Lesson 4:
- view tells us that by running the functions, no data will be saved/changed. pure tells us that not only does the function not save any data to the blockchain, but it also doens’t read any data from the blockchain. Both of these don’t cost any gas to call if they’re called from outside the contract, but the do cost gas if called internally by another function because the calling function is eventually making changes on the blockchain.
- The function modifiers can all be stacked together on a function definition, as follows:
function test() external view onlyOwner anotherModifier { // Some function-y stuff } - The payable modifier:
They are a special type of functions that can recieve Ether.
In Ethereum, because both the money (Ether), the data (transaction payload), and the contract code itself all live on Ethereum, it’s possible for you to call
a function and pay money to the contract at the same time.
This allows us to have some really cool logic, such as: requiring a certain payment to the contract in order to execute a function.
Here’s an example:
contract OnlineStore { function buySomething () external payable { require(msg.value == 0.01 ether); //ether is an inbuilt uint; transferStuff(msg.sender); } }msg.value is a way to see how much Ether was sent to the contract. If a function is not marked as payable, and you try to send Ether to it, the function will reject your transaction.
- The payment can only be done to a data type that’s called address payable.
Example: function withdraw() external onlyOwner() { address payable _owner = address(uint16(owner())); _owner.transfer(address(this).balance); } address(this).balance will return the total balance stored on the contract. - A make-shift way of generating random numbers in Solidity is as follows:
// Generate a random number between 1 and 100: uint randNonce = 0; uint random = uint(keccak256(abi.encodePacked(now, msg.sender, randNonce))) % 100; randNonce++; uint random2 = uint(keccak256(abi.encodePacked(now, msg.sender, randNonce))) % 100;But, this is unsafe, because technically, this can be hacked. How? Read on: In Ethereum, when you call a function on a contract, you broadcast it to a node or nodes on the network as a transaction. The nodes on the network then collect a bunch of transactions, try to be the first to solve a computationally-intensive mathematical problem as a “Proof of Work”, and then publish that group of transactions along with their Proof of Work (PoW) as a block to the rest of the network. Once a node has solved the PoW, the other nodes stop trying to solve the PoW, verify that the other node’s list of transactions are valid, and then accept the block and move on to trying to solve the next block. This makes our random number function exploitable. Let’s say we had a coin flip contract —heads you double your money, tails you lose everything. Let’s say it used the above random function to determine heads or tails. (random >= 50 is heads, random < 50 is tails). If I were running a node, I could publish a transaction only to my own node and not share it. I could then run the coin flip function to see if I won — and if I lost, choose not to include that transaction in the next block I’m solving. I could keep doing this indefinitely until I finally won the coin flip and solved the next block, and profit.
- One relatively safe method is to use an Oracle to access a random number from outside the Ethereum blockchain
Solidity Code from Lesson 4
zombieattack.sol
pragma solidity >=0.5.0 <0.6.0;
import "./zombiehelper.sol";
contract ZombieAttack is ZombieHelper {
uint randNonce = 0;
uint attackVictoryProbability = 70;
function randMod(uint _modulus) internal returns(uint) {
randNonce++;
return uint(keccak256(abi.encodePacked(now, msg.sender, randNonce))) % _modulus;
}
function attack(uint _zombieId, uint _targetId) external ownerOf(_zombieId) {
Zombie storage myZombie = zombies[_zombieId];
Zombie storage enemyZombie = zombies[_targetId];
uint rand = randMod(100);
if (rand <= attackVictoryProbability) {
myZombie.winCount++;
myZombie.level++;
enemyZombie.lossCount++;
feedAndMultiply(_zombieId, enemyZombie.dna, "zombie");
} else {
myZombie.lossCount++;
enemyZombie.winCount++;
_triggerCooldown(myZombie);
}
}
}
zombiehelper.sol
pragma solidity >=0.5.0 <0.6.0;
import "./zombiefeeding.sol";
contract ZombieHelper is ZombieFeeding {
uint levelUpFee = 0.001 ether;
modifier aboveLevel(uint _level, uint _zombieId) {
require(zombies[_zombieId].level >= _level);
_;
}
function withdraw() external onlyOwner {
address _owner = owner();
_owner.transfer(address(this).balance);
}
function setLevelUpFee(uint _fee) external onlyOwner {
levelUpFee = _fee;
}
