openzeppelin-contracts/contracts/crowdsale/emission/AllowanceCrowdsale.sol

pragma solidity ^0.4.21;

import "../Crowdsale.sol";
import "../../token/ERC20/ERC20.sol";
import "../../math/SafeMath.sol";


/**
 * @title AllowanceCrowdsale
 * @dev Extension of Crowdsale where tokens are held by a wallet, which approves an allowance to the crowdsale.
 */
contract AllowanceCrowdsale is Crowdsale {
  using SafeMath for uint256;

  address public tokenWallet;

  /**
   * @dev Constructor, takes token wallet address. 
   * @param _tokenWallet Address holding the tokens, which has approved allowance to the crowdsale
   */
  function AllowanceCrowdsale(address _tokenWallet) public {
    require(_tokenWallet != address(0));
    tokenWallet = _tokenWallet;
  }

  /**
   * @dev Checks the amount of tokens left in the allowance.
   * @return Amount of tokens left in the allowance
   */
  function remainingTokens() public view returns (uint256) {
    return token.allowance(tokenWallet, this);
  }

  /**
   * @dev Overrides parent behavior by transferring tokens from wallet.
   * @param _beneficiary Token purchaser
   * @param _tokenAmount Amount of tokens purchased
   */
  function _deliverTokens(address _beneficiary, uint256 _tokenAmount) internal {
    token.transferFrom(tokenWallet, _beneficiary, _tokenAmount);
  }
}