API to consume Flows safely in Compose [230557927]

Fixed

Feature Request

Status Update

No update yet.

Description

mv...@google.com

created issue #1

Apr 27, 2022 06:00AM

Background

The current Android architecture guidance recommends collecting flows in the UI in a lifecycle-aware manner. This allows the app to save expensive resources when not needed, for example, when the app is in the background.

Keeping expensive resources (e.g. Firabase queries, location or network updates, etc.) alive when not needed could impact the performance of the app and user's device.

To make it easier for developers to comply with the recommended best practices, APIs that do this should be added to the Lifecycle library.

Limitations with StateFlow

Ideally, there should exist APIs for both Flow and StateFlow types. The latter is important since most flows consumed in Compose code will be of that type. If a helper function for StateFlow doesn't exist, the UI (Compose code) would need to give those new functions an initial value (same as with Compose's produceState or Flow's stateIn operator). That violates layering responsibilities since deciding the initial value belongs to the ViewModel/state holder, not the UI.

However, as stated in the refdocs, the StateFlow interface is not stable for inheritance in 3rd party libraries, which makes it a tough call to create extensions functions on the type even though only the current value would be used.

Proposed implementation #1

The collectAsStateWithLifecycle composable function is added to a new lifecycle-runtime-compose artifact.

@Composable
fun <T> Flow<T>.collectAsStateWithLifecycle(
    initialValue: T,
    lifecycle: Lifecycle = LocalLifecycleOwner.current.lifecycle,
    minActiveState: Lifecycle.State = Lifecycle.State.STARTED
): State<T> {
    val currentValue = remember(this) { initialValue }
    return produceState(
        key1 = currentValue, key2 = lifecycle, key3 = minActiveState,
        initialValue = currentValue
    ) {
        lifecycle.repeatOnLifecycle(minActiveState) {
            this@collectAsStateWithLifecycle.collect {
                this@produceState.value = it
            }
        }
    }
}

Typically, this collectAsStateWithLifecycle API would be used like:

@Composable
fun MyScreen(
    viewModel: MyViewModel = viewModel()
) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle(viewModel.uiState.value)
}

Proposed implementation #2

To avoid passing the initialValue every time for StateFlow types, another alternative could be the rememberStateWithLifecycle API. This API would also be added to a new lifecycle-runtime-compose artifact.

@Composable
fun <T> rememberStateWithLifecycle(
    stateFlow: StateFlow<T>,
    lifecycle: Lifecycle = LocalLifecycleOwner.current.lifecycle,
    minActiveState: Lifecycle.State = Lifecycle.State.STARTED
): State<T> {
    val initialValue = remember(stateFlow) { stateFlow.value }
    return produceState(
        key1 = stateFlow, key2 = lifecycle, key3 = minActiveState,
        initialValue = initialValue
    ) {
        lifecycle.repeatOnLifecycle(minActiveState) {
            stateFlow.collect {
                this@produceState.value = it
            }
        }
    }
}

This would be used as follows:

@Composable
fun MyScreen(
    viewModel: MyViewModel = viewModel()
) {
    val uiState by rememberStateWithLifecycle(viewModel.uiState)
}

This API would also offer a method overload that can take a Flow and an initialValue as parameters.

Comments

mv...@google.com <mv...@google.com> #2Apr 27, 2022 12:42PM

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

commit c60f33e229e31ab328ef6b59dab63b264954831c
Author: Alexandre Elias <aelias@google.com>
Date: Fri Jul 10 16:23:09 2020

Semantics no-op cleanups

Partly in response to lmr's broad code review, I did a pass of
superficial API/implementation cleanup. The main changes are:

- I changed each Boolean SemanticsProperty where false is equivalent to
not being present to take "Unit" instead. This is conceptually
clearer: it avoids questions like "can I cancel out a semantics from a
merged child by setting it to false?" Because "property = Unit" looks
weird, I also changed the style of these to "property()".

- I moved the Semantics id generator closer to where it's used, in
SemanticsModifierCore. I made it internal and an AtomicInt.
(Note that integer ids are heavily used in the Android
AccessibilityNodeInfo APIs so I can't simply remove them entirely.)

- I deleted dead code. Some examples include SemanticsHintOverrides,
a public API not connected to anything, and SemanticsPropertyKey
merge() open method which is never called. (In both cases I have
a different plan in mind for accessibility.)

Fixes: 145951226
Fixes: 145955412
Test: existing tests
Relnote: "Single-value semantics properties now use a calling style.
For example, 'semantics { hidden = true }' is now written as:
'semantics { hidden() }'."

