From 04b06f59463aa246399271b2cc56a8553616aa59 Mon Sep 17 00:00:00 2001
From: Yang <ychescale9@gmail.com>
Date: Wed, 6 Dec 2023 22:10:22 +1100
Subject: [PATCH] Pager scale transition effect.

---
 .../kstreamlined/android/KSActivity.kt        | 43 +++++++++++++++++--
 1 file changed, 39 insertions(+), 4 deletions(-)

diff --git a/android/app/src/main/java/io/github/reactivecircus/kstreamlined/android/KSActivity.kt b/android/app/src/main/java/io/github/reactivecircus/kstreamlined/android/KSActivity.kt
index f3a4bb88..75c03d08 100644
--- a/android/app/src/main/java/io/github/reactivecircus/kstreamlined/android/KSActivity.kt
+++ b/android/app/src/main/java/io/github/reactivecircus/kstreamlined/android/KSActivity.kt
@@ -4,13 +4,17 @@ import android.os.Bundle
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
+import androidx.compose.animation.core.EaseInOutQuart
+import androidx.compose.animation.core.tween
 import androidx.compose.foundation.ExperimentalFoundationApi
+import androidx.compose.foundation.background
 import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.navigationBarsPadding
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.pager.HorizontalPager
+import androidx.compose.foundation.pager.PagerState
 import androidx.compose.foundation.pager.rememberPagerState
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
@@ -19,8 +23,10 @@ import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.graphicsLayer
 import androidx.compose.ui.graphics.toArgb
 import androidx.compose.ui.unit.dp
+import androidx.compose.ui.util.lerp
 import androidx.core.splashscreen.SplashScreen.Companion.installSplashScreen
 import dagger.hilt.android.AndroidEntryPoint
 import io.github.reactivecircus.kstreamlined.android.designsystem.component.NavigationIsland
@@ -32,6 +38,7 @@ import io.github.reactivecircus.kstreamlined.android.designsystem.foundation.ico
 import io.github.reactivecircus.kstreamlined.android.designsystem.foundation.icon.Kotlin
 import io.github.reactivecircus.kstreamlined.android.feature.home.HomeScreen
 import io.github.reactivecircus.kstreamlined.android.feature.savedforlater.SavedForLaterScreen
+import kotlin.math.absoluteValue
 
 @AndroidEntryPoint
 class KSActivity : ComponentActivity() {
@@ -52,7 +59,11 @@ class KSActivity : ComponentActivity() {
                     window.navigationBarColor = navigationBarColor
                 }
 
-                Box(modifier = Modifier.fillMaxSize()) {
+                Box(
+                    modifier = Modifier
+                        .fillMaxSize()
+                        .background(KSTheme.colorScheme.background)
+                ) {
                     var selectedNavItem by rememberSaveable { mutableStateOf(NavItemKey.Home) }
 
                     val pagerState = rememberPagerState(pageCount = { NavItemKey.entries.size })
@@ -64,16 +75,27 @@ class KSActivity : ComponentActivity() {
                     ) {
                         when (it) {
                             NavItemKey.Home.ordinal -> {
-                                HomeScreen()
+                                HomeScreen(
+                                    modifier = Modifier.pagerScaleTransition(it, pagerState)
+                                )
                             }
+
                             NavItemKey.Saved.ordinal -> {
-                                SavedForLaterScreen()
+                                SavedForLaterScreen(
+                                    modifier = Modifier.pagerScaleTransition(it, pagerState)
+                                )
                             }
                         }
                     }
 
                     LaunchedEffect(selectedNavItem) {
-                        pagerState.animateScrollToPage(selectedNavItem.ordinal)
+                        pagerState.animateScrollToPage(
+                            page = selectedNavItem.ordinal,
+                            animationSpec = tween(
+                                durationMillis = 400,
+                                easing = EaseInOutQuart,
+                            ),
+                        )
                     }
 
                     NavigationIsland(
@@ -106,6 +128,19 @@ class KSActivity : ComponentActivity() {
     }
 }
 
+@OptIn(ExperimentalFoundationApi::class)
+private fun Modifier.pagerScaleTransition(page: Int, pagerState: PagerState) = graphicsLayer {
+    val pageOffset = (pagerState.currentPage - page) + pagerState.currentPageOffsetFraction
+    lerp(
+        start = 0.8f,
+        stop = 1f,
+        fraction = 1f - pageOffset.absoluteValue.coerceIn(0f, 1f),
+    ).also { scale ->
+        scaleX = scale
+        scaleY = scale
+    }
+}
+
 enum class NavItemKey {
     Home,
     Saved,