using Android Jetpack Navigation "by navGraphViewModels" API causes crash when you enable the option of "dont keep activity" [213504272]

Fixed

Bug

Status Update

No update yet.

Description

ch...@gmail.com

created issue #1

Jan 7, 2022 07:27AM

I have two Activities, MainActivity and SecondActivity. Using Navigation architecture in MainActivity, which has two fragments, such as HomeFragment and DetailFragment, the Navigation diagram is declared as follows:

<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/navigation_home_flow"
    app:startDestination="@id/home_root_fragment">

    <fragment
        android:id="@id/home_root_fragment"
        android:name=".home.HomeFragment"
        android:label="HomeFragment">

    </fragment>

    <fragment
        android:id="@id/detail_fragment"
        android:name=".home.DetailFragment"
        android:label="DetailFragment">
    </fragment>
    
</navigation>

Generate ViewModels in the DetailFragment onCreate with "by navGraphViewModels".

class BaseImmerseFragment : BaseFragment() {

    companion object {
        const val TAG = "DetailFragment"
    }

    val vm by navGraphViewModels<ViewModel>(R.id.navigation_home_flow)


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        //init vm
        LogUtils.d(TAG, "init $vm")
    }
}

There is a button in DetailFragment, clicking it will jump to SecondActivity, when "Do not keep background Activity" is enabled, pressing the back button in SecondActivity will trigger the program crash：

Process: xxxxx, PID: 24236
java.lang.RuntimeException: Unable to start activity ComponentInfo{xxxxxx/xxxx.MainActivity}: java.lang.IllegalStateException: You cannot access the NavBackStackEntry's ViewModels until it is added to the NavController's back stack (i.e., the Lifecycle of the NavBackStackEntry reaches the CREATED state).
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3739)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3892)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2267)
    at com.didichuxing.doraemonkit.kit.timecounter.instrumentation.ProxyHandlerCallback.handleMessage(ProxyHandlerCallback.java:44)
    at android.os.Handler.dispatchMessage(Handler.java:103)
    at android.os.Looper.loop(Looper.java:217)
    at android.app.ActivityThread.main(ActivityThread.java:7993)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:502)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:955)
 Caused by: java.lang.IllegalStateException: You cannot access the NavBackStackEntry's ViewModels until it is added to the NavController's back stack (i.e., the Lifecycle of the NavBackStackEntry reaches the CREATED state).
    at androidx.navigation.NavBackStackEntry.getViewModelStore(NavBackStackEntry.kt:174)
    at xxxxx.DetailFragment$special$$inlined$navGraphViewModels$default$2.invoke(NavGraphViewModelLazy.kt:59)
    at xxxxx.DetailFragment$special$$inlined$navGraphViewModels$default$2.invoke(NavGraphViewModelLazy.kt:58)
    at androidx.lifecycle.ViewModelLazy.getValue(ViewModelProvider.kt:53)
    at androidx.lifecycle.ViewModelLazy.getValue(ViewModelProvider.kt:41)
    at xxxxx.DetailFragment.getVm(DetailFragment.kt:50)
    at xxxxx.DetailFragment.onCreate(DetailFragment.kt:102)
    at androidx.fragment.app.Fragment.performCreate(Fragment.java:2947)
    at androidx.fragment.app.FragmentStateManager.create(FragmentStateManager.java:455)
    at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:255)
    at androidx.fragment.app.FragmentStore.moveToExpectedState(FragmentStore.java:113)
    at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1331)
    at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:2772)

If I put the vm creation declaration in the onViewCreated lifecycle or later, there will be no crashes.

Does "by navGraphViewModels" necessarily require the user to use it this way?

In my opinion, as long as the NavBackStackEntry viewModelStore corresponding to "R.id.navigation_home_flow" is in the CREATED state, it's OK, not the DetailFragment.

The Navigation version I use is 2.4.0-alpha06.

Thanks in advance if you can address my issue.

Comments

il...@google.com <il...@google.com> #2Jan 7, 2022 07:18PM

Can you explain your use case?

ch...@gmail.com <ch...@gmail.com> #3Jan 10, 2022 04:02AM

I am using Safe Args from Navigation and also using SavedStateHandle to pass in arguments from the args bundle to my ViewModel. This gives me access to them through the constructor. Unfortunately I then have to use string keys to get values out of the SavedStateHandle instead of using the generated SafeArgs.

il...@google.com <il...@google.com> #4Jan 11, 2022 12:42AM

Assigned to il...@google.com.

Dagger Hilt has been released in alpha. It greatly simplifies SavedStateHandle injection into ViewModel. Soon more and more people would want this feature.

ch...@gmail.com <ch...@gmail.com> #5Jan 14, 2022 02:57AM

Currently the combination of Hilt, SavedStateHandle and Safe args is not possible. The closest I've seen is to drop using safe args and just use magic strings. This also not ideal re dependency inversion as now you have to construct a mock SavedStateHandle for unit tests etc.

class ArticleViewModel @ViewModelInject constructor(
        private val repository: ArticleRepository,
        @Assisted savedStateHandle: SavedStateHandle
) {
    // FIXME: oops, magic "articleId" string
    private val articleId: Long = savedStateHandle["articleId"]
            ?: throw IllegalArgumentException("Article ID required")
}

ap...@google.com <ap...@google.com> #6Jan 20, 2022 12:50AM

I thought I needed this, but then I realized that I can just create an empty Bundle, store the Fragment's arguments in it as a Bundle (so a Bundle inside a Bundle) and use that to initialize the SavedStateHandle. Then I can just retrieve the arguments from the SavedStateHandle and use the already existing fromBundle method.

What I like about this is that I get to have immutable init arguments, as long as I store the Fragment's arguments with a key that I would never use for other entries.

With a helper function and a base ViewModel class you can hide this entire process and have ViewModels that expose the arguments object as property/member of the ViewModel itself.

il...@google.com <il...@google.com> #7Jan 20, 2022 12:51AM

Marked as fixed.

Could you provide an example of how you achieve that?

Issue 213504272