Change-Id: Ic1afd12ea22c926babc9662f1804d80b33aa0cfc

M ui/integration-tests/benchmark/src/androidTest/java/androidx/ui/benchmark/test/LayoutNodeModifierBenchmark.kt
M ui/ui-core/api/0.1.0-dev15.txt
M ui/ui-core/api/current.txt
M ui/ui-core/api/public_plus_experimental_0.1.0-dev15.txt
M ui/ui-core/api/public_plus_experimental_current.txt
M ui/ui-core/api/restricted_0.1.0-dev15.txt
M ui/ui-core/api/restricted_current.txt
M ui/ui-core/src/androidAndroidTest/kotlin/androidx/ui/graphics/vector/VectorTest.kt
M ui/ui-core/src/androidAndroidTest/kotlin/androidx/ui/semantics/SemanticsTests.kt
M ui/ui-core/src/androidMain/kotlin/androidx/ui/core/AndroidActuals.kt
M ui/ui-core/src/androidMain/kotlin/androidx/ui/core/AndroidComposeView.kt
M ui/ui-core/src/androidMain/kotlin/androidx/ui/core/AndroidComposeViewAccessibilityDelegateCompat.kt
M ui/ui-core/src/androidMain/kotlin/androidx/ui/core/AndroidPopup.kt
M ui/ui-core/src/commonMain/kotlin/androidx/ui/core/Expect.kt
M ui/ui-core/src/commonMain/kotlin/androidx/ui/core/semantics/SemanticsConfiguration.kt
D ui/ui-core/src/commonMain/kotlin/androidx/ui/core/semantics/SemanticsHintOverrides.kt
M ui/ui-core/src/commonMain/kotlin/androidx/ui/core/semantics/SemanticsModifier.kt
M ui/ui-core/src/commonMain/kotlin/androidx/ui/core/semantics/SemanticsNode.kt
M ui/ui-core/src/commonMain/kotlin/androidx/ui/core/semantics/SemanticsOwner.kt
M ui/ui-core/src/commonMain/kotlin/androidx/ui/core/semantics/SemanticsWrapper.kt
M ui/ui-core/src/commonMain/kotlin/androidx/ui/semantics/SemanticsProperties.kt
M ui/ui-foundation/api/0.1.0-dev15.txt
M ui/ui-foundation/api/current.txt
M ui/ui-foundation/api/public_plus_experimental_0.1.0-dev15.txt
M ui/ui-foundation/api/public_plus_experimental_current.txt
M ui/ui-foundation/api/restricted_0.1.0-dev15.txt
M ui/ui-foundation/api/restricted_current.txt
M ui/ui-foundation/src/main/java/androidx/ui/foundation/Clickable.kt
M ui/ui-foundation/src/main/java/androidx/ui/foundation/Dialog.kt
M ui/ui-foundation/src/main/java/androidx/ui/foundation/Scroller.kt
M ui/ui-foundation/src/main/java/androidx/ui/foundation/selection/Selectable.kt
M ui/ui-foundation/src/main/java/androidx/ui/foundation/selection/Toggleable.kt
M ui/ui-foundation/src/main/java/androidx/ui/foundation/semantics/FoundationSemanticsProperties.kt
M ui/ui-material/src/androidTest/java/androidx/ui/material/ButtonTest.kt
M ui/ui-material/src/androidTest/java/androidx/ui/material/CardTest.kt
M ui/ui-material/src/androidTest/java/androidx/ui/material/CheckboxScreenshotTest.kt
M ui/ui-material/src/androidTest/java/androidx/ui/material/RadioButtonScreenshotTest.kt
M ui/ui-material/src/androidTest/java/androidx/ui/material/ScaffoldTest.kt
M ui/ui-material/src/androidTest/java/androidx/ui/material/SnackbarTest.kt
M ui/ui-material/src/androidTest/java/androidx/ui/material/SurfaceTest.kt
M ui/ui-material/src/androidTest/java/androidx/ui/material/ripple/RippleIndicationTest.kt
M ui/ui-material/src/androidTest/java/androidx/ui/material/textfield/TextFieldScreenshotTest.kt
M ui/ui-material/src/main/java/androidx/ui/material/AppBar.kt
M ui/ui-material/src/main/java/androidx/ui/material/TextFieldImpl.kt
M ui/ui-test/src/androidTest/java/androidx/ui/test/AssertsTest.kt
M ui/ui-test/src/androidTest/java/androidx/ui/test/CallSemanticsActionTest.kt
M ui/ui-test/src/androidTest/java/androidx/ui/test/ErrorMessagesTest.kt
M ui/ui-test/src/androidTest/java/androidx/ui/test/FindersTest.kt
M ui/ui-test/src/androidTest/java/androidx/ui/test/PrintToStringTest.kt
M ui/ui-test/src/androidTest/java/androidx/ui/test/ScrollToTest.kt
M ui/ui-test/src/androidTest/java/androidx/ui/test/TextActionsTest.kt
M ui/ui-test/src/main/java/androidx/ui/test/Actions.kt
M ui/ui-test/src/main/java/androidx/ui/test/Filters.kt
M ui/ui-text/api/0.1.0-dev15.txt
M ui/ui-text/api/current.txt
M ui/ui-text/api/public_plus_experimental_0.1.0-dev15.txt
M ui/ui-text/api/public_plus_experimental_current.txt
M ui/ui-text/api/restricted_0.1.0-dev15.txt
M ui/ui-text/api/restricted_current.txt
M ui/ui-text/src/commonMain/kotlin/androidx/ui/text/CoreTextField.kt
M ui/ui-text/src/commonMain/kotlin/androidx/ui/text/TextSemanticsProperties.kt

https://android-review.googlesource.com/1360099

il...@google.com <il...@google.com> #3Jun 16, 2022 06:10PM

Marked as fixed, reassigned to il...@google.com.

This API was added in aosp/2102573 and will be available in Lifecycle 2.6.0-alpha01.

Issue 230557927