This repository contains the full PRM Smart Contract as well as an example ERC-20 & ERC-223 Compliant Token Contract that interacts with the PRM Smart Contract.
!> The example Token Contract within the project does not represent final code & should be revised upon agreement to reflect your ICO model correctly as it currently contains placeholder values.
Below you can find the estimated gas costs of contract deployments as well as the perspective cost of the contained functions.
The PRM contract contains 2 objects, the PRMToken interface & the PRM smart contract.
The PRMToken interface has a minimal gas foot-print & is a stripped-down token interface of the PRM Token for the PRM contract to communicate with in its future deployment. This is purely to generate the function signatures in a human-readable & secure format instead of using address.call(bytes4(sha3("function_name(types)")), parameters_values).
pragma solidity ^0.4.19;
contract PRMToken {
...
}The PRM Contract implements the necessary methods for an external web service to interact with the blockchain & the data stored within. It manages the sale/auction of photos as well as their ownership rights.
pragma solidity ^0.4.19;
contract PRMToken {
...
}
contract PRM {
...
}The total gas estimation for the publishing the contract on the Ethereum blockchain as of 2/26/2018 is 986843 gas.
Struct's in solidity act as objects with strictly variables contained within. Since the photo assets within the PRMContract contain a lot of variables it makes sense to store them within a struct.
struct PhotoAsset {
//A photo's sale price in Ethereum or the current auction's highest offer
uint256 photoPrice;
//A photo's sale price in PRM Tokens or the current auction's highest offer
uint256 photoTokenPrice;
//The auction's expiry date, if available
uint256 expiryDate;
//The sale type discerning a Resale from a Sale
bool saleType;
//The photo's current owner
address owner;
//The last & highest bidder of the auction, if available
address lastBidder;
}Checks whether the caller of the function is the address that created the smart contract. Used for verification in sensitive functions such as setPRMTokenAddress(address).
modifier isAdmin() {
require(msg.sender == admin);
//Continue executing rest of method body
_;
}Modifiers are called prior to a function's execution, with the
_character marking the placement of a function's body.
Sets the address of the PRM Token contract. This address is stored in memory so as to handle the auction e-scrow as well as token transfer for the purchase of photos.
function setPRMTokenAddress(address _PRMTokenAddress) external isAdmin {
PRMTokenAddress = _PRMTokenAddress;
}The gas estimation of the above function as of 2/23/2018 is 42356 gas.
Retrieves ERC-20 compliant tokens that were accidentally sent to the contract address.
function retrieveAccidentalTransfers(address _tokenAddress) external isAdmin {
require(_tokenAddress != PRMTokenAddress);
PRMToken tokenObject = PRMToken(_tokenAddress);
uint256 fullBalance = tokenObject.balanceOf(this);
tokenObject.transfer(admin, fullBalance);
}The gas estimation of the above function as of 2/23/2018 is 53286 gas.
Retrieves the information associated with a photo ID. The result variable within a web3 call is an array with the values stored in the same order as they appear on the below function
(Photo's price in Ethereum, Photo's price in PRM Tokens, Photo's Auction Expiry Date, Photo's Sale Type, Photo's Owner, Photo's Last Auction Bidder)
function getPhotoInfo(uint256 _photoID) external view returns (uint256, uint256, uint256, bool, address, address) {
PhotoAsset memory photo = photoAssets[_photoID];
return (photo.photoPrice, photo.photoTokenPrice, photo.expiryDate, photo.saleType, photo.owner, photo.lastBidder);
}There is no gas cost associated with retrieving data from the blockchain.
Upload photo to the PRM network & emit an event for the server to log the photo ID on its database.
function photoUpload(uint256 _saleType, uint256 _salePrice, bool _isToken) external {
photoAssets[incrementalID].owner = msg.sender;
if (_saleType == 1) {
photoSale(_salePrice, incrementalID, _isToken);
} else if (_saleType == 2) {
photoResale(_salePrice, incrementalID, _isToken);
} else if (_saleType > 2) {
photoAuction(_salePrice, _saleType, incrementalID, _isToken);
}
PhotoUpload(msg.sender, incrementalID);
incrementalID++;
}The gas estimation of the above function varies depending on the type of sale, if any, used.
