MainActivity: bring activity to focus on login (#273)

Use CustomTabsIntent to launch login, but fall back to regular intent if Chrome is not installed
When logging in, on change in state to login complete, bring activity back to focus

Updates tailscale/corp#18202

Signed-off-by: kari-ts <kari@tailscale.com>

android/src/main/java/com/tailscale/ipn/MainActivity.kt

@@ -16,9 +16,11 @@ import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.result.ActivityResultLauncher
 import androidx.activity.result.contract.ActivityResultContract
+import androidx.browser.customtabs.CustomTabsIntent
 import androidx.compose.animation.core.tween
 import androidx.compose.animation.slideInHorizontally
 import androidx.compose.animation.slideOutHorizontally
+import androidx.core.net.toUri
 import androidx.lifecycle.lifecycleScope
 import androidx.navigation.NavType
 import androidx.navigation.compose.NavHost
@@ -28,6 +30,7 @@ import androidx.navigation.navArgument
 import androidx.navigation.navigation
 import com.tailscale.ipn.Peer.RequestCodes
 import com.tailscale.ipn.mdm.MDMSettings
+import com.tailscale.ipn.ui.model.Ipn
 import com.tailscale.ipn.ui.notifier.Notifier
 import com.tailscale.ipn.ui.theme.AppTheme
 import com.tailscale.ipn.ui.view.AboutView
@@ -52,6 +55,7 @@ import com.tailscale.ipn.ui.viewModel.ExitNodePickerNav
 import com.tailscale.ipn.ui.viewModel.SettingsNav
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.cancel
 import kotlinx.coroutines.launch
 
 class MainActivity : ComponentActivity() {
@@ -64,6 +68,7 @@ class MainActivity : ComponentActivity() {
     @JvmStatic val requestPrepareVPN: Int = 1001
 
     const val WRITE_STORAGE_RESULT = 1000
+    private const val TAG = "Main Activity"
   }
 
   override fun onCreate(savedInstanceState: Bundle?) {
@@ -182,12 +187,43 @@ class MainActivity : ComponentActivity() {
     }
   }
 
-  private fun login(url: String) {
-    // (jonathan) TODO: This is functional, but the navigation doesn't quite work
-    // as expected.  There's probably a better built in way to do this.  This will
-    // unblock in dev for the time being though.
-    val browserIntent = Intent(Intent.ACTION_VIEW, Uri.parse(url))
-    startActivity(browserIntent)
+  private fun login(urlString: String) {
+    // Launch coroutine to listen for state changes. When the user completes login, relaunch
+    // MainActivity to bring the app back to focus.
+    App.getApplication().applicationScope.launch {
+      try {
+        Notifier.state.collect { state ->
+          if (state > Ipn.State.NeedsMachineAuth) {
+            val intent =
+                Intent(applicationContext, MainActivity::class.java).apply {
+                  addFlags(Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TOP)
+                  action = Intent.ACTION_MAIN
+                  addCategory(Intent.CATEGORY_LAUNCHER)
+                }
+            startActivity(intent)
+
+            // Cancel coroutine once we've logged in
+            this@launch.cancel()
+          }
+        }
+      } catch (e: Exception) {
+        Log.e(TAG, "Login: failed to start MainActivity: $e")
+      }
+    }
+
+    val url = urlString.toUri()
+    try {
+      val customTabsIntent = CustomTabsIntent.Builder().build()
+      customTabsIntent.launchUrl(this, url)
+    } catch (e: Exception) {
+      // Fallback to a regular browser if CustomTabsIntent fails
+      try {
+        val fallbackIntent = Intent(Intent.ACTION_VIEW, url)
+        startActivity(fallbackIntent)
+      } catch (e: Exception) {
+        Log.e(TAG, "Login: failed to open browser: $e")
+      }
+    }
   }
 
   override fun onResume() {