solidity/docs/examples/safe-remote.rst

.. index:: purchase, remote purchase, escrow

********************
Safe Remote Purchase
********************

::

    pragma solidity >=0.4.22 <0.6.0;

    contract Purchase {
        uint public value;
        address payable public seller;
        address payable public buyer;
        enum State { Created, Locked, Inactive }
        State public state;

        // Ensure that `msg.value` is an even number.
        // Division will truncate if it is an odd number.
        // Check via multiplication that it wasn't an odd number.
        constructor() public payable {
            seller = msg.sender;
            value = msg.value / 2;
            require((2 * value) == msg.value, "Value has to be even.");
        }

        modifier condition(bool _condition) {
            require(_condition);
            _;
        }

        modifier onlyBuyer() {
            require(
                msg.sender == buyer,
                "Only buyer can call this."
            );
            _;
        }

        modifier onlySeller() {
            require(
                msg.sender == seller,
                "Only seller can call this."
            );
            _;
        }

        modifier inState(State _state) {
            require(
                state == _state,
                "Invalid state."
            );
            _;
        }

        event Aborted();
        event PurchaseConfirmed();
        event ItemReceived();

        /// Abort the purchase and reclaim the ether.
        /// Can only be called by the seller before
        /// the contract is locked.
        function abort()
            public
            onlySeller
            inState(State.Created)
        {
            emit Aborted();
            state = State.Inactive;
            seller.transfer(address(this).balance);
        }

        /// Confirm the purchase as buyer.
        /// Transaction has to include `2 * value` ether.
        /// The ether will be locked until confirmReceived
        /// is called.
        function confirmPurchase()
            public
            inState(State.Created)
            condition(msg.value == (2 * value))
            payable
        {
            emit PurchaseConfirmed();
            buyer = msg.sender;
            state = State.Locked;
        }

        /// Confirm that you (the buyer) received the item.
        /// This will release the locked ether.
        function confirmReceived()
            public
            onlyBuyer
            inState(State.Locked)
        {
            emit ItemReceived();
            // It is important to change the state first because
            // otherwise, the contracts called using `send` below
            // can call in again here.
            state = State.Inactive;

            // NOTE: This actually allows both the buyer and the seller to
            // block the refund - the withdraw pattern should be used.

            buyer.transfer(value);
            seller.transfer(address(this).balance);
        }
    }