I'm getting an NPE and I think I'm not understandi...

# compose

[Colton Idle]

I'm getting an NPE and I think I'm not understanding how snapshot state works? In essence I want to show a modal dialog as part of my state. Code inside the thread.

[Colton Idle]

class SignInViewState {
    var email by mutableStateOf("")
    var password by mutableStateOf("")
    var modalError: ModalError? by mutableStateOf(null)
}

data class ModalError(
    val text1: String,
    val text2: String,
)

My compose screen includes this

if (viewModel.state.modalError != null) {
    Dialog(onDismissRequest = { viewModel.state.modalError = null }) {
        MyModal(
            viewModel.state.modalError!!.text1,
            viewModel.state.modalError!!.text2)
    }
}

And this works-ish. My modal does not show. Then I update my modal error in my state, and the pop up shows! But then when I click outside of the dialog I get a crash. My on dismiss request tries to null out the error, but I get an NPE on viewModel.state.modalError!!.text1

[rnett]

As a workaround, try assigning modalError to a local variable and using that. Not sure about the root cause, but that should get around it

[Colton Idle]

Hm.

if (viewModel.state.modalError != null) {
    Dialog(onDismissRequest = { viewModel.state.modalError = null }) {
        MyModal(
            viewModel.state.modalError?.text1.orEmpty(),
            viewModel.state.modalError?.text2.orEmpty())
    }
}

stops the issue from happening, but now I really feel like I don't know what's going on. Can anyone identify what's going on?

[Albert Chang]

I think it's just that the dismiss of the dialog isn't immediate (there's an animation) and during the animation the content of the dialog still have to be shown. This kind of problem also exists when you are using Crossfade, AninatedVisibility, etc.
Using val error = remember { viewModel.state.modalError!! } to solve the problem.

[Colton Idle]

Interesting. This is kind of breaking my mind because I'm trying to figure out where else this would break, and I'm now also confused a bit by why adding an additional val error would help.

[Albert Chang]

It's the remember that helps. It remembers the initial value so even if viewModel.state.modalError becomes null the remembered value doesn't.

[Colton Idle]

@Albert Chang do you think I'm just better off adding an additional field like "showModal"?

class SignInViewState {
    var email by mutableStateOf("")
    var password by mutableStateOf("")
    var showModal by mutableStateOf(false)
    var modalError: ModalError? by mutableStateOf(null)
}

Adding a val error = remember { viewModel.state.modalError!! } almost seems too "clever" and I'm not sure it's really self explanatory on what's actually going on.

[Doris Liu]

This is a case of needing to retain the data for the exit animation. To achieve that, you could either do val error = remember { viewModel.state.modalError!! } inside the dialog content, so that the error isn't retained after the content is disposed. Or, you could retain that data in your ViewModel, and wait for the onDisposed signal to do the cleanup. Either way works. It's a matter of personal preference.

[Colton Idle]

Thanks everyone. This was driving me a little insane so I really appreciate it.

Readability and intent is really important for me and my team. Following Doris' first suggestion I think I could change this

if (viewModel.state.modalError != null) {
    Dialog(onDismissRequest = { viewModel.state.modalError = null }) {
        MyModal(
            viewModel.state.modalError!!.text1,
            viewModel.state.modalError!!.text2)
    }
}

to this

if (viewModel.state.modalError != null) {
    Dialog(onDismissRequest = { viewModel.state.modalError = null }) {
        val error = remember { viewModel.state.modalError!! }
        MyModal(error.text1, error.text2)
    }
}

@Doris Liu I am not following your second option "you could retain that data in your ViewModel". How would that look?

[Doris Liu]

Re: "you could retain that data in your ViewModel" . How would that look?
You'd need to have a separate show/hide Boolean in the state, similar to what you proposed above. It could be used to determine when to dismiss the dialog. The actual data for the error is only reset after the dialog has been disposed:

if (viewModel.state.showError) {    
    Dialog(onDismissRequest = { viewModel.state.showError = false }) {
        MyModal(
            viewModel.state.modalError!!.text1,
            viewModel.state.modalError!!.text2)
        DisposableEffect(viewModel) {
            // retain data until disposed
            onDisposed { viewModel.state.modelError = null } 
        }
    }
}

[Doris Liu]

On another look, your original code below

if (viewModel.state.modalError != null) {
    Dialog(onDismissRequest = { viewModel.state.modalError = null }) {
...

should have the Dialog removed from the tree when modalError = null and hence the if-condition evaluates to false. Is the dialog dismissed through something other than a gesture? Could you post the full NPE stack strace?