The gas estimation of an empty photo upload with no form of sale as of 2/23/2018 is 64548 gas.
The gas estimation of subsequent empty photo uploads with no form of sale as of 2/23/2018 is 49548 gas.
All metrics have began with an empty photo upload to avoid any confusion regarding the median values.
Enable the ownership sale of a photo by specifying the amount of Ether or Tokens to price the photo at.
function photoSale(uint256 _salePrice, uint256 _photoID, bool _isToken) public {
require(msg.sender == photoAssets[_photoID].owner);
if (_isToken) {
photoAssets[_photoID].photoTokenPrice = _salePrice;
} else {
photoAssets[_photoID].photoPrice = _salePrice;
}
}The gas estimation of the above function as of 2/23/2018 is 42232 gas.
The total gas estimation of the photoUpload(uint256 _saleType, uint256 _salePrice, bool _isToken) function if this option is specified is 65970 gas as of 2/23/2018.
Enable the resale of a photo by specifying the amount of Ether or Tokens to price the photo at.
function photoResale(uint256 _resalePrice, uint256 _photoID, bool _isToken) public {
PhotoAsset storage asset = photoAssets[_photoID];
require(msg.sender == asset.owner);
if (_isToken) {
asset.photoTokenPrice = _resalePrice;
} else {
asset.photoPrice = _resalePrice;
}
asset.saleType = true;
}The gas estimation of the above function as of 2/25/2018 is 47479 gas.
The total gas estimation of the photoUpload(uint256 _saleType, uint256 _salePrice, bool _isToken) function if this option is specified is 75223 gas as of 2/25/2018.
Enable the auction of a photo by specifying the amount of Ether or Tokens to price the photo at as well as the auction's duration in hours.
function photoAuction(uint256 _startingBid, uint256 _auctionDuration, uint256 _photoID, bool _isToken) public {
PhotoAsset storage asset = photoAssets[_photoID];
require(msg.sender == asset.owner);
if (_isToken) {
asset.photoTokenPrice = _startingBid;
} else {
asset.photoPrice = _startingBid;
}
asset.expiryDate = now + (1 hours * _auctionDuration);
}The gas estimation of the above function as of 2/25/2018 is 62399 gas.
The total gas estimation of the photoUpload(uint256 _saleType, uint256 _salePrice, bool _isToken) function if this option is specified is 90050 gas as of 2/25/2018.
Place a bid on an on-going auction either in PRM tokens or Ether.
function bidOnAuction(uint256 _photoID, uint256 _tokenAmount) external payable {
PhotoAsset storage asset = photoAssets[_photoID];
assert(asset.expiryDate > now);
if (_tokenAmount > asset.photoTokenPrice) {
PRMToken tokenObject = PRMToken(PRMTokenAddress);
tokenObject.refundAndBid(asset.lastBidder, asset.photoTokenPrice, msg.sender, _tokenAmount);
asset.photoTokenPrice = _tokenAmount;
asset.lastBidder = msg.sender;
PhotoBid(_photoID, _tokenAmount);
} else if (msg.value > asset.photoPrice) {
asset.lastBidder.transfer(asset.photoPrice);
asset.photoPrice = msg.value;
asset.lastBidder = msg.sender;
PhotoBid(_photoID, msg.value);
} else {
revert();
}
}The gas estimation of the above function as of 2/26/2018 is 57461 gas for Ethereum bids.
The gas estimation of the above function as of 2/26/2018 is 98664 gas for PRM Token bids.
Purchase usage right of a photo currently offered up for re-sale with either PRM Tokens or Ether.
function purchaseUsageRight(uint256 _photoID, uint256 _tokenAmount) external payable {
PhotoAsset storage asset = photoAssets[_photoID];
assert(asset.saleType);
if (_tokenAmount == asset.photoTokenPrice && asset.photoTokenPrice > 0) {
PRMToken tokenObject = PRMToken(PRMTokenAddress);
tokenObject.saleTransfer(msg.sender, _tokenAmount, asset.owner);
PhotoRelease(msg.sender, _photoID);
} else if (msg.value == asset.photoPrice && asset.photoPrice > 0) {
asset.owner.transfer(msg.value);
PhotoRelease(msg.sender, _photoID);
} else {
revert();
}
}The gas estimation of the above function as of 2/25/2018 is 32540 gas for Ethereum purchases.
