Fee Abstraction support

Since we provide a solution for applications/businesses, we don't want application users to take care of fees.

During the contract execution we provide a possibility to refund amount of fees to be charged from the user's wallet.

How it works?

• We fund native tokens to the Smart Contract address during the deployment (endowment parameter) or later by transfer native tokens.
• Before the Smart Contract execution we estimate amount of fees to be paid by user's wallet/account.
• During the Smart Contract execution we refund tokens from Smart Contract address to the user's wallet (who submit Smart Contract call transaction).

You can use either Polkadot UI to test this or do it programmatically:

1. Send assets from user's account to the application account with refund using Polkadot UI:

Use Tutorial to configure Node, run Polkadot UI and deploy Smart Contract.

Contract call for the transfer function consist of transactionFee parameter: Let's keep it 0 for now and click Execute button.

Here we can see estimated fees for this transaction: You can find it above the sending from my account block. So, 125.0002 nano Unit will be charged.

Click Cancel and specify this exact amount of tokens to be transferred: Now we can click Execute button and Sign and Submit on the next window.

We are done!

2. Send assets programmatically:

Let's create transaction object first:

const transferObj = await this.contract.tx.transfer(
  anyValue,
  gasLimitAuto,
  destinationAccountPublicKey,
  amount,
);

Now we can get estimated fees using helper:

const info = await transferObj.paymentInfo(this.userKeyring);
const partialFee = info.partialFee.toBn();

After we got the value we can re-create transferObject like this:

const transferObj = await this.contract.tx.transfer(
  anyValue,
  gasLimitAuto,
  destinationAccountPublicKey,
  amount,
  partialFee,
);

Now we'are ready to submit transaction to the network:

transferObj.signAndSend(this.userKeyring, <callback>);

Try this out using Test Scripts!