Photo by Xuan Nguyen / Unsplash

Background

俗話說的好 畢圈一日 人間一年..... 開始學 Smart Contract吧, part 4.

Election

$ mkdir election
$ cd election
$ truffle unbox pet-shop
$ tocuh contracts/Election.sol

Election.sol

pragma solidity ^0.4.24;

contract Election {

    // Model a Candidate
    struct Candidate {
        uint id;
        string name;
        uint voteCount;
    }

    // Store accounts that have voted
    mapping(address => bool) public voters;
    // Read/write Candidates
    mapping(uint => Candidate) public candidates;
    // Store Candidates Count
    uint public candidatesCount;

    // Constructor
    function Election () public {
        addCandidate("Candidate 1");
        addCandidate("Candidate 2");
    }

    function addCandidate (string _name) private {
        candidatesCount ++;
        candidates[candidatesCount] = Candidate(candidatesCount, _name, 0);
    }

    function vote (uint _candidateId) public {
        // require that they haven't voted before
        require(!voters[msg.sender]);

        // require a valid candidate
        require(_candidateId > 0 && _candidateId <= candidatesCount);

        // record that voter has voted
        voters[msg.sender] = true;

        // update candidate vote Count
        candidates[_candidateId].voteCount ++;
    }
}

Deploy

$ touch migrations/2_deploy_contracts.js

2_deploy_contracts.js

var Election = artifacts.require("./Election.sol");

module.exports = function(deployer) {
  deployer.deploy(Election);
};
$ truffle migrate --reset

Testing

$ touch test/election.js

election.js

var Election = artifacts.require("./Election.sol");

contract("Election", function(accounts) {
  var electionInstance;

  it("initializes with two candidates", function() {
    return Election.deployed().then(function(instance) {
      return instance.candidatesCount();
    }).then(function(count) {
      assert.equal(count, 2);
    });
  });
  
  it("it initializes the candidates with the correct values", function() {
    return Election.deployed().then(function(instance) {
      electionInstance = instance;
      return electionInstance.candidates(1);
    }).then(function(candidate) {
      assert.equal(candidate[0], 1, "contains the correct id");
      assert.equal(candidate[1], "Candidate 1", "contains the correct name");
      assert.equal(candidate[2], 0, "contains the correct votes count");
      return electionInstance.candidates(2);
    }).then(function(candidate) {
      assert.equal(candidate[0], 2, "contains the correct id");
      assert.equal(candidate[1], "Candidate 2", "contains the correct name");
      assert.equal(candidate[2], 0, "contains the correct votes count");
    });
  });

  it("allows a voter to cast a vote", function() {
    return Election.deployed().then(function(instance) {
      electionInstance = instance;
      candidateId = 1;
      return electionInstance.vote(candidateId, { from: accounts[0] });
    }).then(function(receipt) {
      return electionInstance.voters(accounts[0]);
    }).then(function(voted) {
      assert(voted, "the voter was marked as voted");
      return electionInstance.candidates(candidateId);
    }).then(function(candidate) {
      var voteCount = candidate[2];
      assert.equal(voteCount, 1, "increments the candidate's vote count");
    })
  });
  
  it("throws an exception for invalid candidates", function() {
    return Election.deployed().then(function(instance) {
      electionInstance = instance;
      return electionInstance.vote(99, { from: accounts[1] })
    }).then(assert.fail).catch(function(error) {
      assert(error.message.indexOf('revert') >= 0, "error message must contain revert");
      return electionInstance.candidates(1);
    }).then(function(candidate1) {
      var voteCount = candidate1[2];
      assert.equal(voteCount, 1, "candidate 1 did not receive any votes");
      return electionInstance.candidates(2);
    }).then(function(candidate2) {
      var voteCount = candidate2[2];
      assert.equal(voteCount, 0, "candidate 2 did not receive any votes");
    });
  });

  it("throws an exception for double voting", function() {
    return Election.deployed().then(function(instance) {
      electionInstance = instance;
      candidateId = 2;
      electionInstance.vote(candidateId, { from: accounts[1] });
      return electionInstance.candidates(candidateId);
    }).then(function(candidate) {
      var voteCount = candidate[2];
      assert.equal(voteCount, 1, "accepts first vote");
      // Try to vote again
      return electionInstance.vote(candidateId, { from: accounts[1] });
    }).then(assert.fail).catch(function(error) {
      assert(error.message.indexOf('revert') >= 0, "error message must contain revert");
      return electionInstance.candidates(1);
    }).then(function(candidate1) {
      var voteCount = candidate1[2];
      assert.equal(voteCount, 1, "candidate 1 did not receive any votes");
      return electionInstance.candidates(2);
    }).then(function(candidate2) {
      var voteCount = candidate2[2];
      assert.equal(voteCount, 1, "candidate 2 did not receive any votes");
    });
  });
});
$ truffle test

