Receive ERC20 token as value in function of smart contract

Question

I want to receive an erc20 token as value for a function.
Basically, the function caller defines an amount which will then be transferred from their balance to the smart contract. Which transfer pattern should I use for this?

As an example, Why will this function not accept my ERC20 token input?

contract Transform {

    InputToken inputToken;
    OutputToken outputToken;

    event Transformed(address from, uint256 input, uint256 output);

    constructor(address _inputToken, address _outputToken) public {
    inputToken = InputToken(_inputToken); 
    outputToken = OutputToken(_outputToken); 
}

    function transformTokens(uint256 _amount) external payable {
        require(_amount > 0);

        uint256 input = _amount; //It's reverting here?? I don't see the issue
        uint256 rate = 1000;

        uint256 output = input.div(rate);
        uint256 excess;

        inputToken.transferFrom(msg.sender, address(this), input);
        outputToken.mint(msg.sender, output);

        Transformed(msg.sender, input, output);
    }
}

Answer 1

For a standard ERC20 this will not work

inputToken.transfer(address(this), input);

The contract is transferring tokens to itself.

To fix this you have to use transferFrom

inputToken.transferFrom(msg.sender, address(this), input);

For it to work the token's owner has to call approve to the contract address.

In javascript will be something like this

inputToken.methods.approve(contractAddress, amount, { from: tokenOwner });

Note: A contract cannot approve on behalf of someone else. Only the token's owner can call approve.