The gas estimation of the above function as of 2/25/2018 is 38508 gas for PRM Token purchases.
Purchase ownership right of a photo currently offered up for sale with either PRM Tokens or Ether.
function purchaseOwnershipRight(uint256 _photoID, uint256 _tokenAmount) external payable {
PhotoAsset storage asset = photoAssets[_photoID];
assert(!asset.saleType && asset.expiryDate == 0);
if (_tokenAmount == asset.photoTokenPrice && asset.photoTokenPrice > 0) {
PRMToken tokenObject = PRMToken(PRMTokenAddress);
tokenObject.saleTransfer(msg.sender, _tokenAmount, asset.owner);
asset.photoTokenPrice = 0;
PhotoOwnershipTransfer(msg.sender, asset.owner, _photoID);
asset.owner = msg.sender;
} else if (msg.value == asset.photoPrice && asset.photoPrice > 0) {
asset.owner.transfer(msg.value);
asset.photoPrice = 0;
PhotoOwnershipTransfer(msg.sender, asset.owner, _photoID);
asset.owner = msg.sender;
} else {
revert();
}
}The gas estimation of the above function as of 2/25/2018 is 28639 gas for Ethereum purchases.
The gas estimation of the above function as of 2/25/2018 is 34624 gas for PRM Token purchases.
Finalize an auction that has expired & reward the owner with the highest bet either in ETH or PRM Tokens.
function finalizeAuction(uint256 _photoID) external {
PhotoAsset storage asset = photoAssets[_photoID];
assert(asset.expiryDate < now && (asset.lastBidder == msg.sender || (msg.sender == asset.owner && asset.lastBidder == 0x0)));
if (asset.photoPrice > 0) {
if (asset.lastBidder != 0x0) {
asset.owner.transfer(asset.photoPrice);
PhotoOwnershipTransfer(asset.lastBidder, asset.owner, _photoID);
asset.owner = asset.lastBidder;
asset.lastBidder = 0x0;
}
asset.photoPrice = 0;
asset.expiryDate = 0;
} else if (asset.photoTokenPrice > 0) {
if (asset.lastBidder != 0x0) {
PRMToken tokenObject = PRMToken(PRMTokenAddress);
tokenObject.releaseBid(asset.owner, asset.photoTokenPrice);
PhotoOwnershipTransfer(asset.lastBidder, asset.owner, _photoID);
asset.owner = asset.lastBidder;
asset.lastBidder = 0x0;
}
asset.photoTokenPrice = 0;
asset.expiryDate = 0;
}
}The gas estimation of the above function as of 2/26/2018 is 40576 gas for Ethereum purchases.
The gas estimation of the above function as of 2/26/2018 is 56389 gas for PRM Token purchases.
The fallback function is called whenever Ether is sent to the smart contract without any bytecode associated with it. It currently reverts the transaction & returns the Ether to the sender.
function() public payable {
revert();
}The PRM Token contract is an example ERC-223 that also includes the functions used by the PRM address. Gas analysis has been coupled to the PRM Contract since the gas costs are reflected on its function calls.
Only unique functions will be included in this analysis (Meaning ERC-20 functions such as transfer() won't be analyzed).
pragma solidity ^0.4.19;
contract PRMToken {
...
}The total gas estimation for the publishing the contract on the Ethereum blockchain as of 2/23/2018 is 1313625 gas.
Sets the address of the PRM contract. This address is stored in memory so as to allow the contract to transfer tokens between users & be used as e-scrow.
function setPRMAddress(address _PRMAddress) external isAdmin {
PRMAddress = _PRMAddress;
}The gas estimation of the above function as of 2/23/2018 is 29012 gas.
Refunds the latest PRM bid on an auction & e-scrows the new bid amount.
function refundAndBid(address _previousBidder, uint256 _previousBid, address _newBidder, uint256 _newBid) external isPRM {
if (_previousBidder == 0x0) {
balances[msg.sender] += _newBid;
} else {
balances[msg.sender] += (_newBid - _previousBid);
}
balances[_previousBidder] += _previousBid;
balances[_newBidder] = safeSub(balances[_newBidder], _newBid);
}This function is called within the PRMContract & as such, its gas expenditure can only be calculated within the functions it is called as a whole.
