【CryptoZombies】lesson1 Chapter 13: Events

Events are a way for your contract to communicate that something happened on the blockchain to your app front-end, which can be 'listening' for certain events and take action when they happen.

Example:

// declare the event
event IntegersAdded(uint x, uint y, uint result);

function add(uint _x, uint _y) public returns (uint) {
 uint result = _x + _y;
 // fire an event to let the app know the function was called:
 emit IntegersAdded(_x, _y, result);
 return result;
}

Your app front-end could then listen for the event. A javascript implementation would look something like:

YourContract.IntegersAdded(function(error, result) {
 // do something with result
})

test

We want an event to let our front-end know every time a new zombie was created, so the app can display it.

1.Declare an event called NewZombie. It should pass zombieId (a uint), name (a string), and dna (a uint).

2.Modify the _createZombie function to fire the NewZombie event after adding the new Zombie to our zombies array.

3.You're going to need the zombie's id. array.push() returns a uint of the new length of the array - and since the first item in an array has index 0, array.push() - 1 will be the index of the zombie we just added. Store the result of zombies.push() - 1 in a uint called id, so you can use this in the NewZombie event in the next line.

ゾンビのidが必要。array.push()は新しい長さのuint配列を返し、配列の最初のインデックスは0であるため、array.push() - 1 が追加したゾンビのインデックス。そこで、zombies.push() - 1結果をidという名前のuintに格納し、次の行で作成するNewZombieイベントで使用できるようにしなさい。

pragma solidity >=0.5.0 <0.6.0;

contract ZombieFactory {

   // declare our event here

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

   struct Zombie {
       string name;
       uint dna;
   }

   Zombie[] public zombies;

   function _createZombie(string memory _name, uint _dna) private {
       zombies.push(Zombie(_name, _dna));
       // and fire it here
   }

   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 {
       uint randDna = _generateRandomDna(_name);
       _createZombie(_name, randDna);
   }

}

↓

pragma solidity >=0.5.0 <0.6.0;

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;

   function _createZombie(string memory _name, uint _dna) private {
       uint id = zombies.push(Zombie(_name, _dna)) - 1;
       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 {
       uint randDna = _generateRandomDna(_name);
       _createZombie(_name, randDna);
   }

}

event(トランザクションへのログ出力)を実行するときはemitをつける必要がある。

chapter10以上は難易度UP。むずかしい、、、

implementation：実装

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