Client-Side Application

$ vim src/index.html

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>Election Results</title>

    <!-- Bootstrap -->
    <link href="css/bootstrap.min.css" rel="stylesheet">
  </head>
  <body>
    <div class="container" style="width: 650px;">
      <div class="row">
        <div class="col-lg-12">
          <h1 class="text-center">Election Results</h1>
          <hr/>
          <br/>
          <div id="loader">
            <p class="text-center">Loading...</p>
          </div>
          <div id="content" style="display: none;">
            <table class="table">
              <thead>
                <tr>
                  <th scope="col">#</th>
                  <th scope="col">Name</th>
                  <th scope="col">Votes</th>
                </tr>
              </thead>
              <tbody id="candidatesResults">
              </tbody>
            </table>
            <form onSubmit="App.castVote(); return false;">
                <div class="form-group">
                <label for="candidatesSelect">Select Candidate</label>
                <select class="form-control" id="candidatesSelect">
                </select>
                </div>
                <button type="submit" class="btn btn-primary">Vote</button>
                <hr />
            </form>
            <hr/>
            <p id="accountAddress" class="text-center"></p>
          </div>
        </div>
      </div>
    </div>
    <!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
    <!-- Include all compiled plugins (below), or include individual files as needed -->
    <script src="js/bootstrap.min.js"></script>
    <script src="js/web3.min.js"></script>
    <script src="js/truffle-contract.js"></script>
    <script src="js/app.js"></script>
  </body>
</html>    
$ vim src/js/app.js

app.js

App = {
  web3Provider: null,
  contracts: {},
  account: '0x0',

  init: function() {
    return App.initWeb3();
  },

  initWeb3: function() {
    if (typeof web3 !== 'undefined') {
      // If a web3 instance is already provided by Meta Mask.
      App.web3Provider = web3.currentProvider;
      web3 = new Web3(web3.currentProvider);
   } else {
      // Specify default instance if no web3 instance provided
      App.web3Provider = new Web3.providers.HttpProvider('http://localhost:7545');
      web3 = new Web3(App.web3Provider);
    }
    return App.initContract();
  },

  initContract: function() {
    $.getJSON("Election.json", function(election) {
      // Instantiate a new truffle contract from the artifact
      App.contracts.Election = TruffleContract(election);
      // Connect provider to interact with contract
      App.contracts.Election.setProvider(App.web3Provider);

      return App.render();
    });
  },
  render: function() {
    var electionInstance;
    var loader = $("#loader");
    var content = $("#content");

    loader.show();
    content.hide();

    // Load account data
    web3.eth.getCoinbase(function(err, account) {
      if (err === null) {
        App.account = account;
        $("#accountAddress").html("Your Account: " + account);
      }
    });
      // Load contract data
    App.contracts.Election.deployed().then(function(instance) {
      electionInstance = instance;
      return electionInstance.candidatesCount();
    }).then(function(candidatesCount) {
      var candidatesResults = $("#candidatesResults");
      candidatesResults.empty();

      var candidatesSelect = $('#candidatesSelect');
      candidatesSelect.empty();

      for (var i = 1; i <= candidatesCount; i++) {
        electionInstance.candidates(i).then(function(candidate) {
          var id = candidate[0];
          var name = candidate[1];
          var voteCount = candidate[2];

          // Render candidate Result
          var candidateTemplate = "<tr><th>" + id + "</th><td>" + name + "</td><td>" + voteCount + "</td></tr>"
          candidatesResults.append(candidateTemplate);

          // Render candidate ballot option
          var candidateOption = "<option value='" + id + "' >" + name + "</ option>"
          candidatesSelect.append(candidateOption);
        });
      }
      return electionInstance.voters(App.account);
    }).then(function(hasVoted) {

      // Do not allow a user to vote
      if(hasVoted) {
        $('form').hide();
      }
      loader.hide();
      content.show();
    }).catch(function(error) {
      console.warn(error);
    });
  },
  castVote: function() {
    var candidateId = $('#candidatesSelect').val();
    App.contracts.Election.deployed().then(function(instance) {
      return instance.vote(candidateId, { from: App.account });
    }).then(function(result) {
      // Wait for votes to update
      $("#content").hide();
      $("#loader").show();
    }).catch(function(err) {
      console.error(err);
    });
  }
};

$(function() {
  $(window).load(function() {
    App.init();
  });
});

單純複製貼上紀錄一下讓之後比較好查詢.

Reference