How to update activity views from a fragment recyclerView - android-activity

Question

I have MainActivity which has AppBar (with a "cart" TextView) and FrameLayout (with Fragment which contain a RecyclerView). This RecyclerView has a Button to increase Cart Value count. And I want that count to update Cart value in MainActivity.
I found this one but without full explanation "Updating views of Activity or Fragment from a RecyclerView adapter".

Answer 1

No better solution than this ViewModel
First of all add this dependency in build.gradle : implementation 'android.arch.lifecycle:extensions:1.1.1'
Your ViewModel class will look like this :
class MyViewModel: ViewModel() {
val mCartInfo: MutableLiveData<Int> = MutableLiveData()
private var cnt = 0
fun addToCartCounter() {
//TODO Apply your logic here
mCartInfo.value = cnt++
}
}
What is LiveData? See here
After that you've to initialize your ViewModel and observe the changed data, all these things are going to be done in your AppBar's Activity :
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val myViewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)
//These will observe any changes happen in ViewModel's MutableLiveData(it'll directly jump inside Observer)
//Observer is main important thing
myViewModel.mCartInfo.observe(this, Observer {
//tvCartCounter is Toolbar's TextView
tvCartCounter.text = it.toString()
})
}
}
Get ViewModel in your Fragment and your button click which calls a ViewModel's method:
class BlankFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// Inflate the layout for this fragment
val rootView = inflater.inflate(R.layout.fragment_blank, container, false)
//Get your Activity's ViewModel
val myViewModel = ViewModelProviders.of(requireActivity()).get(MyViewModel::class.java)
//Button click
rootView.btnAddToCart.setOnClickListener {
//It'll call ViewModel's method
myViewModel.addToCartCounter()
}
return rootView
}
}
Whenever addToCartCounter() calls on your button click it'll add value in MutableLiveData soon after it'll triggered your observer.