我先后写了4篇关于区块链和以太坊的文章，比较零散，有需要的同学可以翻看历史记录。继上一篇发行一个简单的代币之后，接下来准备写三篇在以太坊上开发Dapp的文章，想学习区块链开发的同学可以关注下公众号贤庭漫步。
上一篇文章我们实现了一个简单的代币MyToken，作为一个demo入门，大家应该都了解了代币发行的基本流程。如果要实际发行MyToken的话，则需要做更多的工作，如实现合约的买卖功能等。
ERC20代币
先放上合约代码。
pragma solidity ^0.4.16;
interface tokenRecipient {
function receiveApproval(
address _from,
uint256 _value,
address _token,
bytes _extraData) public;
}
contract TokenERC20 { // Public variables of the token
string public name;
string public symbol;
// 18 decimals is the strongly suggested default, avoid changing it
uint8 public decimals = 18;
uint256 public totalSupply;
// This creates an array with all balances
mapping (address => uint256) public balanceOf;
// This generates a public event on the blockchain that will notify clients
mapping (address => mapping (address => uint256)) public allowance;
event Transfer(address indexed from, address indexed to, uint256 value);
// This notifies clients about the amount burnt
event Burn(address indexed from, uint256 value);
/**
* Constructor function
*
* Initializes contract with initial supply tokens to the creator of the contract
*/
function TokenERC20(
uint256 initialSupply,
string tokenName,
string tokenSymbol
) public {
// Update total supply with the decimal amount
totalSupply = initialSupply * 10 ** uint256(decimals);
balanceOf[msg.sender] = totalSupply;
name = tokenName;
symbol = tokenSymbol;
}
/**
* Internal transfer, only can be called by this contract
*/
function _transfer(address _from, address _to, uint _value) internal {
// Prevent transfer to 0x0 address. Use burn() instead
require(_to != 0x0);
// Check if the sender has enough
require(balanceOf[_from] >= _value);
// Check for overflows
require(balanceOf[_to] + _value > balanceOf[_to]);
// Save this for an assertion in the future
uint previousBalances = balanceOf[_from] + balanceOf[_to];
// Subtract from the sender
balanceOf[_from] -= _value;
// Add the same to the recipient
balanceOf[_to] += _value;
Transfer(_from, _to, _value);
// Asserts are used to use static analysis to find bugs in your code. They should never fail
assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
}
/**
* Transfer tokens
*
* Send `_value` tokens to `_to` from your account
*
* @param _to The address of the recipient
* @param _value the amount to send
*/
function transfer(address _to, uint256 _value) public {
_transfer(msg.sender, _to, _value);
}
/**
* Transfer tokens from other address
*
* Send `_value` tokens to `_to` on behalf of `_from`
*
* @param _from The address of the sender
* @param _to The address of the recipient
* @param _value the amount to send
*/
function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
require(_value <= allowance[_from][msg.sender]); // Check allowance
allowance[_from][msg.sender] -= _value;
_transfer(_from, _to, _value);
return true;
}
/**
* Set allowance for other address
*
* Allows `_spender` to spend no more than `_value` tokens on your behalf
*
* @param _spender The address authorized to spend
* @param _value the max amount they can spend
*/
function approve(address _spender, uint256 _value) public
returns (bool success) {
allowance[msg.sender][_spender] = _value;
return true;
}
/**
* Set allowance for other address and notify
*
* Allows `_spender` to spend no more than `_value` tokens on your behalf, and then ping the contract about it
*
* @param _spender The address authorized to spend
* @param _value the max amount they can spend
* @param _extraData some extra information to send to the approved contract
*/
function approveAndCall(address _spender, uint256 _value, bytes _extraData)
public returns (bool success) {
tokenRecipient spender = tokenRecipient(_spender);
if (approve(_spender, _value)) {
spender.receiveApproval(msg.sender, _value, this, _extraData);
return true;
}
}
/**
* Destroy tokens
*
* Remove `_value` tokens from the system irreversibly
*
* @param _value the amount of money to burn
*/
function burn(uint256 _value) public returns (bool success) {
require(balanceOf[msg.sender] >= _value); // Check if the sender has enough
balanceOf[msg.sender] -= _value; // Subtract from the sender
totalSupply -= _value; // Updates totalSupply
Burn(msg.sender, _value);
return true;
}
/**
* Destroy tokens from other account
*
* Remove `_value` tokens from the system irreversibly on behalf of `_from`.