Transfer the required PRM tokens from one account to the other to complete a sale.
function saleTransfer(address _buyer, uint256 _amount, address _owner) external isPRM {
balances[_owner] += _amount;
balances[_buyer] = safeSub(balances[_buyer], _amount);
}This function is called within the PRMContract & as such, its gas expenditure can only be calculated within the functions it is called as a whole.
Release the PRM tokens held in e-scrow to the owner of the auctioned photo.
function releaseBid(address _owner, uint256 _bid) external isPRM {
balances[_owner] += _bid;
//Underflow impossible to occur due to Smart Contract workflow
balances[msg.sender] -= _bid;
}This function is called within the PRMContract & as such, its gas expenditure can only be calculated within the functions it is called as a whole.
Below one can find web3 code snippets for the various functions required to complete the photo upload workflow & photo purchase workflow.
For the purposes of the following snippets the web3 version 0.20.4.
Properly operating with the Metamask extension means that one should take care of certain cases & initialize the contract correctly. These cases include a the absence of the plugin, locked-by-passphrase account & an invalid network id.
In order to set-up our local web3 instance we require the web3 object to be defined & an Ethereum node to operate with. Both are provided by the in-page web3 Metamask module in addition to the web3 object. Since we want consistency across our deployments initializing a local web3 instance is better for version control.
The below code snippet detects the existance of Metamask (Or any service provider injecting the web3 module such as the Parity browser) & initializes a local web3 instance via the browserified module I have provided. Should the web3 module be absent, the user is alerted of the error & redirected to Metamask's main page.
let localWeb3;
if (typeof window.web3 === 'undefined' || typeof window.web3.currentProvider === 'undefined') {
alert("No Web3 Detected. Consider installing Metamask!");
setTimeout(function(){
window.location = "https://metamask.io/";
},4000);
} else {
localWeb3 = new Web3(window.web3.currentProvider);
}In order to interact with smart contracts deployed on the Ethereum blockchain one should first create an instance of the contract's interface. In order to do this, the two things required are the contract ABI in JSON format & the contract address. A snippet can be found below.
let localWeb3;
let contractInstance;
if (typeof window.web3 === 'undefined' || typeof window.web3.currentProvider === 'undefined') {
...;
} else {
localWeb3 = new Web3(window.web3.currentProvider);
let contractAddress = "0x8cc5da3bae2f7222888c11fca0459c7ea4c848c5";
let contractABI = [{"constant":true,"inputs":[],"name":"name","outputs":[{"name":"_name","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_spender","type":"address"},{"name":"_value","type":"uint256"}],"name":"approve","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"totalSupply","outputs":[{"name":"_totalSupply","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_referrer","type":"address"}],"name":"refer","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"_nickname","type":"bytes32"}],"name":"referByRNS","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"_from","type":"address"},{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transferFrom","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"decimals","outputs":[{"name":"_decimals","type":"uint8"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"referralReward","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_nickname","type":"bytes32"}],"name":"reserveRNS","outputs":[],"payable":true,"stateMutability":"payable","type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"}],"name":"balanceOf","outputs":[{"name":"balance","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"symbol","outputs":[{"name":"_symbol","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transfer","outputs":[{"name":"success","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_value","type":"uint256"},{"name":"_data","type":"bytes"}],"name":"transfer","outputs":[{"name":"success","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"_sender","type":"address"},{"name":"_value","type":"uint256"},{"name":"_data","type":"bytes"}],"name":"tokenFallback","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"retrieveInfo","outputs":[{"name":"_totalSupply","type":"uint256"},{"name":"_referralReward","type":"uint256"},{"name":"referrals","type":"uint256"},{"name":"balance","type":"uint256"},{"name":"nickname","type":"bytes32"},{"name":"isReferred","type":"bool"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"},{"name":"_spender","type":"address"}],"name":"allowance","outputs":[{"name":"remaining","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_token","type":"address"}],"name":"claimTokens","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"admin","outputs":[{"name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"inputs":[],"payable":false,"stateMutability":"nonpayable","type":"constructor"},{"anonymous":false,"inputs":[{"indexed":true,"name":"_from","type":"address"},{"indexed":true,"name":"_to","type":"address"},{"indexed":false,"name":"_value","type":"uint256"}],"name":"Transfer","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"name":"_owner","type":"address"},{"indexed":true,"name":"_spender","type":"address"},{"indexed":false,"name":"_value","type":"uint256"}],"name":"Approval","type":"event"}];
contractInstance = localWeb3.eth.contract(contractABI).at(contractAddress);
}The ABI can be acquired via an external resource stored on the server & parsed via the JSON.parse() method to avoid ugly hard-coded code.
