Stop a slow chain refresh from showing the wrong wallet balance

apps/ios/Koins/AppModel.swift

@@ -23,6 +23,9 @@ final class AppModel: ObservableObject {
   private var refreshTask: Task<Void, Never>?
   private var lastSample: (height: UInt64, date: Date)?
   private var lastCheckpoint = Date.distantPast
+  /// Invalidates in-flight refreshes: switching chains (or starting a newer
+  /// refresh) must stop an older request from writing its result to the UI.
+  private var refreshToken = UUID()
 
   var wallet: WalletRecord? {
     chain == .xrp ? xrpWallet : xmrWallet
@@ -93,6 +96,7 @@ final class AppModel: ObservableObject {
 
   func select(_ newChain: Chain) async {
     chain = newChain
+    refreshToken = UUID()
     errorMessage = nil
     statusMessage = nil
     balance = Balance(total: "0", unlocked: "", funded: false)
@@ -200,14 +204,21 @@ final class AppModel: ObservableObject {
 
   func refresh(force: Bool = false) async {
     guard hasWallet else { return }
+    let token = UUID()
+    refreshToken = token
+    let requestChain = chain
     errorMessage = nil
     if chain == .xrp, let wallet = xrpWallet {
       do {
         async let balanceCall = XRPWalletService.shared.balance(address: wallet.address)
         async let txCall = XRPWalletService.shared.transactions(address: wallet.address)
-        balance = try await balanceCall
-        transactions = try await txCall
+        let fetchedBalance = try await balanceCall
+        let fetchedTransactions = try await txCall
+        guard !isStale(token, requestChain) else { return }
+        balance = fetchedBalance
+        transactions = fetchedTransactions
       } catch {
+        guard !isStale(token, requestChain) else { return }
         errorMessage = error.localizedDescription
       }
       // The Monero wallet keeps scanning while XRP is on screen, so keep its
@@ -223,22 +234,36 @@ final class AppModel: ObservableObject {
     do {
       // Sync progress is cheap and polled every tick; the balance and transfer
       // list touch the wallet's data files, so they are refreshed less often.
-      updateSyncStatus(try await MoneroService.shared.syncStatus())
+      let status = try await MoneroService.shared.syncStatus()
+      if !isStale(token, requestChain) {
+        updateSyncStatus(status)
+      }
       refreshTick += 1
       await checkpointWalletIfSyncing()
       if force || refreshTick == 1 || refreshTick % 4 == 0 {
         async let balanceCall = MoneroService.shared.balance()
         async let txCall = MoneroService.shared.transactions()
         async let accountsCall = MoneroService.shared.accounts()
-        balance = try await balanceCall
-        transactions = try await txCall
-        moneroAccounts = try await accountsCall
+        let fetchedBalance = try await balanceCall
+        let fetchedTransactions = try await txCall
+        let fetchedAccounts = try await accountsCall
+        guard !isStale(token, requestChain) else { return }
+        balance = fetchedBalance
+        transactions = fetchedTransactions
+        moneroAccounts = fetchedAccounts
       }
     } catch {
+      guard !isStale(token, requestChain) else { return }
       errorMessage = error.localizedDescription
     }
   }
 
+  /// True when a newer refresh has started or the user moved to another chain
+  /// while this request was in flight.
+  private func isStale(_ token: UUID, _ requestChain: Chain) -> Bool {
+    token != refreshToken || requestChain != chain
+  }
+
   /// Tracks how fast blocks are being scanned so the UI can show an estimate.
   private func updateSyncStatus(_ status: SyncStatus) {
     syncStatus = status