contract MyToken is ERCToken {

    using SafeMath for uint256;
    string public name;
    string public symbol;
    uint8 public decimals=18;

    mapping (address => mapping (address => uint256)) internal allowed;

  //构造方法
  function MyToken (
        string tokenName,
        string tokenSymbol,
        uint256 initSupply
    ) public {
        totalSupply = initSupply * 10 ** 18;  // Update total supply with the decimal amount 例如发10个token, 这个值应该是 10*10^decimals
        balanceOf[msg.sender] = totalSupply;                   // Give the creator all initial tokens
        name = tokenName;                                      // Set the name for display purposes
        symbol = tokenSymbol;
     }

    function greet() view public returns (string) {
        return 'hello';
    }

   /**
     * 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 previousbalanceOf = balanceOf[_from].add(balanceOf[_to]);

        // Subtract from the sender
        balanceOf[_from] = balanceOf[_from].sub(_value);
        // Add the same to the recipient
        balanceOf[_to] =balanceOf[_to].add(_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].add(balanceOf[_to]) == previousbalanceOf);
    }


    /**
     * 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 returns (bool success){
        _transfer(msg.sender, _to, _value);
        return true;
    }

    function transferAndCall(address _to, uint256 _value, bytes _data)
        public
        returns (bool success) {
        _transfer(msg.sender,_to, _value);
        if(_isContract(_to))
        {
            TransferRecipient spender = TransferRecipient(_to);
            if(!spender.tokenFallback(msg.sender, _value, _data))
            {
                revert();
            }
        }
        return true;
    }

    function _isContract(address _addr) private view returns (bool is_contract) {
      uint length;
      assembly {
            //retrieve the size of the code on target address, this needs assembly
            length := extcodesize(_addr)
      }
      return (length>0);
    }

    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowed[_from][msg.sender]);     // Check allowance
        allowed[_from][msg.sender]= allowed[_from][msg.sender].sub(_value);
        _transfer(_from, _to, _value);
        return true;
    }

    function allowance(address _owner,address _spender) public view returns(uint256){
        return allowed[_owner][_spender];

    }

   
    function approve(address _spender, uint256 _value) public  returns (bool) {
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
        return true;
    }

    function approveAndCall(address _spender, uint256 _value, bytes _extraData)
        public
        returns (bool success) {

        allowed[msg.sender][_spender] = _value;
        if(_isContract(_spender)){
            ApprovalRecipient spender = ApprovalRecipient(_spender);
            if(!spender.approvalFallback(msg.sender, _value, _extraData)){
                revert();
            }
        }
        Approval(msg.sender, _spender, _value);
        return true;

    }

}
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 previousbalanceOf = balanceOf[_from].add(balanceOf[_to]);

    // Subtract from the sender
    balanceOf[_from] = balanceOf[_from].sub(_value);
    // Add the same to the recipient
    balanceOf[_to] =balanceOf[_to].add(_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].add(balanceOf[_to]) == previousbalanceOf);
}

/**
 * Internal transfer, only can be called by this contract
 */