function levelUp(uint _zombieId) external payable {
require(msg.value == levelUpFee);
zombies[_zombieId].level++;
}
function changeName(uint _zombieId, string calldata _newName) external aboveLevel(2, _zombieId) ownerOf(_zombieId) {
zombies[_zombieId].name = _newName;
}
function changeDna(uint _zombieId, uint _newDna) external aboveLevel(20, _zombieId) ownerOf(_zombieId) {
zombies[_zombieId].dna = _newDna;
}
function getZombiesByOwner(address _owner) external view returns(uint[] memory) {
uint[] memory result = new uint[](ownerZombieCount[_owner]);
uint counter = 0;
for (uint i = 0; i < zombies.length; i++) {
if (zombieToOwner[i] == _owner) {
result[counter] = i;
counter++;
}
}
return result;
}
}
zombiefactory.sol
pragma solidity >=0.5.0 <0.6.0;
import "./ownable.sol";
contract ZombieFactory is Ownable {
event NewZombie(uint zombieId, string name, uint dna);
uint dnaDigits = 16;
uint dnaModulus = 10 ** dnaDigits;
uint cooldownTime = 1 days;
struct Zombie {
string name;
uint dna;
uint32 level;
uint32 readyTime;
uint16 winCount;
uint16 lossCount;
}
Zombie[] public zombies;
mapping (uint => address) public zombieToOwner;
mapping (address => uint) ownerZombieCount;
function _createZombie(string memory _name, uint _dna) internal {
uint id = zombies.push(Zombie(_name, _dna, 1, uint32(now + cooldownTime), 0, 0)) - 1;
zombieToOwner[id] = msg.sender;
ownerZombieCount[msg.sender]++;
emit NewZombie(id, _name, _dna);
}
function _generateRandomDna(string memory _str) private view returns (uint) {
uint rand = uint(keccak256(abi.encodePacked(_str)));
return rand % dnaModulus;
}
function createRandomZombie(string memory _name) public {
require(ownerZombieCount[msg.sender] == 0);
uint randDna = _generateRandomDna(_name);
randDna = randDna - randDna % 100;
_createZombie(_name, randDna);
}
}
zombiefeeding.sol
pragma solidity >=0.5.0 <0.6.0;
import "./ownable.sol";
contract ZombieFactory is Ownable {
event NewZombie(uint zombieId, string name, uint dna);
uint dnaDigits = 16;
uint dnaModulus = 10 ** dnaDigits;
uint cooldownTime = 1 days;
struct Zombie {
string name;
uint dna;
uint32 level;
uint32 readyTime;
uint16 winCount;
uint16 lossCount;
}
Zombie[] public zombies;
mapping (uint => address) public zombieToOwner;
mapping (address => uint) ownerZombieCount;
function _createZombie(string memory _name, uint _dna) internal {
uint id = zombies.push(Zombie(_name, _dna, 1, uint32(now + cooldownTime), 0, 0)) - 1;
zombieToOwner[id] = msg.sender;
ownerZombieCount[msg.sender]++;
emit NewZombie(id, _name, _dna);
}
function _generateRandomDna(string memory _str) private view returns (uint) {
uint rand = uint(keccak256(abi.encodePacked(_str)));
return rand % dnaModulus;
}
function createRandomZombie(string memory _name) public {
require(ownerZombieCount[msg.sender] == 0);
uint randDna = _generateRandomDna(_name);
randDna = randDna - randDna % 100;
_createZombie(_name, randDna);
}
}
ownable.sol
pragma solidity >=0.5.0 <0.6.0;
/**
* @title Ownable
* @dev The Ownable contract has an owner address, and provides basic authorization control
* functions, this simplifies the implementation of "user permissions".
*/
contract Ownable {
address private _owner;
event OwnershipTransferred(
address indexed previousOwner,
address indexed newOwner
);
/**
* @dev The Ownable constructor sets the original `owner` of the contract to the sender
* account.
*/
constructor() internal {
_owner = msg.sender;
emit OwnershipTransferred(address(0), _owner);
}
/**
* @return the address of the owner.
*/
function owner() public view returns(address) {
return _owner;
}
/**
* @dev Throws if called by any account other than the owner.
*/
modifier onlyOwner() {
require(isOwner());
_;
}
/**
* @return true if `msg.sender` is the owner of the contract.
*/
function isOwner() public view returns(bool) {
return msg.sender == _owner;
}
/**
* @dev Allows the current owner to relinquish control of the contract.
* @notice Renouncing to ownership will leave the contract without an owner.
* It will not be possible to call the functions with the `onlyOwner`
* modifier anymore.
*/
function renounceOwnership() public onlyOwner {
emit OwnershipTransferred(_owner, address(0));
_owner = address(0);
}
/**
* @dev Allows the current owner to transfer control of the contract to a newOwner.
* @param newOwner The address to transfer ownership to.
*/
function transferOwnership(address newOwner) public onlyOwner {
_transferOwnership(newOwner);
}
/**
* @dev Transfers control of the contract to a newOwner.
* @param newOwner The address to transfer ownership to.
*/
function _transferOwnership(address newOwner) internal {
require(newOwner != address(0));
emit OwnershipTransferred(_owner, newOwner);
_owner = newOwner;
}
}