《Netkiller Blockchain 手札》
作者正在找工作，联系方式 13113668890
Mr. Neo Chan, 陈景峯(BG7NYT)
中国广东省深圳市望海路半岛城邦三期 518067 +86 13113668890
<netkiller@msn.com>
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2.10.5. 资产投资与份额持有
传统艺术品投资门槛非常高，一是用户不知道从哪些渠道可以投资，二是艺术品价值过高，三是艺术品简单难。这导致了投资艺术品门槛过高。 Token 能实现份额化，实现人人参与，人人持有，P2P交易。
例如某机构上链一件艺术品，用户可以投资该艺术品的一定份额，可以转让他持有的权益。且交易去中心化，不受任何机构，管理者的制约。
下面的合约可以展示如何分割艺术品份额，最终达到链上资产的份额分割和持有与交易。
pragma solidity ^0.4.25;
/**
* @title SafeMath
* @dev Math operations with safety checks that revert on error
*/
library SafeMath {
/**
* @dev Multiplies two numbers, reverts on overflow.
*/
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
// Gas optimization: this is cheaper than requiring 'a' not being zero, but the
// benefit is lost if 'b' is also tested.
// See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
if (a == 0) {
return 0;
}
uint256 c = a * b;
require(c / a == b);
return c;
}
/**
* @dev Integer division of two numbers truncating the quotient, reverts on division by zero.
*/
function div(uint256 a, uint256 b) internal pure returns (uint256) {
require(b > 0); // Solidity only automatically asserts when dividing by 0
uint256 c = a / b;
// assert(a == b * c + a % b); // There is no case in which this doesn't hold
return c;
}
/**
* @dev Subtracts two numbers, reverts on overflow (i.e. if subtrahend is greater than minuend).
*/
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
require(b <= a);
uint256 c = a - b;
return c;
}
/**
* @dev Adds two numbers, reverts on overflow.
*/
function add(uint256 a, uint256 b) internal pure returns (uint256) {
uint256 c = a + b;
require(c >= a);
return c;
}
/**
* @dev Divides two numbers and returns the remainder (unsigned integer modulo),
* reverts when dividing by zero.
*/
function mod(uint256 a, uint256 b) internal pure returns (uint256) {
require(b != 0);
return a % b;
}
}
contract Ownable {
address public owner;
event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
constructor() public {
owner = msg.sender;
}
modifier onlyOwner() {
require(msg.sender == owner);
_;
}
function transferOwnership(address newOwner) public onlyOwner {
require(newOwner != address(0));
emit OwnershipTransferred(owner, newOwner);
owner = newOwner;
}
}
contract NetkillerAssetsToken is Ownable {
using SafeMath for uint256;
string public name;
string public symbol;
uint public decimals;
uint256 public totalSupply;
mapping(address => mapping(string => uint256)) internal balances;
mapping(string => address) internal tokens;
event Transfer(address indexed _from, address indexed _to, string indexed _tokenId);
event Burn(address indexed from, string _tokenId);
constructor(
string tokenName,
string tokenSymbol,
uint decimalUnits
) public {
owner = msg.sender;
name = tokenName;
symbol = tokenSymbol;
decimals = decimalUnits;
totalSupply = 0;
}
function add(address _owner, string _tokenId) onlyOwner returns(bool status){
balances[_owner][_tokenId] = 100 * 10 ** uint256(decimals);
tokens[_tokenId] = _owner;
totalSupply = totalSupply.add(1);
return true;
}
function balanceOf(address _owner, string _tokenId) constant returns(uint balance){
return balances[_owner][_tokenId];
}
function ownerOf(string _tokenId) constant returns (address owner) {
return tokens[_tokenId];
}
function transfer(address _to, string _tokenId){
address _from = msg.sender;
uint256 amount = balances[_from][_tokenId];
transfer(_to, amount, _tokenId);
}
function transfer(address _to, uint256 _value, string _tokenId){
require(msg.sender == ownerOf(_tokenId));
require(msg.sender != _to);
require(_to != address(0));
address _from = msg.sender;
uint256 amount = balances[_from][_tokenId];
require(amount >= _value);
balances[_from][_tokenId] = balances[_from][_tokenId].sub(_value);
balances[_to][_tokenId] = balances[_to][_tokenId].add(_value);
tokens[_tokenId] = _to;
emit Transfer(_from, _to, _tokenId);
}
function burn(address _owner, string _tokenId) onlyOwner public returns (bool success) {
require(balances[_owner][_tokenId] > 0 && balances[_owner][_tokenId] == 100 * 10 ** uint256(decimals));
balances[_owner][_tokenId] = 0;
tokens[_tokenId] = address(0);
totalSupply = totalSupply.sub(1);
emit Burn(msg.sender, _tokenId);
return true;
}
}
由于 ERC721 不太符合我的需求，所以我结合 ERC20 和 ERC721 写出了我的合约。合约尽量保持了ERC20的使用习惯，函数定义尽量兼容 ERC20。
我们来看下面的构造方法，每个种类的物品一个合约，例如字画，陶瓷，青铜器。
constructor(
string tokenName,
string tokenSymbol,
uint decimalUnits
) public {
owner = msg.sender;
name = tokenName;
symbol = tokenSymbol;
decimals = decimalUnits;
totalSupply = 0;
}
通过下面函数，添加资产到 Token，使链上资产与Token绑定。
function add(address _owner, string _tokenId) onlyOwner returns(bool status){
balances[_owner][_tokenId] = 100 * 10 ** uint256(decimals);
tokens[_tokenId] = _owner;
totalSupply = totalSupply.add(1);
return true;
}
balances[_owner][_tokenId] = 100 * 10 ** uint256(decimals); 初始化份额是 100 表示 100%
totalSupply = totalSupply.add(1); 物品件数加一。可以用于统计链上资产的数量。
下面函数是查询资产的持有人
function ownerOf(string _tokenId) constant returns (address owner) {
return tokens[_tokenId];
}
下面函数是，权益转让和权益份额转让。
function transfer(address _to, string _tokenId){
address _from = msg.sender;
uint256 amount = balances[_from][_tokenId];
transfer(_to, amount, _tokenId);
}
function transfer(address _to, uint256 _value, string _tokenId){
require(msg.sender == ownerOf(_tokenId));
require(msg.sender != _to);
require(_to != address(0));
address _from = msg.sender;
uint256 amount = balances[_from][_tokenId];
require(amount >= _value);
balances[_from][_tokenId] = balances[_from][_tokenId].sub(_value);
balances[_to][_tokenId] = balances[_to][_tokenId].add(_value);
tokens[_tokenId] = _to;
emit Transfer(_from, _to, _tokenId);
}
接下来，我们就是可以开发 Dapp 钱包了，在钱包中实现资产的转移交易。
这个合约可以一直到 EOS 上，Hyperledger Fabric 不可以，因为 Fabric 没有锁的机制，会导致计算出错。
