Is it possible for a contract to erc20 approve another contract?

Question

I want my contract (#1) to be able to approve another contract (#2) to transfer erc20 tokens from #1 to #2. Is this possible?

Answer 1

Yes, a contract can approve another contract to make transfer from its tokens.

contract A {
    ERC20 token;
    function approveAndCall(B recipient, uint amount) public {
        // Approve recipient to spend amount tokens
        require(token.approve(address(recipient), amount), "Approve has failed");

        // Call contract B to process payment
        require(recipient.processPayment(amount), "Payment failed");
    }
}

Recipient contract

contract B {
    ERC20 token;
    uint256 public constant ticketPrice = 10 ** 18; // 1 token assuming 18 decimals
    function processPayment(uint amount) public return bool {
        // Check price
        if (amount >= ticketPrice) {
            // Revert if tokens cannot be transfered
            require(token.transferFrom(msg.sender, address(this), ticketPrice), "Transfer failed");
            // Generate event when ticket is sold
            emit TicketSold(msg.sender, ticketPrice);
            return true;
        } else {
            // Return false to make caller detect a payment failure
            return false;
        }
    }
}