This smart contract enables peer to peer swapping of any tokens adhering to the ERC721 standard.

How it works

• The user who wants to swap a non-fungible token escrows it to the smart contract with the escrowToken(address _contractAddr, uint256 _tokenId, string _description) function. _contractAddr and _tokenId identifies the token to be escrowed. _description is an optional string to display on a marketplace. (Token must have been approved for transfer by this contract using the approve function of the ERC-721 contract). This returns the index of the token in the list of all escrowed tokens. All escrowed tokens are open for offers.
• To make an offer, the second user also escrows their token and then calls the makeOffer(uint _requestedIndex, uint _offeredIndex, int _exchangeValue, uint _expiresIn) function. Here the user wants to swap _requestedIndex for _offeredIndex with an _exchangeValue. Take note that these tokens can be of different types. Exchange value can be 0, positive, negative. An exchange value of 0 means the offerer wants to swap the tokens without any value transfer. Positive exchange value means the offerer is offering some funds in the addition to the offered token, whereas a negative exchange value means the offerer is requesting funds in part of the deal. (If a positive value is given, the exact amount of funds must have been sent with the transaction.) _expiresIn parameters sets the expiration block of the offer. This returns the offer index, which can be used later to cancel the offer by calling the cancelOffer(uint _offerId) function.
• If a token owner likes an offer, they can take it using the takeOffer function. The trade will be made according to the offer terms, so if the offer has a negative exchange value, the exact amount specified must be sent with this transaction which will be forwarded to the offerer. If it has a positive exchange value the taker will receive the funds. In the case of a 0 exchange value no funds will be transferred.
• Finally, the token owners can withdraw any escrowed token anytime using the withdrawToken function.

Code walkthrough

To be written

TODO

• Add buyNowPrice to listings
• Make swapping tokens optional, i.e. an offer can be made only with funds
• Refund excess funds instead of reverting
• Users should be able to easily access their own offers (made and received)
• Test events
• Swap multiple tokens in one transaction
• Measure gas usage
• Add tests for "makeOfferWithRefunds"

Notes

• Each test in a test suite (contract block) depend on previous tests meaning they cannot be run separately.
• Also see this issue with truffle, where tests fail randomly.