A locked account can prevent you from accessing the Metamask's public address & disable the ability to sign transactions, resulting in numerous errors within your JS code. In order to avoid that, one should first alert the user to unlock his account & then create a watcher that disables itself once the account has been unlocked. If the account is unlocked, updating your local instance's defaultAccount variable will ensure synchronous signature request instead of asynchronous. A snippet can be found below.
let localWeb3;
if (typeof window.web3 === 'undefined' || typeof window.web3.currentProvider === 'undefined') {
...;
} else {
localWeb3 = new Web3(window.web3.currentProvider);
...;
window.web3.eth.getAccounts(function (err, accounts) {
if (!accounts[0]) {
alert("Please unlock your Metamask wallet");
watchAccountUnlock();
} else {
localWeb3.eth.defaultAccount = accounts[0];
}
});
}
function watchAccountUnlock() {
let toClear = setInterval(function() {
window.web3.eth.getAccounts(function (err, accounts) {
if (accounts[0] != localWeb3.eth.defaultAccount) {
localWeb3.eth.defaultAccount = accounts[0];
clearInterval(toClear);
}
});
},500);
}Since the Metamask & any user-reliant web3 injected module can connect to any Ethereum node the user wishes, a malicious party could connect the user to a different network (Such as Ethereum Classic) in an attempt to deceive or steal funds. Or a user may attempt to fool the server by connecting to a testnet which will result in a failure anyhow. For completeness' sake, a snippet has been included to detect the network the user is connected to & inform him.
let localWeb3;
let contractInstance;
if (typeof window.web3 === 'undefined' || typeof window.web3.currentProvider === 'undefined') {
...;
} else {
localWeb3 = new Web3(window.web3.currentProvider);
...;
localWeb3.version.getNetwork((err, netId) => {
if (netId != 1) {
alert("Please switch to the main-net");
watchNetChange();
} else {
...;
}
});
}
function watchNetChange() {
let toClear = setInterval(function() {
localWeb3.version.getNetwork((err, netId) => {
if (netId == 1) {
clearInterval(toClear);
}
});
},500);
}All the above checks, with the addition of a getBlockchainData() function called when all checks have been passed, can be found implemented on the snippet below.
let localWeb3;
let contractInstance;
if (typeof window.web3 === 'undefined' || typeof window.web3.currentProvider === 'undefined') {
alert("No Web3 Detected. Consider installing Metamask!");
setTimeout(function(){
window.location = "https://metamask.io/";
},4000);
} else {
localWeb3 = new Web3(window.web3.currentProvider);
let contractAddress = "CONTRACT_ETHEREUM_ADDRESS_HERE";
let contractABI = "PARSED_CONTRACT_ABI_HERE";
contractInstance = localWeb3.eth.contract(contractABI).at(contractAddress);
window.web3.eth.getAccounts(function (err, accounts) {
if (!accounts[0]) {
alert("Please unlock your Metamask wallet");
watchAccountUnlock();
} else {
localWeb3.eth.defaultAccount = accounts[0];
localWeb3.version.getNetwork((err, netId) => {
if (netId != 1) {
alert("Please switch to the main-net");
watchNetChange();
} else {
getBlockchainData();
}
});
}
});
}
function watchAccountUnlock() {
let toClear = setInterval(function() {
window.web3.eth.getAccounts(function (err, accounts) {
if (accounts[0] != localWeb3.eth.defaultAccount) {
localWeb3.eth.defaultAccount = accounts[0];
localWeb3.version.getNetwork((err, netId) => {
if (netId != 1) {
alert("Please switch to the main-net");
watchNetChange();
} else {
getBlockchainData();
}
});
clearInterval(toClear);
}
});
},500);
}
function watchNetChange() {
let toClear = setInterval(function() {
localWeb3.version.getNetwork((err, netId) => {
if (netId == 1) {
getBlockchainData();
clearInterval(toClear);
}
});
},500);
}
function getBlockchainData() {
...;
}Before initiating the upload of the photo to the server, first execute the smart contract's upload function on the user's side. The function takes in 3 arguments, the sale's type, the photo's price & whether it is a token or an Ether sale.
