BackHandler crashes in IDE preview [185693006]

Fixed

Bug

Status Update

No update yet.

Description

za...@gmail.com

created issue #1

Apr 19, 2021 03:01AM

Jetpack Compose release version: 1.0.0-beta04
Android Studio Build: 7.0.0-alpha14

Steps to Reproduce:

Create a composable containing a BackHandler composable.
Try to render the composable in the IDE preview with an @Preview composable.

The preview will throw an exception about there not being any BackDispatcher available.

Because there isn't any back button in the IDE preview, and most previewed composables probably don't care about navigation anyway, I would expect BackHandler to just silently no-op in this case.

Comments

an...@google.com <an...@google.com> May 26, 2021 12:26PM

Assigned to il...@google.com.

il...@google.com <il...@google.com> #2Jun 2, 2021 12:06AM

Reassigned to an...@google.com.

It is expected that BackHandler crashes in cases where no LocalOnBackPressedDispatcherOwner is provided.

Given that LocalOnBackPressedDispatcherOwner.current is nullable, you can certainly wrap your call to BackHandler in a null check if you'd like to specifically disable that functionality when in @Preview. Of course, you could just provide your own OnBackPressedDispatcherOwner by constructing an OnBackPressedDispatcher instance yourself as part of your @Preview.

Andrey - any thoughts on whether we should have @Preview automatically create and set a LocalOnBackPressedDispatcherOwner as we do for LocalViewModelStoreOwner? Who would be the correct assignee for that?

an...@google.com <an...@google.com> #3Jun 2, 2021 03:47PM

Reassigned to il...@google.com.

If you think you want to provide some fake implementation for previews you probably want to provide it somewhere here:

https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/ui/ui-tooling/src/androidMain/kotlin/androidx/compose/ui/tooling/preview/ComposeViewAdapter.kt;l=478;drc=38caeffe2a32b6135f231b83c54b109e30bcd474, and ask Diego for a review.
Note that in previews we do not really support ViewModels in reality. We provide a fake ViewModelStoreOwner, but this implementation just crashes when you try to create a ViewModel. It was a Sergey's idea as usually ViewModels are doing some network/database requests and we don't really support it in the preview mode.

ap...@google.com <ap...@google.com> #4Jun 15, 2021 03:49PM

Project: platform/frameworks/support
Branch: androidx-main

commit 026203513c5a704abeaa34595e7a7e7ab06876f7
Author: Ian Lake <ilake@google.com>
Date: Mon Jun 14 11:47:14 2021

Add an ActivityResultRegistry to Compose Previews

An ActivityResultRegistry is provided by default
by ComponentActivity, but using APIs that depend on
it, like rememberLauncherForActivityResult() would
crash in @Preview. By adding a fake implementation,
developers can preview those Composables.

For cases where they call `launch()` (i.e., the
`startActivityForResult()` equivalent), the fake
throws an IllegalStateException to indicate that
actually starting an activity / permission request
in Preview is not supported.

Relnote: "Compose `@Preview` now provides a
`LocalActivityResultRegistryOwner` that allows you
to preview Composables that use APIs like
`rememberLauncherForActivityResult()` that depend
on that owner existing."
Test: new ComposeViewAdapterTest test passes
BUG: 185693006

Change-Id: Ib13d12f085065adb89cfb731179b9295029e06e9

M compose/ui/ui-tooling/src/androidAndroidTest/kotlin/androidx/compose/ui/tooling/SimpleComposablePreview.kt
M compose/ui/ui-tooling/src/androidAndroidTest/kotlin/androidx/compose/ui/tooling/preview/ComposeViewAdapterTest.kt
M compose/ui/ui-tooling/src/androidMain/kotlin/androidx/compose/ui/tooling/preview/ComposeViewAdapter.kt

https://android-review.googlesource.com/1737073

ap...@google.com <ap...@google.com> #5Jun 15, 2021 03:49PM

Project: platform/frameworks/support
Branch: androidx-main

commit 622947a991ba924d4896965f708be022bf1b35ba
Author: Ian Lake <ilake@google.com>
Date: Mon Jun 14 11:34:13 2021

Add a fake OnBackPressedDispatcher to Compose Previews

Rather than having APIs that depend on
LocalOnBackPressedDispatcher crash in @Preview,
provide a fake implementation by default to mirror
the capabilities that are available by default
in a ComponentActivity.

Relnote: "Compose `@Preview` now provides a
`LocalOnBackPressedDispatcherOwner` that allows you
to preview Composables that use APIs like `BackHandler`
that depend on that owner existing."
Test: new ComposeViewAdapterTest test passes
BUG: 185693006

Change-Id: Ia1c05ea3cadf2fb55cef9c4b8bae0898cfb35ba9

M compose/ui/ui-tooling/src/androidAndroidTest/kotlin/androidx/compose/ui/tooling/SimpleComposablePreview.kt
M compose/ui/ui-tooling/src/androidAndroidTest/kotlin/androidx/compose/ui/tooling/preview/ComposeViewAdapterTest.kt
M compose/ui/ui-tooling/src/androidMain/kotlin/androidx/compose/ui/tooling/preview/ComposeViewAdapter.kt

https://android-review.googlesource.com/1736795

il...@google.com <il...@google.com> #6Jun 15, 2021 04:04PM

Marked as fixed.

We've added default a LocalOnBackPressedDispatcherOwner and LocalActivityResultRegistryOwner to @Preview, so APIs that depend on these existing won't crash out of the box.

za...@gmail.com <za...@gmail.com> #7Jun 15, 2021 04:06PM

Thanks!