[Pager Indicators] Fix: #768

pager-indicators/src/main/java/com/google/accompanist/pager/PagerTab.kt

@@ -49,7 +49,7 @@ fun Modifier.pagerTabIndicatorOffset(
     val targetIndicatorOffset: Dp
     val indicatorWidth: Dp
 
-    val currentTab = tabPositions[pagerState.currentPage]
+    val currentTab = tabPositions[minOf(tabPositions.lastIndex, pagerState.currentPage)]
     val targetPage = pagerState.targetPage
     val targetTab = tabPositions.getOrNull(targetPage)
 

pager/src/main/java/com/google/accompanist/pager/Pager.kt

@@ -209,6 +209,10 @@ internal fun Pager(
         (flingBehavior as? SnappingFlingBehavior)?.animationTarget
     }
 
+    LaunchedEffect(count) {
+        state.currentPage = minOf(count - 1, state.currentPage).coerceAtLeast(0)
+    }
+
     // Once a fling (scroll) has finished, notify the state
     LaunchedEffect(state) {
         // When a 'scroll' has finished, notify the state

pager/src/main/java/com/google/accompanist/pager/PagerState.kt

@@ -133,7 +133,7 @@ class PagerState(
     @get:IntRange(from = 0)
     var currentPage: Int
         get() = _currentPage
-        private set(value) {
+        internal set(value) {
             if (value != _currentPage) {
                 _currentPage = value
                 if (DebugLog) {

pager/src/sharedTest/kotlin/com/google/accompanist/pager/PagerStateUnitTest.kt

@@ -17,11 +17,15 @@
 package com.google.accompanist.pager
 
 import androidx.compose.foundation.text.BasicText
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.setValue
 import androidx.compose.ui.test.junit4.StateRestorationTester
 import androidx.compose.ui.test.junit4.createComposeRule
 import androidx.test.ext.junit.runners.AndroidJUnit4
 import com.google.common.truth.Truth.assertThat
 import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.runBlocking
 import kotlinx.coroutines.test.runBlockingTest
 import org.junit.Ignore
 import org.junit.Rule
@@ -59,4 +63,45 @@ class PagerStateUnitTest {
         assertThat(state.currentPage).isEqualTo(4)
         assertThat(state.pageCount).isEqualTo(10)
     }
+
+    @OptIn(ExperimentalCoroutinesApi::class)
+    @Test
+    fun change_pager_count() {
+        val pagerState = PagerState()
+        var count by mutableStateOf(10)
+
+        composeTestRule.setContent {
+            HorizontalPager(count = count, state = pagerState) { page ->
+                BasicText(text = "Page:$page")
+            }
+        }
+        composeTestRule.waitForIdle()
+
+        // Now scroll to page 8
+        composeTestRule.runOnIdle {
+            runBlocking {
+                pagerState.scrollToPage(8)
+            }
+        }
+
+        // And assert that currentPage and pageCount are correct
+        assertThat(pagerState.currentPage).isEqualTo(8)
+        assertThat(pagerState.pageCount).isEqualTo(10)
+
+        // Change page count to 4
+        count = 4
+        composeTestRule.waitForIdle()
+
+        // And assert that currentPage and pageCount are correct
+        assertThat(pagerState.currentPage).isEqualTo(3)
+        assertThat(pagerState.pageCount).isEqualTo(4)
+
+        // Change page count to 0
+        count = 0
+        composeTestRule.waitForIdle()
+
+        // And assert that currentPage and pageCount are correct
+        assertThat(pagerState.currentPage).isEqualTo(0)
+        assertThat(pagerState.pageCount).isEqualTo(0)
+    }
 }