Preload the login page

WordPress/src/main/AndroidManifest.xml

@@ -1167,6 +1167,9 @@
         <intent>
             <action android:name="android.intent.action.MAIN" />
         </intent>
+        <intent>
+            <action android:name="android.support.customtabs.action.CustomTabsService" />
+        </intent>
         <!-- required for Android 11 (API level 30) or higher -->
         <package android:name="com.jetpack.android" />
     </queries>

WordPress/src/main/java/org/wordpress/android/modules/AppConfigModule.java

@@ -1,7 +1,6 @@
 package org.wordpress.android.modules;
 
 import android.content.Context;
-import android.os.Build;
 import android.util.Base64;
 import androidx.annotation.NonNull;
 

WordPress/src/main/java/org/wordpress/android/ui/accounts/LoginActivity.java

@@ -152,6 +152,10 @@ protected void onCreate(@Nullable Bundle savedInstanceState) {
         // Attempt Login if this activity was created in response to a user confirming login
         mLoginHelper.tryLoginWithDataString(getIntent().getDataString());
 
+        // Start preloading the WordPress.com login page if needed – this avoids visual hitches
+        // when displaying that screen
+        mLoginHelper.bindCustomTabsService(this);
+
         if (mLoginHelper.isLoggedIn()) {
             this.loggedInAndFinish(new ArrayList<Integer>(), true);
             return;
@@ -477,7 +481,7 @@ public void showWPcomLoginScreen(@NonNull Context context) {
                 .setShowTitle(false)
                 .build();
 
-        intent.launchUrl(this, mLoginHelper.loginUri());
+        intent.launchUrl(this, mLoginHelper.getWpcomLoginUri());
     }
 
     @Override

WordPress/src/main/java/org/wordpress/android/ui/accounts/login/WPcomLoginHelper.kt

@@ -1,7 +1,13 @@
 package org.wordpress.android.ui.accounts.login
 
+import android.content.ComponentName
+import android.content.Context
 import android.net.Uri
 import android.util.Log
+import androidx.browser.customtabs.CustomTabsCallback
+import androidx.browser.customtabs.CustomTabsClient
+import androidx.browser.customtabs.CustomTabsServiceConnection
+import androidx.browser.customtabs.CustomTabsSession
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.cancel
 import kotlinx.coroutines.runBlocking
@@ -14,13 +20,12 @@ import kotlin.coroutines.CoroutineContext
 class WPcomLoginHelper @Inject constructor(
     private val loginClient: WPcomLoginClient,
     private val accountStore: AccountStore,
-    private val appSecrets: AppSecrets
+    appSecrets: AppSecrets
 ) {
     private val context: CoroutineContext = Dispatchers.IO
 
-    fun loginUri(): Uri {
-        return loginClient.loginUri(appSecrets.redirectUri)
-    }
+    val wpcomLoginUri = loginClient.loginUri(appSecrets.redirectUri)
+    private val customTabsServiceConnection = ServiceConnection(wpcomLoginUri)
 
     fun tryLoginWithDataString(data: String?) {
         if (data == null) {
@@ -44,7 +49,47 @@ class WPcomLoginHelper @Inject constructor(
         context.cancel()
     }
 
+    fun bindCustomTabsService(context: Context) {
+        customTabsServiceConnection.bind(context)
+    }
+
     private fun codeFromAuthorizationUri(string: String): String? {
         return Uri.parse(string).getQueryParameter("code")
     }
 }
+
+class ServiceConnection(
+    var uri: Uri
+): CustomTabsServiceConnection() {
+    private var client: CustomTabsClient? = null
+    private var session: CustomTabsSession? = null
+
+    override fun onCustomTabsServiceConnected(name: ComponentName, client: CustomTabsClient) {
+        client.warmup(0)
+        this.client = client
+
+        val session = client.newSession(CustomTabsCallback())
+        session?.mayLaunchUrl(uri, null, null)
+        session?.mayLaunchUrl(Uri.parse("https://wordpress.com/log-in/"), null, null)
+
+        this.session = session
+    }
+
+    override fun onServiceDisconnected(name: ComponentName?) {
+        this.client = null
+        this.session = null
+    }
+
+    fun bind(context: Context) {
+        // Do nothing if there is an existing service connection
+        if (this.client != null) {
+            return
+        }
+
+        // Get the default browser package name, this will be null if
+        // the default browser does not provide a CustomTabsService
+        val packageName = CustomTabsClient.getPackageName(context, null)  ?:  return
+
+        CustomTabsClient.bindCustomTabsService(context, packageName, this)
+    }
+}