android: synchronize ipn state and UI (#585)

Pass in intended state to toggleVpn and keep track of progress to avoid redundant updates and ensure that the action taken reflects the user's intent.
This fixes a possible recomposition loop caused by the ipn state and the vpn toggle state getting out of sync.

Updates tailscale/tailscale#14125

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

android/src/main/java/com/tailscale/ipn/ui/view/MainView.kt

@@ -145,13 +145,12 @@ fun MainView(
                 leadingContent = {
                   if (!hideHeader) {
                     TintedSwitch(
-                        onCheckedChange = {
-                          if (!disableToggle.value) {
-                            viewModel.toggleVpn()
-                          }
-                        },
-                        enabled = !disableToggle.value,
-                        checked = isOn)
+                        checked = isOn,
+                        enabled =
+                            !disableToggle.value &&
+                                !viewModel.isToggleInProgress
+                                    .value, // Disable switch if toggle is in progress
+                        onCheckedChange = { desiredState -> viewModel.toggleVpn(desiredState) })
                   }
                 },
                 headlineContent = {
@@ -228,7 +227,7 @@ fun MainView(
                     // action (eg, if the user connected to another VPN).
                     state != Ipn.State.Stopping,
                     user,
-                    { viewModel.toggleVpn() },
+                    { viewModel.toggleVpn(desiredState = !isOn) },
                     { viewModel.login() },
                     loginAtUrl,
                     netmap?.SelfNode,

android/src/main/java/com/tailscale/ipn/ui/viewModel/MainViewModel.kt

@@ -18,7 +18,6 @@ import com.tailscale.ipn.ui.model.Ipn
 import com.tailscale.ipn.ui.model.Ipn.State
 import com.tailscale.ipn.ui.model.Tailcfg
 import com.tailscale.ipn.ui.notifier.Notifier
-import com.tailscale.ipn.ui.util.AndroidTVUtil.isAndroidTV
 import com.tailscale.ipn.ui.util.PeerCategorizer
 import com.tailscale.ipn.ui.util.PeerSet
 import com.tailscale.ipn.ui.util.TimeUtil
@@ -53,6 +52,10 @@ class MainViewModel(private val vpnViewModel: VpnViewModel) : IpnViewModel() {
   private val _vpnToggleState = MutableStateFlow(false)
   val vpnToggleState: StateFlow<Boolean> = _vpnToggleState
 
+  // Keeps track of whether a toggle operation is in progress. This ensures that toggleVpn cannot be
+  // invoked until the current operation is complete.
+  var isToggleInProgress = MutableStateFlow(false)
+
   // Permission to prepare VPN
   private var vpnPermissionLauncher: ActivityResultLauncher<Intent>? = null
 
@@ -184,15 +187,33 @@ class MainViewModel(private val vpnViewModel: VpnViewModel) : IpnViewModel() {
     }
   }
 
-  fun toggleVpn() {
-    val state = Notifier.state.value
-    val isPrepared = vpnViewModel.vpnPrepared.value
+  fun toggleVpn(desiredState: Boolean) {
+    if (isToggleInProgress.value) {
+      // Prevent toggling while a previous toggle is in progress
+      return
+    }
 
-    when {
-      !isPrepared -> showVPNPermissionLauncherIfUnauthorized()
-      state == Ipn.State.Running -> stopVPN()
-      state == Ipn.State.NeedsLogin && isAndroidTV() -> login()
-      else -> startVPN()
+    viewModelScope.launch {
+      isToggleInProgress.value = true
+      try {
+        val currentState = Notifier.state.value
+        val isPrepared = vpnViewModel.vpnPrepared.value
+
+        if (desiredState) {
+          // User wants to turn ON the VPN
+          when {
+            !isPrepared -> showVPNPermissionLauncherIfUnauthorized()
+            currentState != Ipn.State.Running -> startVPN()
+          }
+        } else {
+          // User wants to turn OFF the VPN
+          if (currentState == Ipn.State.Running) {
+            stopVPN()
+          }
+        }
+      } finally {
+        isToggleInProgress.value = false
+      }
     }
   }