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 class 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)
}
}
Related
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.
Long time ago the normal and suggested way on how to communicate between fragments were to create a interface that was extended in Activity. Basically, if I wanted to get data from Fragment B to Fragment C, I had to do Fragment B to MainActivity and then MainActivity to Fragment C. In my view - total mess.
Just started to use LiveData and thought of using it instead of callbacks - for instance:
FragmentInteraction.kt
val onOkayButtonClicked = SingleLiveEvent<Void>()
val onCancelButtonClicked = SingleLiveEvent<Void>()
FragmentA.kt
onOkayButtonClicked.call()
FragmentB.kt
onOkayButtonClicked.observe(viewLifecycleOwner, Observer {
// do whatever u want
})
Does this approach has any down-sides? Is there an easier / more elegant way to approach this?
You could have only one ViewModel(lifecycleOwner = activity), that would have all the liveDataEvents you need for the interactions. You will act on the liveData from the onClick events:
okayButton.setOnClickListener { viewModel.onOkayButtonClicked() }
This LiveData will be observed by the NavigationComponent, or similar you are using for handling fragment transactions, and it will do the transitions for you.
All these transitions I recommend doing them on the Activity level.
how to make button to open another fragment. being within a fragment. kotlin
I'm starting in kotlin and I'm having a hard time trying to open a fragment with a button, how do I?
You need to use FragmentManager and FragmentTransaction to add your fragment on the fly. you can call a function similar to this in your button's onClick method. But it is recommended for the parent activity to handle each fragment's lifecycle and the fragments
are not supposed to interact each other. The following is taken from the developer docs, that can be found here.
"Often you will want one Fragment to communicate with another, for example to change the content based on a user event. All Fragment-to-Fragment communication is done either through a shared ViewModel or through the associated Activity. Two Fragments should never communicate directly."
fun createFragmentonTheFly(){
var mFragmentTransaction: FragmentTransaction = getSupportFragmentManager().beginTransaction()
mFragmentTransaction.add(R.id.fr_container,new ProductListFragment())
mFragmentTransaction.commit()
}
The best way to do it would be to add an interface let say onFragmentDetachedLisetner and add one method replaceFragment() or something and make your Activity implement this interaface and had it replace as soon as the fragment is detached and make your fragment that contains your button finish itself when user clicks the button, then your activity will replace it with the one you wanted to start. And also consider reusing fragments, as that is the main purpose of fragments at the first place.
I want to separate logic fragment from activity but the problem is I make api call and save data in fragment. And when user click a item in fragment. I need to send parcelable data to other fragment to show detail info about item.
Is launching fragment in fragment anti pattern for android ?
I would like to hear some opinion about this matter.
Yes, is totally an anti-pattern, remember that you need to see the Activity as a container and fragments as independent sub-screens, so is the Activity responsibility to manage the fragments. I.e.: If you have a Post activity you can have a PostText fragment, a PostImage fragment and all of that is manage by the activity, every fragment is attached to an Activity.
It is not a common practice to have a nested fragment inside a fragment even it can be done. However, it would be better to have an activity as the centric container for all your fragments. You can use EventBus (GreenRobot / Otto) to separate the concerns and do all the API calls in another class and send the results by subscribing to this event.
I have 4 tabs in an Activity.
Each of them is a Fragment. And every Fragment has a ListView.
So, if i change the ListView in Fragment, it must change the ListView in all other Fragments ie.., Tabs.
The problem i face is while creating the interface instance.
It takes it's own onClick() method.
In case i want a callback to the parent activity i could have done that by overriding onAttach. But how to make a callback to a Fragment?
From Developers site:
Often you will want one Fragment to communicate with another, for example to change the content based on a user event. All Fragment-to-Fragment communication is done through the associated Activity. Two Fragments should never communicate directly.
So, make a callback to the Activity which in turn makes a callback to other fragments??
Thank You
It's pretty simple,all You need is steps below:
1) From onClick method in your first fragment make a function call of activity:
((IYourActivityInterface) getActivty()).activityMethod();
2) In your activity find fragment by tag or id and run it's method:
public void activityMethod(){
Fragment tabFragment = getFragmentManager().findFragmentByTag("second_fragment");
// or Fragment tabFragment = getFragmentManager().findFragmentById(R.id.frag);
if (tabFragment!=null){
((IFragmentInterface) tabFragment).fragmentMethod();
}
}
Hope this is what you are looking for.)