[fenix] For https://github.com/mozilla-mobile/fenix/issues/20440 - Forces TopSitePager bind if no. of pages changed

app/src/main/java/org/mozilla/fenix/home/sessioncontrol/SessionControlAdapter.kt

@@ -86,8 +86,15 @@ sealed class AdapterItem(@LayoutRes val viewType: Int) {
          * See https://github.com/mozilla-mobile/fenix/pull/20189#issuecomment-877124730
          */
         override fun getChangePayload(newItem: AdapterItem): Any? {
-            val newTopSites = (newItem as? TopSitePager) ?: return null
+            val newTopSites = (newItem as? TopSitePager)
-            val oldTopSites = (this as? TopSitePager) ?: return null
+            val oldTopSites = (this as? TopSitePager)
+
+            if (newTopSites == null || oldTopSites == null ||
+                (newTopSites.topSites.size > TopSitePagerViewHolder.TOP_SITES_PER_PAGE)
+                != (oldTopSites.topSites.size > TopSitePagerViewHolder.TOP_SITES_PER_PAGE)
+            ) {
+                return null
+            }
 
             val changed = mutableSetOf<Pair<Int, TopSite>>()
 

app/src/test/java/org/mozilla/fenix/home/sessioncontrol/SessionControlAdapterTest.kt

@@ -0,0 +1,78 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.home.sessioncontrol
+
+import io.mockk.every
+import io.mockk.mockk
+import mozilla.components.feature.top.sites.TopSite
+import mozilla.components.feature.top.sites.TopSite.Type.FRECENT
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+import org.mozilla.fenix.home.sessioncontrol.AdapterItem.CollectionItem
+import org.mozilla.fenix.home.sessioncontrol.AdapterItem.TopSitePager
+import org.mozilla.fenix.home.sessioncontrol.AdapterItem.TopSitePagerPayload
+
+class SessionControlAdapterTest {
+
+    @Test
+    fun `WHEN getChangePayload called with wrong type THEN return null`() {
+        val newItem: AdapterItem = CollectionItem(mockk(), mockk(relaxed = true))
+
+        val result = TopSitePager(mockk()).getChangePayload(newItem)
+
+        assertNull(result)
+    }
+
+    @Test
+    fun `GIVEN topSitePager with 5 topSites WHEN getChangePayload with 10 items THEN return null`() {
+        val newItem = TopSitePager(mockk(relaxed = true))
+        val topSitePager = TopSitePager(mockk(relaxed = true))
+        every { topSitePager.topSites.size } returns 5
+        every { newItem.topSites.size } returns 10
+
+        val result = topSitePager.getChangePayload(newItem)
+
+        assertNull(result)
+    }
+
+    @Test
+    fun `GIVEN topSitePager with 10 topSites WHEN getChangePayload with 5 items THEN return null`() {
+        val newItem = TopSitePager(mockk(relaxed = true))
+        val topSitePager = TopSitePager(mockk(relaxed = true))
+        every { topSitePager.topSites.size } returns 10
+        every { newItem.topSites.size } returns 5
+
+        val result = topSitePager.getChangePayload(newItem)
+
+        assertNull(result)
+    }
+
+    @Test
+    fun `GIVEN two topSites WHEN getChangePayload called with one changed item THEN return TopSitePagerPayload with changes`() {
+        val topSite0 = TopSite(-1, "topSite0", "", 0, FRECENT)
+        val topSite1 = TopSite(-1, "topSite1", "", 0, FRECENT)
+        val topSiteChanged = TopSite(-1, "changed", "", 0, FRECENT)
+        val topSitePager = TopSitePager(listOf(topSite0, topSite1))
+        val newItem = TopSitePager(listOf(topSite0, topSiteChanged))
+
+        val result = topSitePager.getChangePayload(newItem)
+
+        assertEquals(TopSitePagerPayload(setOf(Pair(1, topSiteChanged))), result)
+    }
+
+    @Test
+    fun `GIVEN two topSites WHEN getChangePayload called with one removed THEN return TopSitePagerPayload with removed item`() {
+        val topSite0 = TopSite(-1, "topSite0", "", 0, FRECENT)
+        val topSite1 = TopSite(-1, "topSite1", "", 0, FRECENT)
+        val topSiteRemoved = TopSite(-1, "REMOVED", "", 0, FRECENT)
+        val topSitePager = TopSitePager(listOf(topSite0, topSite1))
+        val newItem = TopSitePager(listOf(topSite0))
+
+        val result = topSitePager.getChangePayload(newItem)
+
+        assertEquals(TopSitePagerPayload(setOf(Pair(1, topSiteRemoved))), result)
+    }
+}