如何使用Android导航组件将值传递回之前的片段目的地？

Question

let sat I have some destinations like this

from fragment A --> to fragment B --> to fragment C

I can use Safe Args to pass data from fragment A to fragment B. and also using safe args from fragment B to fragment C.

what if I want to bring a string that generated in fragment C back to fragment B or to fragment A ?

to navigate from fragment C to fragment B, I use code:

Navigation.findNavController(fragmentView).navigateUp()

and to navigate from fragment C to fragment A, I use code:

Navigation.findNavController(view).popBackStack(R.id.fragmentA, false)

so how to pass a value back to previous fragment destination ? do I have to make an action back from fragment C to fragment B and pass the value through safe args ?

really need your help. Thank you very much :)

Answer 1

You can use the below snippet for sending data to previous fragment.

Fragment A observing the data:

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    findNavController().currentBackStackEntry?.savedStateHandle?.getLiveData<String>("key")?.observe(viewLifecycleOwner) { data ->
        // Do something with the data.
    }
}

Fragment B sending the data:

findNavController().previousBackStackEntry?.savedStateHandle?.set("key", data)
findNavController().popBackStack()

I also wrote extensions for this.

fun <T : Any> Fragment.setBackStackData(key: String,data : T, doBack : Boolean = true) {
    findNavController().previousBackStackEntry?.savedStateHandle?.set(key, data)
    if(doBack)
    findNavController().popBackStack()
}

fun <T : Any> Fragment.getBackStackData(key: String, result: (T) -> (Unit)) {
    findNavController().currentBackStackEntry?.savedStateHandle?.getLiveData<T>(key)?.observe(viewLifecycleOwner) {
        result(it)
    }
}

Usage:

Fragment A:

getBackStackData<String>("key") { data ->
    // Do something with the data.
}

Fragment B:

setBackStackData("key",data)

    

Note: I am using String as data. You may use any other type of variable.

Note: If you are going to use a Class as data, don't forget to add @Parcelize annotation and extend Parcelable .

Answer 2

You can create two buttons in every fragment, one to move to the next fragment (FragmentA to FragmentB) and others to move back (FragmentB to FragmentA). Then you'll be able to pass data from A to B and B to A back using safe args in the button onClick() method like

@Override
public void onClick(View view) {
int amount = 20;
ConfirmationAction action = SpecifyAmountFragmentDirections.confirmationAction();
action.setAmount(amount);
Navigation.findNavController(view).navigate(action);
}

Answer 3

i have just came across the same problem and have several options i can propose (currently considering myself which ones is best for my use case).

1. create a navigation action from fragment C back to fragment A. note that if that's all you do, your stack will now be: ABCA. to avoid this, you can use popUpTo and PopUpToInclusive . see the documentation .

the disadvantage of this method is that fragment A will be cleared no matter what, and would have to re-load (and fetch all its' data again).

2. use a shared ViewModel between for your fragments, and observe the required value with LiveData . in this scenario, fragment C will update a LiveData value in the shared ViewModel , which fragment A observes. now when you go back to fragment A, your observer will be triggered with the value set by fragment C.

3. similar to option 2, if you are using an external data source (like a Room database) you can observe changes using LiveData in fragment A. all fragment C has to do in this case is update the data source, and fragment A will automatically be updated through the LiveData