智能合约Solidity学习（第二课）

@@@ Mapping 映射

这是solidity语言中提供的一种强大的数据结构，作用就像传统面向对象编程语言C++、Java中的map一样。

关键字：mapping

// For a financial app, storing a uint that holds the user's account balance:
mapping (address => uint) public accountBalance;
// Or could be used to store / lookup usernames based on userId
mapping (uint => string) userIdToName;

@@@ 关键字 address

是一种账户地址，以太坊中有两类账户：用户账户与合约账户，在solidity中均是用address来声明定义。

其值是一种16进制的字符串,类似下面这样：

0x0cE446255506E92DF41614C46F1d6df9Cc969183

@@@ 全局变量 msg.sender

全局变量是在solidity中的任何function均可直接访问的变量。msg.sender的值是函数调用者的账户地址，这里的调用者既可以是具体链上的某个用户，也可以是其他智能合约。

使用例子

mapping (address => uint) favoriteNumber;

function setMyNumber(uint _myNumber) public {
  // Update our `favoriteNumber` mapping to store `_myNumber` under `msg.sender`
  favoriteNumber[msg.sender] = _myNumber;
  // ^ The syntax for storing data in a mapping is just like with arrays
}

function whatIsMyNumber() public view returns (uint) {
  // Retrieve the value stored in the sender's address
  // Will be `0` if the sender hasn't called `setMyNumber` yet
  return favoriteNumber[msg.sender];
}

@@@关键字require

类似高级语言中的断言，用来判断条件是否满足，不满足则将停止继续运行并且向外抛出异常。

用法：

require(keccak256(_name) == keccak256("Vitalik"));

如果变量_name的hash值跟"Vitalik"的hash值不相等，则会抛出异常。

@@@合约继承(inheritance)

关键字: is

概念类似其他高级语言，如C++,Java, 子合约会继续父合约的public方法。

例子：

contract Doge {
  function catchphrase() public returns (string) {
    return "So Wow CryptoDoge";
  }
}

contract BabyDoge is Doge {
  function anotherCatchphrase() public returns (string) {
    return "Such Moon BabyDoge";
  }
}

@@@ 关键字import

用于引用其他solidity代码文件，类似于C++中的#include的作用。

例子：

import "./someothercontract.sol";

contract newContract is SomeOtherContract {

}

@@@关键字 storage VS memory

storage : 用此关键字修饰的变量，意味着会持久化存储在区块链上，类似中电脑中的硬盘存储；

memory: 用此关键字修饰的变量，意味着只是存储在内存中，在不同的外部函数调用 当前合约间会被擦除。

     大部分情况，不用我们显示的使用这两种关键字，solidity会有默认的处理方式：比如合约内的变量（函数外的变量），默认就是storage,而函数内的变量，则默认是memory的，当离开函数时，这些变量就会消失。而有一些特殊情况，则是需要显示使用这两个变量的。

例子：

contract SandwichFactory {
  struct Sandwich {
    string name;
    string status;
  }

  Sandwich[] sandwiches;

  function eatSandwich(uint _index) public {
    // Sandwich mySandwich = sandwiches[_index];

    // ^ Seems pretty straightforward, but solidity will give you a warning
    // telling you that you should explicitly declare `storage` or `memory` here.

    // So instead, you should declare with the `storage` keyword, like:
    Sandwich storage mySandwich = sandwiches[_index];
    // ...in which case `mySandwich` is a pointer to `sandwiches[_index]`
    // in storage, and...
    mySandwich.status = "Eaten!";
    // ...this will permanently change `sandwiches[_index]` on the blockchain.

    // If you just want a copy, you can use `memory`:
    Sandwich memory anotherSandwich = sandwiches[_index + 1];
    // ...in which case `anotherSandwich` will simply be a copy of the 
    // data in memory, and...
    anotherSandwich.status = "Eaten!";
    // ...will just modify the temporary variable and have no effect 
    // on `sandwiches[_index + 1]`. But you can do this:
    sandwiches[_index + 1] = anotherSandwich;
    // ...if you want to copy the changes back into blockchain storage.
  }
}

@@@函数可见性修饰符

除第一课中提到的public 与private 外，solidity中还有两种函数可见性修饰符：internal 和 external。

internal与private很类似，不同点在于被internal修饰的函数可以被子合约调用，而后者不可以；

external与public类似，不同点在于被external修饰的函数只能在合约外被调用，合约内的方法不能调用此方法。

contract Sandwich {
  uint private sandwichesEaten = 0;

  function eat() internal {
    sandwichesEaten++;
  }
}

contract BLT is Sandwich {
  uint private baconSandwichesEaten = 0;

  function eatWithBacon() public returns (string) {
    baconSandwichesEaten++;
    // We can call this here because it's internal
    eat();
  }
}

