[fenix] For https://github.com/mozilla-mobile/fenix/issues/9506: Add session observer to check conditions for displaying banner.

app/src/main/java/org/mozilla/fenix/browser/BrowserFragment.kt

@@ -157,6 +157,20 @@ class BrowserFragment : BaseBrowserFragment(), UserInteractionHandler {
         }
         session?.register(toolbarSessionObserver, viewLifecycleOwner, autoPause = true)
 
+        if (settings.shouldShowOpenInAppBanner) {
+            session?.register(
+                OpenInAppOnboardingObserver(
+                    context = context,
+                    navController = findNavController(),
+                    settings = settings,
+                    appLinksUseCases = context.components.useCases.appLinksUseCases,
+                    container = browserToolbarView.view.parent as ViewGroup
+                ),
+                owner = this,
+                autoPause = true
+            )
+        }
+
         if (!settings.userKnowsAboutPwas) {
             session?.register(
                 PwaOnboardingObserver(

app/src/main/java/org/mozilla/fenix/browser/OpenInAppOnboardingObserver.kt

@@ -0,0 +1,64 @@
+/* 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.browser
+
+import android.content.Context
+import android.view.ViewGroup
+import androidx.navigation.NavController
+import mozilla.components.browser.session.Session
+import mozilla.components.feature.app.links.AppLinksUseCases
+import mozilla.components.support.ktx.kotlin.tryGetHostFromUrl
+import org.mozilla.fenix.R
+import org.mozilla.fenix.ext.nav
+import org.mozilla.fenix.utils.Settings
+
+/**
+ * Displays an [InfoBanner] when a user visits a website that can be opened in an installed native app.
+ */
+class OpenInAppOnboardingObserver(
+    private val context: Context,
+    private val navController: NavController,
+    private val settings: Settings,
+    private val appLinksUseCases: AppLinksUseCases,
+    private val container: ViewGroup
+) : Session.Observer {
+
+    private var sessionDomainForDisplayedBanner: String? = null
+    private var infoBanner: InfoBanner? = null
+
+    override fun onUrlChanged(session: Session, url: String) {
+        sessionDomainForDisplayedBanner?.let {
+            if (url.tryGetHostFromUrl() != it) {
+                infoBanner?.dismiss()
+            }
+        }
+    }
+
+    override fun onLoadingStateChanged(session: Session, loading: Boolean) {
+        val appLink = appLinksUseCases.appLinkRedirect
+
+        if (!loading &&
+            settings.shouldShowOpenInAppBanner &&
+            appLink(session.url).hasExternalApp()
+        ) {
+            infoBanner = InfoBanner(
+                context = context,
+                message = context.getString(R.string.open_in_app_cfr_info_message),
+                dismissText = context.getString(R.string.open_in_app_cfr_negative_button_text),
+                actionText = context.getString(R.string.open_in_app_cfr_positive_button_text),
+                container = container
+            ) {
+                val directions = BrowserFragmentDirections.actionBrowserFragmentToSettingsFragment(
+                    preferenceToScrollTo = context.getString(R.string.pref_key_open_links_in_external_app)
+                )
+                navController.nav(R.id.browserFragment, directions)
+            }
+
+            infoBanner?.showBanner()
+            sessionDomainForDisplayedBanner = session.url.tryGetHostFromUrl()
+            settings.shouldShowOpenInAppBanner = false
+        }
+    }
+}