*
* @param _from the address of the sender
* @param _value the amount of money to burn
*/
function burnFrom(address _from, uint256 _value) public returns (bool success) {
require(balanceOf[_from] >= _value); // Check if the targeted balance is enough
require(_value <= allowance[_from][msg.sender]); // Check allowance
balanceOf[_from] -= _value; // Subtract from the targeted balance
allowance[_from][msg.sender] -= _value; // Subtract from the sender's allowance
totalSupply -= _value; // Update totalSupply
Burn(_from, _value);
return true;
}
}
ERC20合约中提供了更多的函数，如approve()，sendFrom()等。sendFrom()函数允许其他人从我们的账户中转账给其他账户，这在现实场景中是真实存在的，如财务人员用公司的账户给员工发工资等。但我们必须给予限额，避免他人把我们账户的钱全部转走了。approve()函数就提供了这个功能，但approve()函数不会通知对方操作限额，所以如果你在设置限额的时候希望对方可以收到通知，可以使用approveAndCall()。
基于安全性考虑，转账必须只有合约本身才可调用，所以这里把转账功能单独出来并声明为internal。
/* Internal transfer, can only be called by this contract */
function _transfer(address _from, address _to, uint _value) internal {
require (_to != 0x0); // Prevent transfer to 0x0 address. Use burn() instead
require (balanceOf[_from] >= _value); // Check if the sender has enough
require (balanceOf[_to] + _value > balanceOf[_to]); // Check for overflows
require(!frozenAccount[_from]); // Check if sender is frozen
require(!frozenAccount[_to]); // Check if recipient is frozen
balanceOf[_from] -= _value; // Subtract from the sender
balanceOf[_to] += _value; // Add the same to the recipient
Transfer(_from, _to, _value);
}
所有涉及转账功能的函数，都需要先做各自的数据安全性检查，然后调用_transfer()函数实现转账。
代币管理
所有的dapp都是分布式的，但为了防止代币被乱用、失去控制，我们需要实现相关管理功能，如冻结用户，冻结资产，发行更多代币等。与一般app不同的是，所有这些规则都需要在代币部署前完成，一经部署，规则就无法改变了。
管理员可以是一个账户，也可以是另一个合约。如果合约实现是一个去中心化组织的话，将可以有效限制合约拥有者的权利。
我们使用继承来实现代币管理合约。
contract owned {
address public owner;
function owned() {
owner = msg.sender;
}
modifier onlyOwner {
require(msg.sender == owner);
_;
}
function transferOwnership(address newOwner) onlyOwner {
owner = newOwner;
}
}
这个合约定义了谁拥有合约，并提供一个modifier修饰器，被修饰的函数只有拥有者才有调用权限。接下来让代币合约继承owed合约就可以了。
contract TokenERC20 is owned {
/* the rest of the contract as usual */
这样我们就实现了一个简单的管理合约。
代币发行模型
目前市场上很多的代币发行策略要么是通胀模型，要么是固定最大值。接下来我们实现这个功能。
首先，定义一个状态变量totalSupply，在构造函数中赋值。
contract TokenERC20 {
uint256 public totalSupply;
function TokenERC20(...) {
totalSupply = initialSupply;
...
}
...
}
接下来添加一个代币发行函数：
function mintToken(address target, uint256 mintedAmount) onlyOwner {
balanceOf[target] += mintedAmount;
totalSupply += mintedAmount;
Transfer(0, owner, mintedAmount);
Transfer(owner, target, mintedAmount);
}
函数通过给目标地址增加一定数量的代币，从而增加代币的整体供应量。
冻结账户资产
几乎所有的资产发行都存在冻结功能，冻结一定数量的资产可以获得更多的代币收益。我们也给代币实现冻结功能。添加如下代码：
mapping (address => bool) public frozenAccount;
event FrozenFunds(address target, bool frozen);
function freezeAccount(address target, bool freeze) onlyOwner {
frozenAccount[target] = freeze;
FrozenFunds(target, freeze);
}
所有账户默认不会被冻结，owner可以通过调用freezenAccount()来冻结某些账户。被冻结的账户不能转账，在transfer()函数中添加一行代码：
function transfer(address _to, uint256 _value) {
require(!frozenAccount[msg.sender]);
代币交易
一个完善的代币应该可以用以太币购买，也可以兑换成以太币。接下来为我们的代币实现交易功能。
首先，定义买卖价格：
uint256 public sellPrice;
uint256 public buyPrice;
function setPrices(uint256 newSellPrice, uint256 newBuyPrice) onlyOwner {
sellPrice = newSellPrice;
buyPrice = newBuyPrice;
}
接下来定义买卖函数：
function buy() payable returns (uint amount){
amount = msg.value / buyPrice; // calculates the amount
require(balanceOf[this] >= amount); // checks if it has enough to sell
balanceOf[msg.sender] += amount; // adds the amount to buyer's balance
balanceOf[this] -= amount; // subtracts amount from seller's balance
Transfer(this, msg.sender, amount); // execute an event reflecting the change
return amount; // ends function and returns
}
function sell(uint amount) returns (uint revenue){
require(balanceOf[msg.sender] >= amount); // checks if the sender has enough to sell
balanceOf[this] += amount; // adds the amount to owner's balance
balanceOf[msg.sender] -= amount; // subtracts the amount from seller's balance
revenue = amount * sellPrice;
msg.sender.transfer(revenue); // sends ether to the seller: it's important to do this last to prevent recursion attacks
Transfer(msg.sender, this, amount); // executes an event reflecting on the change
return revenue; // ends function and returns
}
这里买卖的价格单位不是Ether，而是wei，1 ether = 10^18 wei。
最后
为了代码结构清晰，我们把本文添加的代币功能放在一个新合约MyAdvancedToken中，并继承owned和TokenERC20，最后的代码如下：
pragma solidity ^0.4.16;
