For mozilla-mobile#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

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

@@ -143,6 +143,7 @@ open class FenixApplication : LocaleAwareApplication() {
             }
         }
 
+        prefetchForHomeFragment()
         setupLeakCanary()
         if (settings().isTelemetryEnabled) {
             components.analytics.metrics.start(MetricServiceType.Data)
@@ -229,6 +230,13 @@ 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

@@ -215,6 +215,12 @@ class HomeFragment : Fragment() {
             sessionControlInteractor,
             homeViewModel
         )
+
+        // This has to be called separately from the consumeFrom block since the coroutine doesn't
+        // allow our UI to be updated right away and delays our start up. The init block allows us to
+        // force our UI to render with the data available in our store at fragment creation time.
+        sessionControlView?.update(homeFragmentStore.state)
+
         activity.themeManager.applyStatusBarTheme(activity)
 
         view.consumeFrom(homeFragmentStore, viewLifecycleOwner) {