How to verify the type/name of token received in a smart contract?

Question

Here is a sample contract which is designed to take xyz token. How can the contract verify that the sent is the xyz token?

pragma "^0.7.0"

contract myContract {
  address public payable owner;
  uint public value;  //<<==xyz token

  constructor(uint _value) payable {
    owner = msg.sender;
    value = _value;  //<<== how does the contract verify it is the _value of xyz token?
  }
}

Answer 1

To receive ERC20 payments you usually use the combination of user's approval and transferFrom from the contract.

1. User calls token's approve function with the recipient contract address. Using a wallet, perhaps Metamask. In javascript it will look like:

    token.approve(recipientContactAddress, amount, { from: userAddress });
   

2. User calls a function from the recipient contract, deposit, let's call that function deposit.

    recipientContract.deposit(amount, { from: userAddress });
   

3. Recipient contract calls token's transferFrom

    function deposit(uint256 amount) public {
        require(token.transferFrom(msg.sender, address(this), amount), "transferFrom failed");
    }
   

Answer 2

In this post, the answer for case 1 There is a contract A and a contract B and they need to communicate. A is not B and B is not A, but A needs enough information about B in order to communicate with it, and the reverse. answers my question.