Truffle test case to unit test withdrawal pattern

Question

How can I test the withdraw() function in a truffle javascript test case for the below contract?

The problem is, If I use the msg.sender from the same contract, the before and after balance will be same.

Is there a way to create a new test contract and use it to call the withdraw function from the javascript test case?

pragma solidity ^0.4.18;

import "./ConvertLib.sol";

contract MetaCoin {
    mapping (address => uint) balances;

    event NewDeposit(address _from, uint256 _value);

    function MetaCoin() public {
        balances[tx.origin] = 10000;
    }

    function getBalance(address addr) public view returns(uint) {
        return balances[addr];
    }

    function deposit() payable {
        balances[msg.sender] += msg.value;
        NewDeposit(msg.sender, msg.value);
    }

    function withdraw() {

        if ( ! (msg.sender.call.value(balances[msg.sender])() ) ) {
            throw;
        }

        balances[msg.sender] = 0; // Please don't worry about re-entrancy for now.
    }                   
}

Withdrawal Pattern

Answer 1

For your test you can crate a DepositorWithdrawer contract able to call deposit/withdraw from MetaCoin. When the DepositorWithdrawer contract is calling MetaCoin, msg.sender will be the contract address.

The process would look something like this.

MetaCoin mt = new MetaCoin();
DepositorWithdrawer w = new DepositorWithdrawer();
w.transfer(10 ether);
w.deposit(mt);
// do your assert here
w.withdraw(mt);
// do your assert here

Personally, I would prefer doing this as a JavaScript Test. There you have better control about who is sending the transaction and you don't have to code unnecessary contracts.