Keep Monero scan progress across app restarts

apps/ios/Koins/AppModel.swift

@@ -21,6 +21,7 @@ final class AppModel: ObservableObject {
   private var refreshTick = 0
   private var refreshTask: Task<Void, Never>?
   private var lastSample: (height: UInt64, date: Date)?
+  private var lastCheckpoint = Date.distantPast
 
   var wallet: WalletRecord? {
     chain == .xrp ? xrpWallet : xmrWallet
@@ -212,6 +213,7 @@ final class AppModel: ObservableObject {
       // progress current here too.
       if moneroReady, let status = try? await MoneroService.shared.syncStatus() {
         updateSyncStatus(status)
+        await checkpointWalletIfSyncing()
       }
       return
     }
@@ -222,6 +224,7 @@ final class AppModel: ObservableObject {
       // list touch the wallet's data files, so they are refreshed less often.
       updateSyncStatus(try await MoneroService.shared.syncStatus())
       refreshTick += 1
+      await checkpointWalletIfSyncing()
       if force || refreshTick == 1 || refreshTick % 4 == 0 {
         async let balanceCall = MoneroService.shared.balance()
         async let txCall = MoneroService.shared.transactions()
@@ -334,4 +337,20 @@ final class AppModel: ObservableObject {
     errorMessage = nil
     statusMessage = nil
   }
+
+  /// Called when the app leaves the foreground: iOS can suspend or kill it at
+  /// any point, so flush the wallet while there is still time.
+  func persistWallet() async {
+    guard moneroReady else { return }
+    await MoneroService.shared.save()
+  }
+
+  /// monero-cpp only writes the wallet file from save(), and its sync loop
+  /// never does, so without this a restart re-scans from the beginning.
+  private func checkpointWalletIfSyncing() async {
+    guard moneroReady, syncStatus?.syncing == true else { return }
+    guard Date().timeIntervalSince(lastCheckpoint) >= 60 else { return }
+    lastCheckpoint = Date()
+    await MoneroService.shared.save()
+  }
 }