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LazyListScrollProcessor manages user and programmatic scrolls (#1619)
* wip * LazyListScrollProcessor manages user and programmatic scrolls --------- Co-authored-by: Jesse Wilson <[email protected]>
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...src/commonTest/kotlin/app/cash/redwood/lazylayout/api/ScrollItemIndexSerializationTest.kt
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| @@ -0,0 +1,41 @@ | ||
| /* | ||
| * Copyright (C) 2023 Square, Inc. | ||
| * | ||
| * Licensed under the Apache License, Version 2.0 (the "License"); | ||
| * you may not use this file except in compliance with the License. | ||
| * You may obtain a copy of the License at | ||
| * | ||
| * http://www.apache.org/licenses/LICENSE-2.0 | ||
| * | ||
| * Unless required by applicable law or agreed to in writing, software | ||
| * distributed under the License is distributed on an "AS IS" BASIS, | ||
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| * See the License for the specific language governing permissions and | ||
| * limitations under the License. | ||
| */ | ||
| package app.cash.redwood.lazylayout.api | ||
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| import assertk.assertThat | ||
| import assertk.assertions.isEqualTo | ||
| import kotlin.test.Test | ||
| import kotlinx.serialization.json.Json | ||
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| class ScrollItemIndexSerializationTest { | ||
| private val json = Json { | ||
| ignoreUnknownKeys = true | ||
| useArrayPolymorphism = true | ||
| } | ||
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| @Test | ||
| fun encodeAndDecode() { | ||
| assertRoundTrip( | ||
| ScrollItemIndex(3, 7), | ||
| """{"id":3,"index":7}""", | ||
| ) | ||
| } | ||
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| private fun assertRoundTrip(value: ScrollItemIndex, encoded: String) { | ||
| assertThat(json.encodeToString(ScrollItemIndex.serializer(), value)).isEqualTo(encoded) | ||
| assertThat(json.decodeFromString(ScrollItemIndex.serializer(), encoded)).isEqualTo(value) | ||
| } | ||
| } |
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68 changes: 68 additions & 0 deletions
68
...idget/src/commonMain/kotlin/app/cash/redwood/lazylayout/widget/LazyListScrollProcessor.kt
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,68 @@ | ||
| /* | ||
| * Copyright (C) 2023 Square, Inc. | ||
| * | ||
| * Licensed under the Apache License, Version 2.0 (the "License"); | ||
| * you may not use this file except in compliance with the License. | ||
| * You may obtain a copy of the License at | ||
| * | ||
| * http://www.apache.org/licenses/LICENSE-2.0 | ||
| * | ||
| * Unless required by applicable law or agreed to in writing, software | ||
| * distributed under the License is distributed on an "AS IS" BASIS, | ||
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| * See the License for the specific language governing permissions and | ||
| * limitations under the License. | ||
| */ | ||
| package app.cash.redwood.lazylayout.widget | ||
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| import app.cash.redwood.lazylayout.api.ScrollItemIndex | ||
|
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| public abstract class LazyListScrollProcessor { | ||
| /** To notify guest code of user scrolls. */ | ||
| private var onViewportChanged: ((Int, Int) -> Unit)? = null | ||
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| /** We can't scroll to this index until we have enough data for it to display! */ | ||
| private var deferredProgrammaticScrollIndex: Int = -1 | ||
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| /** Once we receive a user scroll, we stop forwarding programmatic scrolls. */ | ||
| private var userHasScrolled = false | ||
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| public fun onViewportChanged(onViewportChanged: (Int, Int) -> Unit) { | ||
| this.onViewportChanged = onViewportChanged | ||
| } | ||
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| public fun scrollItemIndex(scrollItemIndex: ScrollItemIndex) { | ||
| // Defer until we have data in onEndChanges(). | ||
| deferredProgrammaticScrollIndex = scrollItemIndex.index | ||
| } | ||
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| public fun onEndChanges() { | ||
| // Do nothing: we don't have deferred scrolls. | ||
| if (deferredProgrammaticScrollIndex == -1) return | ||
|
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| // Do nothing: we don't do programmatic scrolls if the user has already scrolled manually. | ||
| if (userHasScrolled) return | ||
|
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| // Do nothing: we can't scroll to this item because it hasn't loaded yet! | ||
| if (contentSize() <= deferredProgrammaticScrollIndex) return | ||
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| // Do a programmatic scroll! | ||
| programmaticScroll(deferredProgrammaticScrollIndex) | ||
| deferredProgrammaticScrollIndex = -1 | ||
| } | ||
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| /** | ||
| * React to a user-initiated scroll. Callers should not call this function for programmatic | ||
| * scrolls. | ||
| */ | ||
| public fun onUserScroll(firstIndex: Int, lastIndex: Int) { | ||
| if (firstIndex > 0) userHasScrolled = true | ||
| onViewportChanged?.invoke(firstIndex, lastIndex) | ||
| } | ||
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| /** Returns the number of items we're scrolling over. */ | ||
| public abstract fun contentSize(): Int | ||
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| /** Perform a programmatic scroll. */ | ||
| public abstract fun programmaticScroll(firstIndex: Int) | ||
| } |
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