1 ERC721 tokens

在这个游戏里面，我们使用ERC721 tokens标准，通常的情况下，使用的是ERC20 tokens。

有兴趣的童学可以研究一下两个标准的不同。

ERC721 tokens有两种方式交易"金币"的方式。虽然在这里我用的"金币"一词，但是可以是如何东西，你的加密猫，你的无敌英雄角色。

下面的 transfer 和 approve + takeOwnership 是ERC721 tokens标准里面的两个交易接口。完成的功能相同。

function transfer(address _to, uint256 _tokenId) public;

function approve(address _to, uint256 _tokenId) public;

function takeOwnership(uint256 _tokenId) public;

2 Event

Event的调用方法：

定义一个Event

contract ERC721 {

  event Transfer(address indexed _from, address indexed _to, uint256 _tokenId);

  event Approval(address indexed _owner, address indexed _approved, uint256 _tokenId);

  function balanceOf(address _owner) public view returns (uint256 _balance);

  function ownerOf(uint256 _tokenId) public view returns (address _owner);

  function transfer(address _to, uint256 _tokenId) public;

  function approve(address _to, uint256 _tokenId) public;

  function takeOwnership(uint256 _tokenId) public;

}

调用一个Event

function _transfer(address _from, address _to, uint256 _tokenId) private {

    ownerZombieCount[_to]++;

    ownerZombieCount[_from]--;

    zombieToOwner[_tokenId] = _to;

    Transfer(_from, _to, _tokenId);

  }

3 mapping

定义一个mapping

mapping (uint => address) public zombieToOwner;

   mapping (address => uint) ownerZombieCount;

4 require

require(newOwner != address(0));

5 SafeMath

OpenZeppelin提供了一个库：SafeMath，可以解决overflow问题。
overflow也叫做溢出。

Let’s say we have a uint8, which can only have 8 bits. That means the largest number we can store is binary 11111111 (or in decimal, 2^8 - 1 = 255).

可以这样使用：

using SafeMath for uint256;

uint256 a = 5;
uint256 b = a.add(3); // 5 + 3 = 8 a自动成为函数的第一个参数

uint256 c = a.mul(2); // 5 * 2 = 10

这里是原代码：

library SafeMath {

  function mul(uint256 a, uint256 b) internal pure returns (uint256) {

    if (a == 0) {

      return 0;

    }

    uint256 c = a * b;

    assert(c / a == b);

    return c;

  } 

 function div(uint256 a, uint256 b) internal pure returns (uint256) {

    // assert(b > 0); // Solidity automatically throws 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;

  }

  function sub(uint256 a, uint256 b) internal pure returns (uint256) { 

   assert(b <= a);

    return a - b;

  }

  function add(uint256 a, uint256 b) internal pure returns (uint256) {

    uint256 c = a + b;

    assert(c >= a);

    return c;

  }

}

可以这样代替已有的算数符号

Ex. Instead of doing:
myUint++;

We would do:
myUint = myUint.add(1);

6 自定义 library

在Solidity里面，可以将几个library定义到同一文件里面。
可以这样调用：

using SafeMath16 for uint16;

/**

 * @title SafeMath16

 * @dev SafeMath library implemented for uint16

 */

library SafeMath16 {

  function mul(uint16 a, uint16 b) internal pure returns (uint16) {

    if (a == 0) {

      return 0;

    }

    uint16 c = a * b;

    assert(c / a == b);

    return c;

  }

  function div(uint16 a, uint16 b) internal pure returns (uint16) {

    // assert(b > 0); // Solidity automatically throws when dividing by 0

    uint16 c = a / b;

    // assert(a == b * c + a % b); // There is no case in which this doesn't hold

    return c;

  }

  function sub(uint16 a, uint16 b) internal pure returns (uint16) {

    assert(b <= a);

    return a - b;

  }

  function add(uint16 a, uint16 b) internal pure returns (uint16) {

    uint16 c = a + b;

    assert(c >= a);

    return c;

  }

}

我们终于完成了这一章节的学习。下一章节我们要学习如何发布到ETH网络上。