openzeppelin-contracts/contracts/payment/escrow/Escrow.sol

pragma solidity ^0.4.24;

import "../../math/SafeMath.sol";
import "../../ownership/Secondary.sol";

 /**
 * @title Escrow
 * @dev Base escrow contract, holds funds designated for a payee until they
 * withdraw them.
 * @dev Intended usage: This contract (and derived escrow contracts) should be a
 * standalone contract, that only interacts with the contract that instantiated
 * it. That way, it is guaranteed that all Ether will be handled according to
 * the Escrow rules, and there is no need to check for payable functions or
 * transfers in the inheritance tree. The contract that uses the escrow as its
 * payment method should be its primary, and provide public methods redirecting
 * to the escrow's deposit and withdraw.
 */
contract Escrow is Secondary {
  using SafeMath for uint256;

  event Deposited(address indexed payee, uint256 weiAmount);
  event Withdrawn(address indexed payee, uint256 weiAmount);

  mapping(address => uint256) private _deposits;

  function depositsOf(address payee) public view returns (uint256) {
    return _deposits[payee];
  }

  /**
  * @dev Stores the sent amount as credit to be withdrawn.
  * @param payee The destination address of the funds.
  */
  function deposit(address payee) public onlyPrimary payable {
    uint256 amount = msg.value;
    _deposits[payee] = _deposits[payee].add(amount);

    emit Deposited(payee, amount);
  }

  /**
  * @dev Withdraw accumulated balance for a payee.
  * @param payee The address whose funds will be withdrawn and transferred to.
  */
  function withdraw(address payee) public onlyPrimary {
    uint256 payment = _deposits[payee];

    _deposits[payee] = 0;

    payee.transfer(payment);

    emit Withdrawn(payee, payment);
  }
}