run createSparkSend in isolate

lib/wallets/wallet/wallet_mixin_interfaces/spark_interface.dart

@@ -304,7 +304,9 @@ mixin SparkInterface on Bip39HDWallet, ElectrumXInterface {
     );
     extractedTx.setPayload(Uint8List(0));
 
-    final spend = LibSpark.createSparkSendTransaction(
+    final spend = await compute(
+      _createSparkSend,
+      (
         privateKeyHex: privateKey.toHex,
         index: kDefaultSparkIndex,
         recipients: txData.recipients
@@ -327,9 +329,11 @@ mixin SparkInterface on Bip39HDWallet, ElectrumXInterface {
         serializedCoins: serializedCoins,
         allAnonymitySets: allAnonymitySets,
         idAndBlockHashes: idAndBlockHashes
-          .map((e) => (setId: e.groupId, blockHash: base64Decode(e.blockHash)))
+            .map(
+                (e) => (setId: e.groupId, blockHash: base64Decode(e.blockHash)))
             .toList(),
         txHash: extractedTx.getHash(),
+      ),
     );
 
     for (final outputScript in spend.outputScripts) {
@@ -853,6 +857,63 @@ String base64ToReverseHex(String source) =>
         .map((e) => e.toRadixString(16).padLeft(2, '0'))
         .join();
 
+/// Top level function which should be called wrapped in [compute]
+Future<
+    ({
+      Uint8List serializedSpendPayload,
+      List<Uint8List> outputScripts,
+      int fee,
+    })> _createSparkSend(
+    ({
+      String privateKeyHex,
+      int index,
+      List<
+          ({
+            String address,
+            int amount,
+            bool subtractFeeFromAmount
+          })> recipients,
+      List<
+          ({
+            String sparkAddress,
+            int amount,
+            bool subtractFeeFromAmount,
+            String memo
+          })> privateRecipients,
+      List<
+          ({
+            String serializedCoin,
+            String serializedCoinContext,
+            int groupId,
+            int height,
+          })> serializedCoins,
+      List<
+          ({
+            int setId,
+            String setHash,
+            List<({String serializedCoin, String txHash})> set
+          })> allAnonymitySets,
+      List<
+          ({
+            int setId,
+            Uint8List blockHash,
+          })> idAndBlockHashes,
+      Uint8List txHash,
+    }) args) async {
+  final spend = LibSpark.createSparkSendTransaction(
+    privateKeyHex: args.privateKeyHex,
+    index: args.index,
+    recipients: args.recipients,
+    privateRecipients: args.privateRecipients,
+    serializedCoins: args.serializedCoins,
+    allAnonymitySets: args.allAnonymitySets,
+    idAndBlockHashes: args.idAndBlockHashes,
+    txHash: args.txHash,
+  );
+
+  return spend;
+}
+
 /// Top level function which should be called wrapped in [compute]
 Future<List<SparkCoin>> _identifyCoins(
     ({