For #11660: added prefetch for topsites and update in onCreateView() (#11668)

* For #11660:added prefetch for topsites

TopSites will be prefetched with observerOnce (wrapper around observerForever).
Also, the SessionControlView.update() is called right away instead of waiting from consumeFrom
in the HomeFragment.onCreateView() which will allow the UI to render all at once on its first
perform traversal

* Removed the submitList(null) since it retriggered a drawing on lower end device

app/src/main/java/org/mozilla/fenix/FenixApplication.kt

@@ -142,6 +142,7 @@ open class FenixApplication : LocaleAwareApplication() {
             }
         }
 
+        prefetchForHomeFragment()
         setupLeakCanary()
         if (settings().isTelemetryEnabled) {
             components.analytics.metrics.start(MetricServiceType.Data)
@@ -228,6 +229,13 @@ open class FenixApplication : LocaleAwareApplication() {
         // no-op, LeakCanary is disabled by default
     }
 
+    // This is for issue https://github.com/mozilla-mobile/fenix/issues/11660. We prefetch our info for startup
+    // so that we're sure that we have all the data available as our fragment is launched.
+    private fun prefetchForHomeFragment() {
+        StrictMode.allowThreadDiskReads().resetPoliciesAfter {
+            components.core.topSiteStorage.prefetch()
+        }
+    }
     private fun setupPush() {
         // Sets the PushFeature as the singleton instance for push messages to go to.
         // We need the push feature setup here to deliver messages in the case where the service

app/src/main/java/org/mozilla/fenix/components/TopSiteStorage.kt

@@ -14,6 +14,7 @@ import mozilla.components.feature.top.sites.TopSite
 import mozilla.components.feature.top.sites.TopSiteStorage
 import mozilla.components.support.locale.LocaleManager
 import org.mozilla.fenix.R
+import org.mozilla.fenix.ext.observeOnce
 import org.mozilla.fenix.ext.settings
 import org.mozilla.fenix.settings.SupportUtils
 import org.mozilla.fenix.settings.advanced.getSelectedLocale
@@ -86,4 +87,10 @@ class TopSiteStorage(private val context: Context) {
             context.settings().defaultTopSitesAdded = true
         }
     }
+
+    fun prefetch() {
+        getTopSites().observeOnce {
+            cachedTopSites = it
+        }
+    }
 }

app/src/main/java/org/mozilla/fenix/ext/LiveData.kt

@@ -0,0 +1,20 @@
+/* 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.ext
+
+import androidx.lifecycle.LiveData
+import androidx.lifecycle.Observer
+
+/**
+ * Observe a LiveData once and unregister from it as soon as the live data returns a value
+ */
+fun <T> LiveData<T>.observeOnce(observer: (T) -> Unit) {
+    observeForever(object : Observer<T> {
+        override fun onChanged(value: T) {
+            removeObserver(this)
+            observer(value)
+        }
+    })
+}

app/src/main/java/org/mozilla/fenix/home/HomeFragment.kt

@@ -215,11 +215,10 @@ class HomeFragment : Fragment() {
             sessionControlInteractor,
             homeViewModel
         )
-        activity.themeManager.applyStatusBarTheme(activity)
 
-        view.consumeFrom(homeFragmentStore, viewLifecycleOwner) {
-            sessionControlView?.update(it)
-        }
+        updateSessionControlView(view)
+
+        activity.themeManager.applyStatusBarTheme(activity)
 
         view.consumeFrom(requireComponents.core.store, viewLifecycleOwner) {
             val tabCount = if (currentMode.getCurrentMode() == Mode.Normal) {
@@ -234,6 +233,20 @@ class HomeFragment : Fragment() {
         return view
     }
 
+    /**
+     * The [SessionControlView] is forced to update with our current state when we call
+     * [HomeFragment.onCreateView] in order to be able to draw everything at once with the current
+     * data in our store. The [View.consumeFrom] coroutine dispatch
+     * doesn't get run right away which means that we won't draw on the first layout pass.
+     */
+    fun updateSessionControlView(view: View) {
+        sessionControlView?.update(homeFragmentStore.state)
+
+        view.consumeFrom(homeFragmentStore, viewLifecycleOwner) {
+            sessionControlView?.update(it)
+        }
+    }
+
     private fun updateLayout(view: View) {
         val shouldUseBottomToolbar = view.context.settings().shouldUseBottomToolbar
 

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

@@ -4,7 +4,6 @@
 
 package org.mozilla.fenix.home.sessioncontrol
 
-import android.os.Build
 import android.view.View
 import androidx.recyclerview.widget.ItemTouchHelper
 import androidx.recyclerview.widget.LinearLayoutManager
@@ -142,13 +141,8 @@ class SessionControlView(
     }
 
     fun update(state: HomeFragmentState) {
-        // Workaround for list not updating until scroll on Android 5 + 6
-        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.N) {
-            sessionControlAdapter.submitList(null)
-        }
 
         val stateAdapterList = state.toAdapterList()
-
         if (homeScreenViewModel.shouldScrollToTopSites) {
             sessionControlAdapter.submitList(stateAdapterList) {