Issue #18521: Focus on page corresponding to selected tab

This first part of #18521 is to focus on the correct pager item first
before we land a patch for `scrollToPosition` of the selected tab.

app/src/main/java/org/mozilla/fenix/tabstray/TabLayoutMediator.kt

@@ -0,0 +1,59 @@
+/* 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.tabstray
+
+import androidx.annotation.VisibleForTesting
+import com.google.android.material.tabs.TabLayout
+import mozilla.components.browser.state.selector.selectedTab
+import mozilla.components.browser.state.store.BrowserStore
+import org.mozilla.fenix.tabstray.TrayPagerAdapter.Companion.POSITION_NORMAL_TABS
+import org.mozilla.fenix.tabstray.TrayPagerAdapter.Companion.POSITION_PRIVATE_TABS
+
+/**
+ * Selected the selected pager depending on the [BrowserStore] state and synchronizes user actions
+ * with the pager position.
+ */
+class TabLayoutMediator(
+    private val tabLayout: TabLayout,
+    private val interactor: TabsTrayInteractor,
+    private val store: BrowserStore
+) {
+
+    /**
+     * Start observing the [TabLayout] and select the current tab for initial state.
+     */
+    fun attach() {
+        tabLayout.addOnTabSelectedListener(TabLayoutObserver(interactor))
+
+        selectActivePage()
+    }
+
+    @VisibleForTesting
+    internal fun selectActivePage() {
+        val selectedTab = store.state.selectedTab ?: return
+
+        val selectedPagerPosition = if (selectedTab.content.private) {
+            POSITION_PRIVATE_TABS
+        } else {
+            POSITION_NORMAL_TABS
+        }
+
+        tabLayout.getTabAt(selectedPagerPosition)?.select()
+    }
+}
+
+/**
+ * An observer for the [TabLayout] used for the Tabs Tray.
+ */
+internal class TabLayoutObserver(
+    private val interactor: TabsTrayInteractor
+) : TabLayout.OnTabSelectedListener {
+    override fun onTabSelected(tab: TabLayout.Tab) {
+        interactor.setCurrentTrayPosition(tab.position)
+    }
+
+    override fun onTabUnselected(tab: TabLayout.Tab) = Unit
+    override fun onTabReselected(tab: TabLayout.Tab) = Unit
+}

app/src/main/java/org/mozilla/fenix/tabstray/TabsTrayFragment.kt

@@ -13,7 +13,6 @@ import androidx.appcompat.app.AppCompatDialogFragment
 import androidx.constraintlayout.widget.ConstraintLayout
 import androidx.navigation.fragment.findNavController
 import com.google.android.material.bottomsheet.BottomSheetBehavior
-import com.google.android.material.tabs.TabLayout
 import kotlinx.android.synthetic.main.component_tabstray2.*
 import kotlinx.android.synthetic.main.component_tabstray2.view.*
 import org.mozilla.fenix.R
@@ -72,6 +71,12 @@ class TabsTrayFragment : AppCompatDialogFragment(), TabsTrayInteractor {
         )
 
         setupPager(view.context, this, browserTrayInteractor)
+
+        TabLayoutMediator(
+            tabLayout = tab_layout,
+            interactor = this,
+            store = requireComponents.core.store
+        ).attach()
     }
 
     override fun setCurrentTrayPosition(position: Int) {
@@ -110,21 +115,5 @@ class TabsTrayFragment : AppCompatDialogFragment(), TabsTrayInteractor {
             adapter = TrayPagerAdapter(context, trayInteractor, browserInteractor)
             isUserInputEnabled = false
         }
-
-        tab_layout.addOnTabSelectedListener(TabLayoutObserver(trayInteractor))
-    }
-}
-
-/**
- * An observer for the [TabLayout] used for the Tabs Tray.
- */
-internal class TabLayoutObserver(
-    private val interactor: TabsTrayInteractor
-) : TabLayout.OnTabSelectedListener {
-    override fun onTabSelected(tab: TabLayout.Tab) {
-        interactor.setCurrentTrayPosition(tab.position)
     }
-
-    override fun onTabUnselected(tab: TabLayout.Tab) = Unit
-    override fun onTabReselected(tab: TabLayout.Tab) = Unit
 }

app/src/test/java/org/mozilla/fenix/tabstray/TabLayoutMediatorTest.kt

@@ -0,0 +1,73 @@
+/* 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.tabstray
+
+import com.google.android.material.tabs.TabLayout
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.verify
+import mozilla.components.browser.state.state.BrowserState
+import mozilla.components.browser.state.state.ContentState
+import mozilla.components.browser.state.state.TabSessionState
+import mozilla.components.browser.state.store.BrowserStore
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.mozilla.fenix.helpers.FenixRobolectricTestRunner
+import org.mozilla.fenix.tabstray.TrayPagerAdapter.Companion.POSITION_NORMAL_TABS
+import org.mozilla.fenix.tabstray.TrayPagerAdapter.Companion.POSITION_PRIVATE_TABS
+
+@RunWith(FenixRobolectricTestRunner::class)
+class TabLayoutMediatorTest {
+
+    @Test
+    fun `page to normal tab position when selected tab is also normal`() {
+        val store = createState("123")
+        val tabLayout: TabLayout = mockk(relaxed = true)
+        val tab: TabLayout.Tab = mockk(relaxed = true)
+        val mediator = TabLayoutMediator(tabLayout, mockk(relaxed = true), store)
+
+        every { tabLayout.getTabAt(POSITION_NORMAL_TABS) }.answers { tab }
+
+        mediator.selectActivePage()
+
+        verify { tab.select() }
+    }
+
+    @Test
+    fun `page to private tab position when selected tab is also private`() {
+        val store = createState("456")
+        val tabLayout: TabLayout = mockk(relaxed = true)
+        val tab: TabLayout.Tab = mockk(relaxed = true)
+        val mediator = TabLayoutMediator(tabLayout, mockk(relaxed = true), store)
+
+        every { tabLayout.getTabAt(POSITION_PRIVATE_TABS) }.answers { tab }
+
+        mediator.selectActivePage()
+
+        verify { tab.select() }
+    }
+
+    private fun createState(selectedId: String) = BrowserStore(
+        initialState = BrowserState(
+            tabs = listOf(
+                TabSessionState(
+                    id = "123",
+                    content = ContentState(
+                        private = false,
+                        url = "https://firefox.com"
+                    )
+                ),
+                TabSessionState(
+                    id = "456",
+                    content = ContentState(
+                        private = true,
+                        url = "https://mozilla.org"
+                    )
+                )
+            ),
+            selectedTabId = selectedId
+        )
+    )
+}