From d7cd0406e503394beca70485d49e9f8e4dbb2b3a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bogusz=20Paw=C5=82owski?= <bogusz.pawlowski@gmail.com>
Date: Sat, 18 Mar 2023 20:36:48 +0100
Subject: [PATCH] Add month changes throttling (#95)

---
 library/build.gradle.kts                      |  2 +-
 .../composecalendar/month/MonthListState.kt   | 15 ++++++++++-----
 .../util/ListStateExtensions.kt               | 19 +++++++++++++++++++
 .../composecalendar/week/WeekListState.kt     | 15 ++++++++++-----
 4 files changed, 40 insertions(+), 11 deletions(-)
 create mode 100644 library/src/main/java/io/github/boguszpawlowski/composecalendar/util/ListStateExtensions.kt

diff --git a/library/build.gradle.kts b/library/build.gradle.kts
index aff0c2f..70dcc37 100644
--- a/library/build.gradle.kts
+++ b/library/build.gradle.kts
@@ -11,7 +11,7 @@ plugins {
 android {
   tasks.withType<KotlinCompile> {
     kotlinOptions {
-      freeCompilerArgs = freeCompilerArgs + "-Xexplicit-api=strict"
+      freeCompilerArgs = freeCompilerArgs + "-Xexplicit-api=strict" + "-Xcontext-receivers"
     }
   }
 }
diff --git a/library/src/main/java/io/github/boguszpawlowski/composecalendar/month/MonthListState.kt b/library/src/main/java/io/github/boguszpawlowski/composecalendar/month/MonthListState.kt
index a3809f4..16c781f 100644
--- a/library/src/main/java/io/github/boguszpawlowski/composecalendar/month/MonthListState.kt
+++ b/library/src/main/java/io/github/boguszpawlowski/composecalendar/month/MonthListState.kt
@@ -6,6 +6,7 @@ import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.snapshotFlow
 import io.github.boguszpawlowski.composecalendar.header.MonthState
+import io.github.boguszpawlowski.composecalendar.util.throttleOnOffset
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.onEach
@@ -21,7 +22,7 @@ internal class MonthListState(
   private val listState: LazyListState,
 ) {
 
-  private val currentlyVisibleMonth by derivedStateOf {
+  private val currentFirstVisibleMonth by derivedStateOf {
     getMonthForPage(listState.firstVisibleItemIndex)
   }
 
@@ -30,16 +31,20 @@ internal class MonthListState(
       moveToMonth(month)
     }.launchIn(coroutineScope)
 
-    snapshotFlow { currentlyVisibleMonth }.onEach { newMonth ->
-      monthState.currentMonth = newMonth
-    }.launchIn(coroutineScope)
+    with(listState) {
+      snapshotFlow { currentFirstVisibleMonth }
+        .throttleOnOffset()
+        .onEach { newMonth ->
+          monthState.currentMonth = newMonth
+        }.launchIn(coroutineScope)
+    }
   }
 
   fun getMonthForPage(index: Int): YearMonth =
     initialMonth.plusMonths((index - StartIndex).toLong())
 
   private fun moveToMonth(month: YearMonth) {
-    if (month == currentlyVisibleMonth) return
+    if (month == currentFirstVisibleMonth) return
     initialMonth.minus(month).let { offset ->
       coroutineScope.launch {
         listState.animateScrollToItem((StartIndex - offset).toInt())
diff --git a/library/src/main/java/io/github/boguszpawlowski/composecalendar/util/ListStateExtensions.kt b/library/src/main/java/io/github/boguszpawlowski/composecalendar/util/ListStateExtensions.kt
new file mode 100644
index 0000000..07c576d
--- /dev/null
+++ b/library/src/main/java/io/github/boguszpawlowski/composecalendar/util/ListStateExtensions.kt
@@ -0,0 +1,19 @@
+package io.github.boguszpawlowski.composecalendar.util
+
+import androidx.compose.foundation.lazy.LazyListState
+import androidx.compose.runtime.snapshotFlow
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.combine
+import kotlinx.coroutines.flow.filter
+import kotlinx.coroutines.flow.map
+
+context(LazyListState) internal fun <T> Flow<T>.throttleOnOffset() =
+  combine(
+    snapshotFlow { firstVisibleItemScrollOffset }
+  ) { newMonth, offset ->
+    newMonth to (offset <= MinimalOffsetForEmit)
+  }.filter { (_, shouldUpdate) ->
+    shouldUpdate
+  }.map { (newValue, _) -> newValue }
+
+private const val MinimalOffsetForEmit = 10
diff --git a/library/src/main/java/io/github/boguszpawlowski/composecalendar/week/WeekListState.kt b/library/src/main/java/io/github/boguszpawlowski/composecalendar/week/WeekListState.kt
index c6d75c4..be8b38c 100644
--- a/library/src/main/java/io/github/boguszpawlowski/composecalendar/week/WeekListState.kt
+++ b/library/src/main/java/io/github/boguszpawlowski/composecalendar/week/WeekListState.kt
@@ -7,6 +7,7 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.snapshotFlow
 import io.github.boguszpawlowski.composecalendar.header.WeekState
 import io.github.boguszpawlowski.composecalendar.month.StartIndex
+import io.github.boguszpawlowski.composecalendar.util.throttleOnOffset
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.flow.launchIn
 import kotlinx.coroutines.flow.onEach
@@ -21,7 +22,7 @@ internal class WeekListState(
   private val listState: LazyListState,
 ) {
 
-  private val currentlyVisibleMonth by derivedStateOf {
+  private val currentlyFirstVisibleMonth by derivedStateOf {
     getWeekForPage(listState.firstVisibleItemIndex)
   }
 
@@ -30,16 +31,20 @@ internal class WeekListState(
       moveToWeek(week)
     }.launchIn(coroutineScope)
 
-    snapshotFlow { currentlyVisibleMonth }.onEach { newMonth ->
-      weekState.currentWeek = newMonth
-    }.launchIn(coroutineScope)
+    with(listState) {
+      snapshotFlow { currentlyFirstVisibleMonth }
+        .throttleOnOffset()
+        .onEach { newMonth ->
+          weekState.currentWeek = newMonth
+        }.launchIn(coroutineScope)
+    }
   }
 
   fun getWeekForPage(index: Int): Week =
     initialWeek.plusWeeks((index - StartIndex).toLong())
 
   private fun moveToWeek(week: Week) {
-    if (week == currentlyVisibleMonth) return
+    if (week == currentlyFirstVisibleMonth) return
     initialWeek.minus(week).let { offset ->
       coroutineScope.launch {
         listState.animateScrollToItem((StartIndex - offset).toInt())