I would like to pass a lambda from fragment A to fragment B when A transitions to B via a findNavController().navigate(R.id.action_a_to_b). The use case is B helps pick an item out to display on screen A.
Something like:
// In A
findNavController().navigate(R.id.action_a_to_b, configBlock: { fragmentB ->
fragmentB.itemSelectedCallback = this::itemSelected
})
I recognize this pattern doesn't quite fit with what Google is pushing (I assume they want shared observed view models with fragments not communicating between each other) but I am not looking to transition to that architecture style yet.
This is not yet possible, however, there is an existing feature request for being able to navigate for a result, which would let you get this type of functionality.
Related
I am searching for postSticky() method replacement. It is being used for simple passing value to previous Fragment, but thing is that I am using BackStackUtil for navigation so instance() method is being called when returning only if stack gets cleared somehow before getting back.Previous Fragment is holding List of items, when next Fragment can modify picked item and yet another one can do something else so it is chain of sticky events when each of these is being passed to previous Fragment. App structure won't let me to apply Coordinator pattern at current stage and also I don't want to attach Bundle to Fragments kept on stack. I was looking for solutions but I couldn't find any. I also don't want to store values in some static fields or SharedPreferences/Data storage.I was thinking about shared ViewModel but I don't really like this idea to be honest, so I would appreciate any ideas or just confirmation if shared VM is the only/best way.
Do you have any other ideas?
In your A fragment, before navigating to B fragment, listen to savedStateHandle:
findNavController()
.currentBackStackEntry
?.savedStateHandle?.getLiveData<Bundle>("DATA_KEY")
?.observe(viewLifecycleOwner) { result ->
// Result from fragment B
}
In your B fragment, before navigating back, set the data to pass to A fragment:
findNavController()
.previousBackStackEntry
?.savedStateHandle
?.set("DATA_KEY", result)
You can remove the observer using:
findNavController()
.currentBackStackEntry
?.savedStateHandle?.remove<Bundle>
Note that here the passed type is Bundle (the type in getLiveData<Bundle>) but you can use any type you want.
I have been facing issues while using live data and shared view model as a medium to interact between my fragments and activity. Here is the issue..
Activity A (Has a view model X shared across two fragments)
---Displays----> Fragment A on startup (dashboard type of view) --- on select in A (viewmodel updated)--> Live data Triggered
--view model X in the activity observes changes and adds Fragment B dynamically to the back stack-->
Fragment B is active now.
Couple of issues, that I'm facing
I see that, on back press from fragment B, back to Fragment A and vice versa, the previous value of the livedata is observed at the beginning before fetching the latest data.
On rotation/ state change, my activity observes for the fragment changes the second time ( kind of same as above)
Any workaround for this ? or is there anything that i'm missing
Thanks in advance..
as a quick workaround I went with this approach wherein I observe my changes only if my viewLifeCycleOwner is in a resumed state.
getViewLifeCycleOwner().getCurrentState() == Lifecycle.State.RESUMED.
Ideally it should be a single event approach, but yea for a quick fix this should work.
I am using Android Navigation Component in my project. What I am not satisfied is the fragment making the decision to do fragment transition to next fragment transition i.e
In my LoginFragment I have this -
viewModel.onLoginPressed(email, password)
.observe(viewLifecycleOwner, Observer {
if (it.userLoggedIn) {
activity?.findNavController(R.id.nav_host_fragment)
?.navigate(R.id.action_loginFragment_to_productsFragment)
}
})
According to me, the view must be dummy and must not do any such decisions of what to do on loginSuccess for example. The viewModel must be responsible for this.
How do I use this navigation component inside viewModel?
The ViewModel doesn't need to know about navigation, but it knows about events and state.
The communication should be more like:
NavHostActivity -> Fragment -> ViewModel
Your Fragment has Views, and click listeners, and state. The user types a user/password and presses a button. This onClick listener will tell the view model -> (pseudo-code) onUserPressedTheLoginButtonWith(username, password)
The ViewModel in turn will receive this, do what it needs (like check if you're already logged or whatever, perhaps the final decision is to navigate to another fragment).
What the ViewModel will do is expose a LiveData like
val navigationEvent = LiveData<...>
So the viewModel will navigationEvent.postValue(...)
The Fragment should observe this viewModel.navigationEvent.observe(...) { }
And in THERE in the fragment it can either navigate directly or -if you have an Interface- use it like:
yourNavigator.navigateTo(...) //either your VM knows the destination or the yourNavitagor has a navigateToLogin() concrete method, this is all "it depends what you like/prefer/etc.".
In summary
Activity Host contains Nav code, irrelevant.
Fragment(s) can communicate to a NavDelegate created by you (and likely injected) or Fragments simply know the details and do it by themselves.
Fragment(s) observe the navigation state/event from the viewModel.
Fragment(s) push events from the UI (clicks, actions, etc.) to this viewModel
The ViewModel decides what to do and updates the "liveData"(s) (there can be more than one type of thing you want to observe, not only navigation).
The Fragment(s) react to this observation and act accordingly, either doing it themselves or delegating (step 2 ^).
That's how I'd do it.
I am trying to use the Navigation architecture component in my toy app.
First I drew the fragments relationship in my "nav_graph.xml".
For example, I drew 3 fragments A, B, and C like below:
A -> B -> C
So I have 2 actions:
action_a_to_b
action_b_to_c
In general, I use the below code to move another fragment.