[Colton Idle]

Oh. This is an interesting update. Let me post the full stacktrace!

[Colton Idle]

2021-06-27 19:10:17.368 10948-10948/com.rollertoaster.app E/AndroidRuntime: FATAL EXCEPTION: main
Process: <http://com.rollertoaster.app|com.rollertoaster.app>, PID: 10948
java.lang.NullPointerException
at com.rollertoaster.app.login.SignInFragment$onCreateView$1$2$1$2.invoke(SignInFragment.kt:102)
at com.rollertoaster.app.login.SignInFragment$onCreateView$1$2$1$2.invoke(SignInFragment.kt:100)
at androidx.compose.runtime.internal.ComposableLambdaImpl.invoke(ComposableLambda.jvm.kt:107)
at androidx.compose.runtime.internal.ComposableLambdaImpl.invoke(ComposableLambda.jvm.kt:34)
at androidx.compose.runtime.RecomposeScopeImpl.compose(RecomposeScopeImpl.kt:140)
at androidx.compose.runtime.ComposerImpl.recomposeToGroupEnd(Composer.kt:2156)
at androidx.compose.runtime.ComposerImpl.skipCurrentGroup(Composer.kt:2399)
at androidx.compose.runtime.ComposerImpl$doCompose$2$5.invoke(Composer.kt:2574)
at androidx.compose.runtime.ComposerImpl$doCompose$2$5.invoke(Composer.kt:2567)
at androidx.compose.runtime.SnapshotStateKt.observeDerivedStateRecalculations(SnapshotState.kt:523)
at androidx.compose.runtime.ComposerImpl.doCompose(Composer.kt:2560)
at androidx.compose.runtime.ComposerImpl.recompose$runtime_release(Composer.kt:2536)
at androidx.compose.runtime.CompositionImpl.recompose(Composition.kt:613)
at androidx.compose.runtime.Recomposer.performRecompose(Recomposer.kt:763)
at androidx.compose.runtime.Recomposer.access$performRecompose(Recomposer.kt:102)
at androidx.compose.runtime.Recomposer$runRecomposeAndApplyChanges$2$2.invoke(Recomposer.kt:446)
at androidx.compose.runtime.Recomposer$runRecomposeAndApplyChanges$2$2.invoke(Recomposer.kt:415)
at androidx.compose.ui.platform.AndroidUiFrameClock$withFrameNanos$2$callback$1.doFrame(AndroidUiFrameClock.android.kt:34)
at androidx.compose.ui.platform.AndroidUiDispatcher.performFrameDispatch(AndroidUiDispatcher.android.kt:109)
at androidx.compose.ui.platform.AndroidUiDispatcher.access$performFrameDispatch(AndroidUiDispatcher.android.kt:41)
at androidx.compose.ui.platform.AndroidUiDispatcher$dispatchCallback$1.doFrame(AndroidUiDispatcher.android.kt:69)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:970)
at android.view.Choreographer.doCallbacks(Choreographer.java:796)
at android.view.Choreographer.doFrame(Choreographer.java:727)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:957)
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

Clicking through the link in the stacktrace puts my cursor on

viewModel.state.modalError!!.text1,

Which is basically the thing that was blowing my mind of how this got past the if statement in the first place (to Doris' point "and hence the if-condition evaluates to false")

[Doris Liu]

This NPE seems strange. @Colton Idle could you isolate the issue in a minimal repro case and file a bug so we can investigate it?

[Colton Idle]

Will do so first thing tomorrow. 👍

[louiscad]

You meant first thing on Monday?

[Colton Idle]

Time zones are hard

[Colton Idle]

@Doris Liu all set! https://issuetracker.google.com/issues/192043119