@@@定义接口合约

contract NumberInterface {
  function getNum(address _myAddress) public view returns (uint);
}

接口使用例子：

contract MyContract {
  address NumberInterfaceAddress = 0xab38... 
  // ^ The address of the FavoriteNumber contract on Ethereum
  NumberInterface numberContract = NumberInterface(NumberInterfaceAddress);
  // Now `numberContract` is pointing to the other contract

  function someFunction() public {
    // Now we can call `getNum` from that contract:
    uint num = numberContract.getNum(msg.sender);
    // ...and do something with `num` here
  }
}

@@@ returns 函数里一次返回多个值 

function getKitty(uint256 _id) external view returns (
    bool isGestating,
    bool isReady,
    uint256 cooldownIndex,
    uint256 nextActionAt,
    uint256 siringWithId,
    uint256 birthTime,
    uint256 matronId,
    uint256 sireId,
    uint256 generation,
    uint256 genes
) {
    Kitty storage kit = kitties[_id];

    // if this variable is 0 then it's not gestating
    isGestating = (kit.siringWithId != 0);
    isReady = (kit.cooldownEndBlock <= block.number);
    cooldownIndex = uint256(kit.cooldownIndex);
    nextActionAt = uint256(kit.cooldownEndBlock);
    siringWithId = uint256(kit.siringWithId);
    birthTime = uint256(kit.birthTime);
    matronId = uint256(kit.matronId);
    sireId = uint256(kit.sireId);
    generation = uint256(kit.generation);
    genes = kit.genes;
}

function multipleReturns() internal returns(uint a, uint b, uint c) {
  return (1, 2, 3);
}

function processMultipleReturns() external {
  uint a;
  uint b;
  uint c;
  // This is how you do multiple assignment:
  (a, b, c) = multipleReturns();
}

// Or if we only cared about one of the values:
function getLastReturnValue() external {
  uint c;
  // We can just leave the other fields blank:
  (,,c) = multipleReturns();
}

@@@ 第二课例子代码

pragma solidity ^0.4.19;

import "./zombiefactory.sol";

contract KittyInterface {
  function getKitty(uint256 _id) external view returns (
    bool isGestating,
    bool isReady,
    uint256 cooldownIndex,
    uint256 nextActionAt,
    uint256 siringWithId,
    uint256 birthTime,
    uint256 matronId,
    uint256 sireId,
    uint256 generation,
    uint256 genes
  );
}

contract ZombieFeeding is ZombieFactory {

  address ckAddress = 0x06012c8cf97BEaD5deAe237070F9587f8E7A266d;
  KittyInterface kittyContract = KittyInterface(ckAddress);

  // Modify function definition here:
  function feedAndMultiply(uint _zombieId, uint _targetDna,string _species) public {
    require(msg.sender == zombieToOwner[_zombieId]);
    Zombie storage myZombie = zombies[_zombieId];
    _targetDna = _targetDna % dnaModulus;
    uint newDna = (myZombie.dna + _targetDna) / 2;
    // Add an if statement here
    if (keccak256(_species) == keccak256("kitty")) {
      newDna = newDna - newDna % 100 + 99;
    }
    _createZombie("NoName", newDna);
  }

  function feedOnKitty(uint _zombieId, uint _kittyId) public {
    uint kittyDna;
    (,,,,,,,,,kittyDna) = kittyContract.getKitty(_kittyId);
    // And modify function call here:
    feedAndMultiply(_zombieId, kittyDna);
  }

}

pragma solidity ^0.4.19;

contract ZombieFactory {

    event NewZombie(uint zombieId, string name, uint dna);

    uint dnaDigits = 16;
    uint dnaModulus = 10 ** dnaDigits;

    struct Zombie {
        string name;
        uint dna;
    }

    Zombie[] public zombies;

    mapping (uint => address) public zombieToOwner;
    mapping (address => uint) ownerZombieCount;

    function _createZombie(string _name, uint _dna) internal {
        uint id = zombies.push(Zombie(_name, _dna)) - 1;
        zombieToOwner[id] = msg.sender;
        ownerZombieCount[msg.sender]++;
        NewZombie(id, _name, _dna);
    }

    function _generateRandomDna(string _str) private view returns (uint) {
        uint rand = uint(keccak256(_str));
        return rand % dnaModulus;
    }

    function createRandomZombie(string _name) public {
        require(ownerZombieCount[msg.sender] == 0);
        uint randDna = _generateRandomDna(_name);
        randDna = randDna - randDna % 100;
        _createZombie(_name, randDna);
    }

}