Cache wallet balance and activity so screens show it instantly

apps/ios/Koins/Support/WalletStore.swift

@@ -63,9 +63,24 @@ enum WalletStore {
 
   static func removeWallet(_ chain: Chain) {
     defaults.removeObject(forKey: walletKey(chain))
+    defaults.removeObject(forKey: cacheKey(chain))
     Keychain.remove(secretKey(chain))
   }
 
+  // MARK: - Cached state (balance, transactions, accounts)
+
+  private static func cacheKey(_ chain: Chain) -> String { "koins.cache.\(chain.rawValue)" }
+
+  static func cachedState(_ chain: Chain) -> ChainCache? {
+    guard let data = defaults.data(forKey: cacheKey(chain)) else { return nil }
+    return try? JSONDecoder().decode(ChainCache.self, from: data)
+  }
+
+  static func saveCachedState(_ state: ChainCache, chain: Chain) {
+    guard let data = try? JSONEncoder().encode(state) else { return }
+    defaults.set(data, forKey: cacheKey(chain))
+  }
+
   static func secret(_ chain: Chain) -> String? {
     Keychain.get(secretKey(chain))
   }