger and start for example transaction to replace it with FragmentB
• implement Navigation component from JetPack. While I've tried it and seems pretty straightforward, I had a problem by not being able to retrieve the current fragment.
• Use a shared ViewModel between fragment and activity, update it from the fragment and observe it in the activity. This would be a "replacement" of the callbacks

2. Since the FAB is in the parent activity, when pressed, I need to be able to interact with the current visible fragment and do an action. For instance, add a new item in a recyclerview inside the fragment. So basically a way to communicate between the activity and fragment There are two ways I can think of how to make this

• If not using Navigation then I can use findFragmentById and retrieve the current fragment and run a public method to trigger the action.
• Using a shared 'ViewMode' between fragment and activity, update it from activity and observe it in the fragment.

So, as you can see, the recommended way to do navigation would be to use the new 'Navigation' architecture component, however, at the moment it lacks a way to retrieve the current fragment instance so I don't know how to communicate between the activity and fragment. This could be achieved with shared ViewModel but here I have a missing piece: I understand that fragment to fragment communication can be made with a shared ViewModel. I think that this makes sense when the fragments have something in common for this, like a Master/Detail scenarion and sharing the same viewmodel is very useful.

But, then talking between activity and ALL fragments, how could a shared ViewModel be used? Each fragment needs its own complex ViewModel. Could it be a GeneralViewModel which gets instantiated in the activity and in all fragments, together with the regular fragment viewmodel, so have 2 viewmodels in each fragment.

Being able to talk between fragments and activity with a viewmodel will make the finding of active fragment unneeded as the viewmodel will provide the needed mechanism and also would allow to use Navigation component.

Any information is gladly received.

Later edit. Here is some sample code based on the comment bellow. Is this a solution for my question? Can this handle both changes between fragments and parent activity and it's on the recommended side.

 private GlobalViewModel ():ViewModel(){

      var eventFromActivity:MutableLiveData<Event>
      var eventFromFragment:MutableLiveData<Event>


      fun setEventFromActivity(event:Event){
          eventFromActivity.value = event
      }

      fun setEventFromFragment(event:Event){
          eventFromFragment.value = event
      }

 } 

Then in my activity

class HomeActivity: AppCompatActivity(){

   onCreate{
       viewModel = ViewModelProviders.of(this, factory)
                .get(GlobalViewModel::class.java)

        viewModel.eventsFromFragment.observe(){
           //based on the Event values, could update toolbar title, could start
           // new fragment, could show a dialog or snackbar
        ....
        }   

     //when need to update the fragment do 
     viewModel.setEventFromActivity(event)
   }
}

Then in all fragments have something like this

class FragmentA:Fragment(){

  onViewCreated(){

       viewModel = ViewModelProviders.of(this, factory)
                .get(GlobalViewModel::class.java)

        viewModel.eventsFromActivity.observe(){
           // based on Event value, trigger a fun from the fragment 
        ....
        }

        viewModelFragment = ViewModelProviders.of(this, factory)
            .get(FragmentAViewModel::class.java)

        viewModelFragment.some.observe(){
        ....
        }   

  //when need to update the activity do 
     viewModel.setEventFromFragment(event)      
  }

}

来源：https://stackoverflow.com/questions/51518972/shared-viewmodel-to-help-communication-between-fragments-and-parent-activity