[Question] Create Smart Contract call transaction offline and submit it later

[evgeny-s]

Hey, guys.
Thanks for this useful lib.
Is it a possibility to create a Smart Contract call offline, wrap it to string let's say, share a transaction string (for example put it to DB), and later unwrap and submit it to the Blockchain?

Thanks!

[emostov]

Hey 👋

I am actually not personally familiar with transactions for contracts, but if they are standard transactions (extrinsics), you can encode the transaction, which turns it into a SCALE encoded hex string.

You can store that hex string in a DB, and you can store it at various stages of construction and should still be able to decode it with polkadot-js and/or the utils from this lib to sign etc. Additionally, polkadot-js is pretty flexible when it comes into reading data so in many cases it can also read in a JSON representation of the transaction.

The final transaction is broadcasted as a hex string so you could easily store that in a DB as well and broadcast it as is.

[azhu003]

Hi.

Have you implemented the offline creation contract call?

[evgeny-s]

Hello, @azhu003
Yes, I was able to do it using extrinsics and polkadot JS lib. Here is the example:

 this.appKeyring = keyring.addFromJson(
 JSON.parse(process.env.APP_WALLET_JSON)
 );

 const gasLimitAuto = -1;
 const anyValue = 0;
 const transferObj = await this.contract.tx.methodName(
 anyValue,
 gasLimitAuto,
 param1,
 param2,
 param3,
 param4,
 param5
 );

 return new Promise((res, rej) => {
 transferObj
 .signAndSend(this.appKeyring, ({ status }) => {
 if (status.isInBlock) {
 const hash = status.asInBlock.toHex();
 this.logger.log(`In block: ${hash}`);
 res(hash);
 }
 })
 .catch((err) => rej(err));
 });

[evgeny-s]

Above - online call.
Below offline:

public async createSignedTransaction(
 privateKey: string,
 destination: string,
 amount: string,
 ): Promise<SignedDataResponse> {
 const gasLimitAuto = -1;
 const anyValue = 0;

 const keyring = new Keyring({type: this.userKeyRingType});
 const userKeyRing = keyring.addFromSeed(hexToU8a(privateKey));

 const tx = await this.contract.tx
 .transfer(anyValue, gasLimitAuto, destination, Number(amount), partialFee)
 .signAsync(userKeyRing);
 const txString = JSON.stringify(tx);
 const signedDataDto = new SignedDataResponse();
 signedDataDto.tx = txString;

 return signedDataDto;
 }


public async submitTx(tx: string, publicKey: string, appId: string): Promise<SubmitTransactionRes> {
 const txn = JSON.parse(tx);

 const txnHash = await this.substrateApi.rpc.author.submitExtrinsic(txn);
 const txnString = JSON.stringify(txnHash);
 const submitTxResponse = new SubmitTransactionRes(txnString);

 this.logger.log(`Tx hash: ${txnHash}`);

 return submitTxResponse;
 }

[evgeny-s]

@azhu003
Hope it helps.

[azhu003]

@evgeny-s thanks😁