4. Write Klaystagram Smart Contract
Background
Contract setup
Set events and data structure
Write functions 4.1.
uploadPhoto
4.2.transferOwnership
4.3.getPhoto
1) Background
We will make a simple contract called "Klaystagram".
PhotoData
struct is defined to store various photo data.User can upload photo and transfer the ownership photo via
uploadPhoto
andtransferOwnership
functions.
2) 스마트 컨트랙트 설정
Specify solidity version. We recommend using 0.5.6 stable version.
We will make use of ERC721 standard to build non-fungible tokens.
Import
ERC721.sol
andERC721Enumerable.sol
Check out detailed information about ERC721 at erc721.org
pragma solidity 0.5.6;
import "./ERC721/ERC721.sol";
import "./ERC721/ERC721Enumerable.sol";
contract Klaystagram is ERC721, ERC721Enumerable {
3) 이벤트 및 데이터 구조 설정
We need to set up an event to keep track of activities on blockchain.
As for data structure, mapping _photoList
takes a uint256 tokenId
to map a specific PhotoData
struct. By defining PhotoUploaded event, transaction receipt will log this event whenever function containing this is called.
event PhotoUploaded (uint indexed tokenId, bytes photo, string title, string location, string description, uint256 timestamp);
mapping (uint256 => PhotoData) private _photoList;
struct PhotoData {
uint256 tokenId; // 1부터 차례로 1씩 증가하는 고유한 토큰 ID
address[] ownerHistory; // 이전 소유자들의 기록
bytes photo; // 이미지의 소스
string title; // 사진 제목
string location; // 사진 촬영 장소
string description; // 사진에 대한 간략한 설명
uint256 timestamp; // 업로드된 시간
}
4) 함수 작성
Let's write some functions that interact with the contract. In this tutorial let us only consider two functions: uploadPhoto
and transferOwnership
. Check out Klaystagram.sol to see the whole set of functions.
4-1) uploadPhoto
uploadPhoto
uploadPhoto
function takes 4 arguments including photo's image source. To keep things simple, tokenId
will start from 1 and will increase by 1.
_mint
function is from ERC721 contract. It creates a new token and assign it to a specific address, which in this case, msg.sender
. In this application, logged in user will create transaction with their own private key. So msg.sender
will be the user's public address.
Finally, initialize PhotoData
struct, locate it inside _photoList
mapping, and push the owner address into ownerHistory
array. And don't forget to emit the event we just created. As mentioned above, this event will be included in transaction receipt.
function uploadPhoto(bytes memory photo, string memory title, string memory location, string memory description) public {
uint256 tokenId = totalSupply() + 1;
_mint(msg.sender, tokenId);
address[] memory ownerHistory;
PhotoData memory newPhotoData = PhotoData({
tokenId : tokenId,
ownerHistory : ownerHistory,
photo : photo,
title: title,
location : location,
description : description,
timestamp : now
});
_photoList[tokenId] = newPhotoData;
_photoList[tokenId].ownerHistory.push(msg.sender);
emit PhotoUploaded(tokenId, photo, title, location, description, now);
}
4-2) transferOwnership
transferOwnership
Let's take a look at transferOwnership
function. When transferring photo ownership, we need to do two things. First, we have to reassign the owner, and then we have to push new owner address into ownerHistory
array.
To do this, transferOwnership
first calls safeTransferFrom
function from ERC721 standard, which eventually calls transferFrom
function. As mentioned above, right after token transfer is successfully done, we have to push new owner information into ownerHistory
array, and that is exactly why transferFrom
is overridden as below.
/**
* @notice safeTransferFrom 함수는 수신자가 ERC721 토큰을 처리할 수 있는지 체크하여 토큰이 유실되는 상황을 줄여줍니다. After checking is done, it will call transferFrom function defined below
*/
function transferOwnership(uint256 tokenId, address to) public returns(uint, address, address, address) {
safeTransferFrom(msg.sender, to, tokenId);
uint ownerHistoryLength = _photoList[tokenId].ownerHistory.length;
return (
_photoList[tokenId].tokenId,
//original owner
_photoList[tokenId].ownerHistory[0],
//previous owner, length cannot be less than 2
_photoList[tokenId].ownerHistory[ownerHistoryLength-2],
//current owner
_photoList[tokenId].ownerHistory[ownerHistoryLength-1]);
}
/**
* @notice Recommend using transferOwnership, which uses safeTransferFrom function
* @dev Override transferFrom function to make sure that every time ownership transfers
* new owner address gets pushed into ownerHistory array
*/
function transferFrom(address from, address to, uint256 tokenId) public {
super.transferFrom(from, to, tokenId);
_photoList[tokenId].ownerHistory.push(to);
}
4-3) getPhoto
getPhoto
Finally, let's make a getter function that fetches data stored in the smart contract. By calling a single function, we want to fetch every information regarding a specific photo. So getPhoto
function takes an index(token id) as an argument and returns every element in PhotoData struct.
function getPhoto(uint tokenId) public view
returns(uint256, address[] memory, bytes memory, string memory, string memory, string memory, uint256) {
require(_photoList[tokenId].tokenId != 0, "Photo does not exist");
return (
_photoList[tokenId].tokenId,
_photoList[tokenId].ownerHistory,
_photoList[tokenId].photo,
_photoList[tokenId].title,
_photoList[tokenId].location,
_photoList[tokenId].description,
_photoList[tokenId].timestamp);
}
This is it, now we can deploy this contract!
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