pragma solidity ^0.4.8;

  // ----------------------------------------------------------------------------------------------
  // Sample fixed supply token contract
  // Enjoy. (c) BokkyPooBah 2017. The MIT Licence.
  // ----------------------------------------------------------------------------------------------

   // ERC Token Standard #20 Interface
  // https://github.com/ethereum/EIPs/issues/20
contract ERC20Interface {
  // 获取总的支持量
  function totalSupply() constant returns (uint256 totalSupply);

  // 获取其他地址的余额
  function balanceOf(address _owner) constant returns (uint256 balance);

  // 向其他地址发送token
  function transfer(address _to, uint256 _value) returns (bool success);

  // 从一个地址想另一个地址发送余额
  function transferFrom(address _from, address _to, uint256 _value) returns (bool success);

  //允许_spender从你的账户转出_value的余额，调用多次会覆盖可用量。某些DEX功能需要此功能
  function approve(address _spender, uint256 _value) returns (bool success);

  // 返回_spender仍然允许从_owner退出的余额数量
  function allowance(address _owner, address _spender) constant returns (uint256 remaining);

  // token转移完成后出发
  event Transfer(address indexed _from, address indexed _to, uint256 _value);

  // approve(address _spender, uint256 _value)调用后触发
  event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}

//继承接口后的实例
contract mkethToken is ERC20Interface {
  string public constant symbol = "Wan Ke Bi"; //单位
  string public constant name = "WKC"; //名称
  uint8 public constant decimals = 18; //小数点后的位数
  uint256 _totalSupply = 1500000000 * 10 ** uint256(decimals); //发行总量

  // 智能合约的所有者
  address public owner;

  // 每个账户的余额
  mapping(address => uint256) balances;

  // 帐户的所有者批准将金额转入另一个帐户。从上面的说明我们可以得知allowed[被转移的账户][转移钱的账户]
  mapping(address => mapping (address => uint256)) allowed;

  // 只能通过智能合约的所有者才能调用的方法
  modifier onlyOwner() {
      if (msg.sender != owner) {
          throw;
      }
      _;
  }

  // 构造函数
  function mkethToken() {
      owner = msg.sender;
      balances[owner] = _totalSupply;
  }

  function totalSupply() constant returns (uint256 totalSupply) {
      totalSupply = _totalSupply;
  }

  // 特定账户的余额
  function balanceOf(address _owner) constant returns (uint256 balance) {
      return balances[_owner];
  }

  // 转移余额到其他账户
  function transfer(address _to, uint256 _amount) returns (bool success) {
      if (balances[msg.sender] >= _amount 
          && _amount > 0
          && balances[_to] + _amount > balances[_to]) {
          balances[msg.sender] -= _amount;
          balances[_to] += _amount;
          Transfer(msg.sender, _to, _amount);
          return true;
      } else {
          return false;
      }
  }

  //从一个账户转移到另一个账户，前提是需要有允许转移的余额
  function transferFrom(
      address _from,
      address _to,
      uint256 _amount
  ) returns (bool success) {
      if (balances[_from] >= _amount
          && allowed[_from][msg.sender] >= _amount
          && _amount > 0
          && balances[_to] + _amount > balances[_to]) {
          balances[_from] -= _amount;
          allowed[_from][msg.sender] -= _amount;
          balances[_to] += _amount;
          Transfer(_from, _to, _amount);
          return true;
      } else {
          return false;
      }
  }

  //允许账户从当前用户转移余额到那个账户，多次调用会覆盖
  function approve(address _spender, uint256 _amount) returns (bool success) {
      allowed[msg.sender][_spender] = _amount;
      Approval(msg.sender, _spender, _amount);
      return true;
  }

  //返回被允许转移的余额数量
  function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
      return allowed[_owner][_spender];
  }
}