In A fragment,
findNavController().navigate(ADirections.actionAToB())
In B fragment,
findNavController().navigate(ADirections.actionBToC())
But you may know, there is another way to navigate.
The fragment id can be used to navigate directly like below:
findNavController().navigate(R.id.a)
In my case, I don't have the action for A to C fragment.
But if I use the below code in my A fragment, I can navigate!
findNavController().navigate(R.id.c)
Is it a bug? or intented?
This is intentional as per the documentation for navigate():
This supports both navigating via an action and directly navigating to a destination.
If you're using Safe Args, then only actions are supported. This ensures that you're only using the connections you've specified in your graph.
I have a workflow with 3 screens. From "screen 1" to access to "screen 2", the user must accept some kind of terms and conditions that I call in my picture "modal". But he only has to accept those conditions once. The next time he is on the first screen, he can go directly to screen 2. The user can chose to NOT accept the terms, and therefore we go back to "screen 1" and do not try to go to "screen 2".
I am wondering how to do it with the new navigation component.
Previously, what I would do it:
On screen 1, check if the user must accept the conditions
If no, start "screen 2" activity
If yes, use startActivityForResult() and wait result from the modal. Mark the terms as accepted. Start "screen 2"
But with the navigation graph, there is no way to start a Fragment to obtain a result.
I could mark the terms as accepted in the "modal" screen and start the "screen 2" from there. The thing is that to access to the screen 2, I need to do a network request. I do not want to duplicate the call to the API and processing its outcome in both "screen 1" and "modal".
Is there a way to go back from "modal" to "screen 1" with some information (user accepted the terms), using Jetpack navigation?
Edit: I currently get around it by using the same flow that Yahya suggests below: using an Activity just for the modal and using startActivityForResult from the "screen 1". I am just wondering if I could continue to use the navigation graph for the whole flow.
In the 1.3.0-alpha04 version of AndroidX Fragment library they introduced new APIs that allow passing data between Fragments.
Added support for passing results between two Fragments via new APIs on FragmentManager. This works for hierarchy fragments (parent/child), DialogFragments, and fragments in Navigation and ensures that results are only sent to your Fragment while it is at least STARTED. (b/149787344)
FragmentManager gained two new methods:
FragmentManager#setFragmentResult(String, Bundle) which you can treat similiarly to the existing Activity#setResult ;
FragmentManager#setFragmentResultListener(String, LifecycleOwner, FragmentResultListener) which allows you to listen/observe result changes.
How to use it?
In FragmentA add FragmentResultListener to the FragmentManager in the onCreate method:
setFragmentResultListener("request_key") { requestKey: String, bundle: Bundle ->
val result = bundle.getString("your_data_key")
// do something with the result
}
In FragmentB add this code to return the result:
val result = Bundle().apply {
putString("your_data_key", "Hello!")
}
setFragmentResult("request_key", result)
Start FragmentB e.g.: by using:
findNavController().navigate(NavGraphDirections.yourActionToFragmentB())
To close/finish FragmentB call:
findNavController().navigateUp()
Now, your FragmentResultListener should be notified and you should receive your result.
(I'm using fragment-ktx to simplify the code above)
There are a couple of alternatives to the shared view model.
fun navigateBackWithResult(result: Bundle) as explained here https://medium.com/google-developer-experts/using-navigation-architecture-component-in-a-large-banking-app-ac84936a42c2
Create a callback.
ResultCallback.kt
interface ResultCallback : Serializable {
fun setResult(result: Result)
}
Pass this callback as an argument (note it has to implement Serializable and the interface needs to be declared in its own file.)
<argument android:name="callback"
app:argType="com.yourpackage.to.ResultCallback"/>
Make framgent A implement ResultCallback, fragment B by will get the arguments and pass the data back through them, args.callback.setResult(x)
It looks like there isn't equivalent for startActivityForResult in Navigation Component right now. But if you're using LiveData and ViewModel you may be interested in this article. Author is using activity scoped ViewModel and LiveData to achieve this for fragments.
Recently (in androidx-navigation-2.3.0-alpha02 ) Google was released a correct way for achieve this behaviour with fragments.
In short: (from release note)
If Fragment A needs a result from Fragment B..
A should get the savedStateHandle from the currentBackStackEntry, call getLiveData providing a key and observe the result.
findNavController().currentBackStackEntry?.savedStateHandle?.getLiveData<Type>("key")?.observe(
viewLifecycleOwner) {result ->
// Do something with the result.
}
B should get the savedStateHandle from the previousBackStackEntry, and set the result with the same key as the LiveData in A
findNavController().previousBackStackEntry?.savedStateHandle?.set("key", result)
Related documentation
There is another alternative workaround. You can use another navigation action from your modal back to screen1, instead of using popBackStack(). On that action you can send whatever data you like to screen one. Use this strategy to make sure the modal screen isn't then kept in the navigation back stack: https://stackoverflow.com/a/54015319/4672107.
The only issue I see with this strategy is that pressing the back button will not send any data back, however most use cases require navigation after a specific user action and and in those situations, this workaround will work.
To get the caller fragment, use something like fragmentManager.putFragment(args, TargetFragment.EXTRA_CALLER, this), and then in the target fragment get the caller fragment using
if (args.containsKey(EXTRA_CALLER)) {
caller = fragmentManager?.getFragment(args, EXTRA_CALLER)
if (caller != null) {
if (caller is ResultCallback) {
this.callback = caller
}
}
}