contract owned {
address public owner;
function owned() public {
owner = msg.sender;
}
modifier onlyOwner {
require(msg.sender == owner);
_;
}
function transferOwnership(address newOwner) onlyOwner public {
owner = newOwner;
}
}
interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) public; }
contract TokenERC20 { // Public variables of the token
string public name;
string public symbol;
uint8 public decimals = 18; // 18 decimals is the strongly suggested default, avoid changing it
uint256 public totalSupply; // This creates an array with all balances
mapping (address => uint256) public balanceOf;
mapping (address => mapping (address => uint256)) public allowance; // This generates a public event on the blockchain that will notify clients
event Transfer(address indexed from, address indexed to, uint256 value); // This notifies clients about the amount burnt
event Burn(address indexed from, uint256 value);
/**
* Constrctor function
*
* Initializes contract with initial supply tokens to the creator of the contract
*/
function TokenERC20(
uint256 initialSupply,
string tokenName,
string tokenSymbol
) public {
totalSupply = initialSupply * 10 ** uint256(decimals); // Update total supply with the decimal amount
balanceOf[msg.sender] = totalSupply; // Give the creator all initial tokens
name = tokenName; // Set the name for display purposes
symbol = tokenSymbol; // Set the symbol for display purposes
}
/**
* Internal transfer, only can be called by this contract
*/
function _transfer(address _from, address _to, uint _value) internal {
// Prevent transfer to 0x0 address. Use burn() instead
require(_to != 0x0);
// Check if the sender has enough
require(balanceOf[_from] >= _value);
// Check for overflows
require(balanceOf[_to] + _value > balanceOf[_to]);
// Save this for an assertion in the future
uint previousBalances = balanceOf[_from] + balanceOf[_to];
// Subtract from the sender
balanceOf[_from] -= _value;
// Add the same to the recipient
balanceOf[_to] += _value;
Transfer(_from, _to, _value);
// Asserts are used to use static analysis to find bugs in your code. They should never fail
assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
}
/**
* Transfer tokens
*
* Send `_value` tokens to `_to` from your account
*
* @param _to The address of the recipient
* @param _value the amount to send
*/
function transfer(address _to, uint256 _value) public {
_transfer(msg.sender, _to, _value);
}
/**
* Transfer tokens from other address
*
* Send `_value` tokens to `_to` in behalf of `_from`
*
* @param _from The address of the sender
* @param _to The address of the recipient
* @param _value the amount to send
*/
function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
require(_value <= allowance[_from][msg.sender]); // Check allowance
allowance[_from][msg.sender] -= _value;
_transfer(_from, _to, _value);
return true;
}
/**
* Set allowance for other address
*
* Allows `_spender` to spend no more than `_value` tokens in your behalf
*
* @param _spender The address authorized to spend
* @param _value the max amount they can spend
*/
function approve(address _spender, uint256 _value) public
returns (bool success) {
allowance[msg.sender][_spender] = _value;
return true;
}
/**
* Set allowance for other address and notify
*
* Allows `_spender` to spend no more than `_value` tokens in your behalf, and then ping the contract about it
*
* @param _spender The address authorized to spend
* @param _value the max amount they can spend
* @param _extraData some extra information to send to the approved contract
*/
function approveAndCall(address _spender, uint256 _value, bytes _extraData)
public returns (bool success) {
tokenRecipient spender = tokenRecipient(_spender);
if (approve(_spender, _value)) {
spender.receiveApproval(msg.sender, _value, this, _extraData);
return true;
}
}
/**
* Destroy tokens
*
* Remove `_value` tokens from the system irreversibly
*
* @param _value the amount of money to burn
*/
function burn(uint256 _value) public returns (bool success) {
require(balanceOf[msg.sender] >= _value); // Check if the sender has enough
balanceOf[msg.sender] -= _value; // Subtract from the sender
totalSupply -= _value; // Updates totalSupply
Burn(msg.sender, _value);
return true;
}
/**
* Destroy tokens from other account
*
* Remove `_value` tokens from the system irreversibly on behalf of `_from`.