The sale's type is represented by an unsigned integer & reflects a simple upload if it is equal to 0, an ownership sale if it is equal to 1, a resale of usage rights if it is equal to 2 & an auction if it is above 2. In order to save gas efficiency, the same unsigned integer used for checking the sale type is also used as the auction's duration, so auctions can only start from 3 hours & up with the sale type reflecting the hours the auction is in effect.
An example snippet for an auction with a duration of 24 hours & a minimum bet of 1 Ether can be found below.
!> Any amount retrieved from the user needs to factor in the number of decimals of the perspective currency. If for example the user wishes to sell the photo at 1 Ether, the number 1 * 10**18 should be fed into the contract function. Accordingly, if the user wishes to sell the photo at 1 PRM Token, the number 1 * 10**numberOfDecimals should be fed into the contract function.
A library handling big numbers should be used browser-side to avoid any overflows or underflows when performing dangerous functions such as
Math.pow(10,18).
let contractInstance; //Contract object acquired from web3
let saleType = 24;
let salePrice = 1 * Math.pow(10,18);
let isToken = false;
contractInstance.photoUpload(saleType, salePrice, isToken, function(err,result) {
if (!err) {
...;
} else {
//err variable contains an error object with an accompanying error message
console.error(err);
alert("TX Rejected/Error Occured");
}
});After successfully broadcasting the transaction to the network, the user then should be prompted to upload his desired picture. While the picture is being uploaded the server should initiate a setInterval on the back-end that waits for the latest unoccupied photoID included in a photo upload event by the user's address.
!> Although someone could potentially include a fake address within his upload payload and attempt to upload a picture without actually registering it on the blockchain by using another user's photoID, it has no use whatsoever as all transactions take place on the blockchain & another person would get the photo reward instead of the malicious party.
The above issue can also be nullified by verifying a signed message by the user's private key but that adds unnecessary overhead to the user experience & is not recommended.
The below back-end snippet takes into account that the API call to the server included the localWeb3.eth.defaultAccount variable.
let contractInstance; //Contract object acquired from web3
let address = request.address;
let watcher = contractInstance.PhotoUpload({_uploader:address});
let photoID;
watcher.watch(function (error, result){
if (!error) {
photoID = result.args._photoID;
successCallback(photoID);
}
});
setTimeout(function(){
watcher.stopWatching();
watcher = null;
errorCallback("Watcher Timeout");
}, 4*15000);//4 * 15 Seconds (Median Ethereum Block Time as of 2/26/2018) reflects 4 blocks having been minedAfter the successCallback has been fired, the uploaded picture can now be assigned the photoID correctly. If the errorCallback is called, the server should delete the image from its database.
After detecting the type of sale the photo is up for on the front-end & executing the appropriate contract object function found in this documentation, the back-end should initiate a watcher that detects if the photo has been released for download to the buyer.
let contractInstance; //Contract object acquired from web3
let address = request.address;
let watcher = contractInstance.PhotoRelease({_viewer:address});
let photoID;
watcher.watch(function (error, result){
if (!error) {
photoID = result.args._photoID;
successCallback(photoID);
}
});
setTimeout(function(){
watcher.stopWatching();
watcher = null;
errorCallback("Watcher Timeout");
}, 4*15000);//4 * 15 Seconds (Median Ethereum Block Time as of 2/26/2018) reflects 4 blocks having been minedSimilar concept to the PhotoUpload event.
Since auctions end asynchronously, the watcher should be initiated when the user attempts to claim the auction within your platform. Only the latest bidder is allowed to finalize the auction.
- Remix IDE - The IDE used for smart contract development
- Ganache - Local blockchain used for gas estimations & debugging
- web3.js - The library used for communicating with the blockchain
- Atom - Used for creating the back-end snippets
- Alexander Papageorgiou - Complete Project Development - alex-ppg