4. Soạn hợp đồng thông minh Klaystagram

1. Background

2. Contract setup

3. Set events and data structure

4. Write functions 4.1. uploadPhoto 4.2. transferOwnership 4.3. getPhoto

1) Hình nền

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 and transferOwnership functions.

2) Thiết lập hợp đồng

• 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 and ERC721Enumerable.sol

  • Check out detailed information about ERC721 at erc721.org

pragma solidity 0.5.6;

nhập "./ERC721/ERC721.sol";
nhập "./ERC721/ERC721Enumerable.sol";

hợp đồng Klaystagram là ERC721, ERC721Enumerable {

3) Đặt sự kiện và cấu trúc dữ liệu

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);

ánh xạ (uint256 => PhotoData) private _photoList;

struct PhotoData {
    uint256 tokenId;                       // id token không trùng lặp, bắt đầu từ 1 và tăng thêm 1
    address[] ownerHistory;                // Lịch sử tất cả những chủ sở hữu trước đây
    bytes photo;                           // Nguồn ảnh
    string title;                          // Tiêu đề ảnh
    string location;                       // Nơi chụp ảnh
    string description;                    // Mô tả ngắn về ảnh
    uint256 timestamp;                     // Thời gian tải lên
}

4) Viết hàm

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 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

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.

/**
  * @ghi chú hàm safeTransferFrom kiểm tra xem người nhận có thể xử lý token ERC721 không,
  * nhờ đó, ít có khả năng bị mất token. Sau khi kiểm tra xong, hàm transferFrom với định nghĩa bên dưới sẽ được gọi
  */
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,
        //chủ sở hữu ban đầu        _photoList[tokenId].ownerHistory[0],
        //người sở hữu trước đây, độ dài không thể nhỏ hơn 2
        _photoList[tokenId].ownerHistory[ownerHistoryLength-2],
        //chủ sở hữu hiện tại
        _photoList[tokenId].ownerHistory[ownerHistoryLength-1]);
}

/**
  * @notice Khuyên dùng transferOwnership có sử dụng hàm safeTransferFrom
  * @dev Viết đề lên hàm transferFrom để đảm bảo rằng mỗi lần chuyển quyền sở hữu
  *  địa chỉ của chủ sở hữu mới được đẩy vào mảng ownerHistory
  */
function transferFrom(address from, address to, uint256 tokenId) public {
    super.transferFrom(from, to, tokenId);
    _photoList[tokenId].ownerHistory.push(to);
}

4-3) 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, "Ảnh không tồn tại");
    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!