Implement smart contract that can spend user's tokens with just one approve

Question

I am looking for a way to implement smart contract that can spend user's ETH or erc-20 tokens with just one approve, without anything else required from user.

USECASE

• 1.User approves token spending(usdt for example) he has 10 USDT.

• 2.WITHOUT USER INTERACTION our smart contract can transfer this 10 usdt from user to wherever else it is coded to transfer it. (This function spend_user_usdt should be invoked with either something like chainlink cron job or manually by contract creator.

• 3. Since User already approved token spending it doesn't require anything to do, tokens are transfered from his wallet automatically

I know this is possible because sometimes smart contracts get hacked and users have to revoke tokens approval, or their tokens are gone without additional manual confirmation.

But I wasn't able to find decent information on it.

PS: Just in case I don't want to write malicious smart contracts - that in not in the zone of my interests. This mechanics could be used to build very interesting projects that simplify user experience in many areas.

Answer 1

This is part of the ERC20 specification.

See here, the transferFrom() function. https://eips.ethereum.org/EIPS/eip-20#methods

Users send a transaction to the token contract to approve your contract to spend an allowance, the $10 over which your contract would have control.

How your contract exercises that control is entirely an application concern. I see no reason why a chainLink keeper or other external signal couldn't trigger the "approved" contract to take action, spending the user's funds.

Hope it helps.

Answer 2

As Rob Hitchens said, this is an already existing feature of Erc20 tokens. However, i would like to add that you can't do that with ETH, since it's not a token. But you can do this with wrapped eth, and it's the same very simple mecanism.