*
* @param _from the address of the sender
* @param _value the amount of money to burn
*/
function burnFrom(address _from, uint256 _value) public returns (bool success) {
require(balanceOf[_from] >= _value); // Check if the targeted balance is enough
require(_value <= allowance[_from][msg.sender]); // Check allowance
balanceOf[_from] -= _value; // Subtract from the targeted balance
allowance[_from][msg.sender] -= _value; // Subtract from the sender's allowance
totalSupply -= _value; // Update totalSupply
Burn(_from, _value);
return true;
}
}
/******************************************/
/* ADVANCED TOKEN STARTS HERE */
/******************************************/
contract MyAdvancedToken is owned, TokenERC20 {
uint256 public sellPrice;
uint256 public buyPrice;
mapping (address => bool) public frozenAccount;
/* This generates a public event on the blockchain that will notify clients */
event FrozenFunds(address target, bool frozen);
/* Initializes contract with initial supply tokens to the creator of the contract */
function MyAdvancedToken(
uint256 initialSupply,
string tokenName,
string tokenSymbol
) TokenERC20(initialSupply, tokenName, tokenSymbol) public {}
/* Internal transfer, only can be called by this contract */
function _transfer(address _from, address _to, uint _value) internal {
require (_to != 0x0); // Prevent transfer to 0x0 address. Use burn() instead
require (balanceOf[_from] >= _value); // Check if the sender has enough
require (balanceOf[_to] + _value > balanceOf[_to]); // Check for overflows
require(!frozenAccount[_from]); // Check if sender is frozen
require(!frozenAccount[_to]); // Check if recipient is frozen
balanceOf[_from] -= _value; // Subtract from the sender
balanceOf[_to] += _value; // Add the same to the recipient
Transfer(_from, _to, _value);
}
/// @notice Create `mintedAmount` tokens and send it to `target`
/// @param target Address to receive the tokens
/// @param mintedAmount the amount of tokens it will receive
function mintToken(address target, uint256 mintedAmount) onlyOwner public {
balanceOf[target] += mintedAmount;
totalSupply += mintedAmount;
Transfer(0, this, mintedAmount);
Transfer(this, target, mintedAmount);
}
/// @notice `freeze? Prevent | Allow` `target` from sending & receiving tokens
/// @param target Address to be frozen
/// @param freeze either to freeze it or not
function freezeAccount(address target, bool freeze) onlyOwner public {
frozenAccount[target] = freeze;
FrozenFunds(target, freeze);
}
/// @notice Allow users to buy tokens for `newBuyPrice` eth and sell tokens for `newSellPrice` eth
/// @param newSellPrice Price the users can sell to the contract
/// @param newBuyPrice Price users can buy from the contract
function setPrices(uint256 newSellPrice, uint256 newBuyPrice) onlyOwner public {
sellPrice = newSellPrice;
buyPrice = newBuyPrice;
}
/// @notice Buy tokens from contract by sending ether
function buy() payable public {
uint amount = msg.value / buyPrice; // calculates the amount
_transfer(this, msg.sender, amount); // makes the transfers
}
/// @notice Sell `amount` tokens to contract
/// @param amount amount of tokens to be sold
function sell(uint256 amount) public {
require(this.balance >= amount * sellPrice); // checks if the contract has enough ether to buy
_transfer(msg.sender, this, amount); // makes the transfers
msg.sender.transfer(amount * sellPrice); // sends ether to the seller. It's important to do this last to avoid recursion attacks
}
}
到这里我们的代币已经开发完成了，使用以太坊钱包部署体验下吧。接下来两篇文章是在以太坊上创建众筹合约和去中心自动化组织DAO，欢迎关注公众号
