What is the best way to tranfer ERC20 token from a contract?

Question

I have a smart contract called "PlayerContract" and a smart contract which implements ERC20 called "MyToken".

in PlayerContract I have a method that accepts an uint256 (represent an amount of MyToken) from users and when we reach a known condition one user win the sum of tokens.

example :

function play (uint256 tokens) public {

   // store values for each participant.

  if (condition) {
   //transfer token to winers from loosers
  }
}

My question is how can "PlayerContract" transfer token from one to onther and what is the best way to do that?

Answer 1

You need to interface playerContract with MyToken.

contract MyToken{

    ...

}

contract playerContract{


    function play (uint256 tokens) public {
        // store values for each participant.
        if (condition) {
            MyToekn instance = MyToken('address of Mytoken');
            instance.transfer('winner address', ntokens); 
        }
    }


}

The idea is that the players are sending tokens to playerContract Address. For the contract to be able to transfer these tokens to the winner it needs to use the function transfer in MyToken, so you define an instance to the contact and now playerContract can use the transfer function.

Hope this helps

EDIT: using a single transaction.

This can be achieved with approveAndCall. The ERC20 contract must have implemented it and something like this is in the standard:

contract ApproveAndCallFallBack {
    function receiveApproval(address from, uint256 tokens, address token, bytes data) public;
}

In you contract the function receive approval must be implemented. You will be getting the information of who is sending the tokens, how much, from which token contract and to whom, you also get a variable data in case you need them to pass data, but this is not the case.

contract MyToken{

...

}

contract playerContract{


function play (uint256 tokens, address from) internal {
    // store values for each participant.
    if (condition) {
        MyToekn instance = MyToken('address of Mytoken');
        instance.transfer('winner address', ntokens); 
    }
}


function receiveApproval(address from, uint256 tokens, address token, bytes data) public{
     require(token=='your token address');
     delete data;
     MyToekn instance = MyToken(token);
     instance.transferFrom(from,address(this), tokens)
     play(tokens, from); 
}

}

The require(token=='your token address'); is there because people may try to pay you with a token different than yours.

Hope this helps.

Note: do not forget that if you have some profit in tokens these will be linked to the contract and you will need to implement a function to move then like in the play function.

Answer 2

import MyToken from './MyToken.sol'

In your function

MyToken(_tokenAddress).transfer(_to, _amount)

So either you have to pass in _tokenAddress as an argument to play(), or set it in a constructor.