LiveData.switchMap should allow to return `null` instead LiveData [132923666]

Fixed

Feature Request

Status Update

No update yet.

Description

ma...@gmail.com

created issue #1

May 17, 2019 10:59AM

Component used: androidx.lifecycle:lifecycle-livedata-ktx
Version used: 2.2.0-alpha01

LiveData.switchMap breaks the nullability contracts of the corresponding Java Transformations.switchMap.
Transformations.switchMap allows returning null, while the Kotlin extension function does not.
I can write

Transformations.switchMap(fooLiveData) { it?.barLiveData }

but I can't write

fooLiveData.switchMap { it?.barLiveData }

since I'm expected to return a non-nullable LiveData<Y>.
To make it compile, I would need to write an ugly

fooLiveData.switchMap { it?.barLiveData ?: MutableLiveData<Bar>(null) }

The null return value is safely handled in the Java implementation, so I believe there should be no reason to force a non-null return type.

Please consider changing the switchMap function signature from
inline fun <X, Y> LiveData<X>.switchMap(crossinline transform: (X) -> LiveData<Y>): LiveData<Y>
to
inline fun <X, Y> LiveData<X>.switchMap(crossinline transform: (X) -> LiveData<Y>?): LiveData<Y>

Comments

pe...@gmail.com <pe...@gmail.com> #2Nov 21, 2019 02:39PM

Project: platform/frameworks/support
Branch: androidx-master-dev

commit b90079595f33f58fece04026a97faa0d243acdb1
Author: Yuichi Araki <yaraki@google.com>
Date: Wed Sep 18 16:55:49 2019

Change the way to detect mismatch between POJO and query

This fixes cursor mismatch warnings with expandProjection.

Bug: 140759491
Test: QueryMethodProcessorTest
Change-Id: I7659002e5e0d1ef60fc1af2a625c0c36da0664d8

M room/compiler/src/main/kotlin/androidx/room/processor/QueryMethodProcessor.kt
M room/compiler/src/main/kotlin/androidx/room/solver/TypeAdapterStore.kt
M room/compiler/src/main/kotlin/androidx/room/solver/query/result/PojoRowAdapter.kt
M room/compiler/src/test/kotlin/androidx/room/processor/QueryMethodProcessorTest.kt
M room/compiler/src/test/kotlin/androidx/room/testing/TestProcessor.kt

https://android-review.googlesource.com/1123258

https://goto.google.com/android-sha1/b90079595f33f58fece04026a97faa0d243acdb1

pe...@gmail.com <pe...@gmail.com> #3Nov 21, 2019 03:24PM

The following changes were cherrypicked through Coastguard (

https://android-build.googleplex.com/coastguard/)
Release Track: (

https://android-build.googleplex.com/coastguard/dashboard/6093116828680192/#/cherrypickHistory/all)
Created: 2019-10-14 17:23:42.610596+00:00
Changes: aosp/1123258

[Deleted User] <[Deleted User]> #4Nov 29, 2019 02:45AM

Project: platform/frameworks/support
Branch: androidx-master-dev

commit bdde5a1a970ddc9007b28de4aa29d60ffa588f08
Author: Yigit Boyar <yboyar@google.com>
Date: Thu Apr 16 16:47:05 2020

Re-factor how errors are dismissed when query is re-written

This CL changes how we handle errors/warnings if query is
re-written.
There was a bug in expandProjection where we would report warnings
for things that Room already fixes automatically (

b/140759491

).
The solution to that problem (I7659002e5e0d1ef60fc1af2a625c0c36da0664d8)
solved it by deferring validating of columns until after re-write
decision is made. Unfortunately, this required changing PojoRowAdapter
to have a dummy mapping until it is validating, make it hard to use
as it does have a non-null mapping which is not useful.

This CL partially reverts that change and instead rely on the log
deferring logic we have in Context. This way, we don't need to break
the stability of PojoRowAdapter while still having the ability to
drop warnings that room fixes. This will also play nicer when we
have different query re-writing options that can use more information
about the query results.

Bug: 153387066
Bug: 140759491
Test: existing tests pass
Change-Id: I2ec967c763d33d7a3ff02c1a13c6953b460d1e5f

M room/compiler/src/main/kotlin/androidx/room/log/RLog.kt
M room/compiler/src/main/kotlin/androidx/room/processor/QueryMethodProcessor.kt
M room/compiler/src/main/kotlin/androidx/room/solver/TypeAdapterStore.kt
M room/compiler/src/main/kotlin/androidx/room/solver/query/result/PojoRowAdapter.kt

https://android-review.googlesource.com/1288456

pe...@gmail.com <pe...@gmail.com> #5May 28, 2020 08:51AM

I just revisited this problem and I think it's working as expected
Tested w. Kotlin 1.3.72 / BuildTools 3.6.3 / LiveData 2.2.0

la...@gtempaccount.com <la...@gtempaccount.com> #6Oct 19, 2020 08:22PM

This is still an issue in 2.3.0-beta01. Returning `null` in the transform updates the Mediator to have no source. This is useful when you want to update only from the most recent valid / non-null value of another LiveData. The fix is to add a single `?` as the OP indicated, so this should be able to be added into a release easily.

il...@google.com <il...@google.com> Nov 18, 2021 10:12PM

Status: New

il...@google.com <il...@google.com> Feb 1, 2022 09:34PM

Assigned to jb...@google.com.

ap...@google.com <ap...@google.com> #7Feb 2, 2022 11:27PM

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

commit b4c21faef83d1b2f6c7fa095dabfdbcf780b5b66
Author: Jeremy Woods <jbwoods@google.com>
Date: Wed Feb 02 13:52:09 2022

Make kotlin switchmap function match java

We need to allow the function parameters the same nullability as in
java.

RelNote: "The `LiveData` `switchMap` function parameter can
now have nullable output."
Test: ./gradlew checkApi
Bug: 132923666

Change-Id: I40396c3279a9506a52a4ab1f81e98af2120ff4e0

M lifecycle/lifecycle-livedata-ktx/src/main/java/androidx/lifecycle/Transformations.kt

https://android-review.googlesource.com/1969579

jb...@google.com <jb...@google.com> #8Feb 2, 2022 11:30PM

Marked as fixed.

This has been fixed internally and will be available in the Lifecycle 2.5.0-alpha02 release.