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I want to make a page for direct sale of my token, just like the one in Loomx, with a fixed token price (changeable later by me) and immediately transfer token to buyer securely after purchase.

I try to "steal" their smart contract but it's not verified and public. When I try search online, most are Crowdsale, which seems to be over complicated.

I also found a direct-corwdsale library, but it seems not being used widely.

Here's my code which is very simple:

pragma solidity ^0.4.24;

import 'http://github.com/OpenZeppelin/openzeppelin-solidity/contracts/ownership/Ownable.sol';
import 'http://github.com/OpenZeppelin/openzeppelin-solidity/contracts/token/ERC20/ERC20.sol';

contract Directsale is Ownable {

    uint public tokensPerEther;
    ERC20 public token;
    bool public saleClosed = false;

    event FundTransfer(address backer, uint amount);

    constructor (
        uint tokenPerEachEther,
        address tokenAddress
    ) public {
        tokensPerEther = tokenPerEachEther * 1 ether;
        token = ERC20(tokenAddress);
    }

    function closeSale() external onlyOwner {
        saleClosed = true;
    }

    function setTokenPrice(uint tokenPerEachEther) external onlyOwner {
       tokensPerEther  = tokenPerEachEther * 1 ether;
    }

    function () payable public {
        require(!saleClosed);
        require(msg.value > 0);
        uint amount = msg.value;
        token.transfer(msg.sender, amount * tokensPerEther);
        emit FundTransfer(msg.sender, amount);
    }

}

Would that be ok and would not cause any security issue?

And, is there any good open-source example including the web3 javascript, so I can study with?

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Just a quick answer to my own question, I turned out using the CappedCrowdsale.sol and AllowanceCrowdsale.sol from OpenZeppelin. It should be safe and stable enough, my final code is here:

pragma solidity ^0.4.24;

import 'openzeppelin-solidity/contracts/crowdsale/emission/AllowanceCrowdsale.sol';
import 'openzeppelin-solidity/contracts/crowdsale/validation/CappedCrowdsale.sol';
import "openzeppelin-solidity/contracts/ownership/HasNoTokens.sol";

contract CubikCrowdsale is CappedCrowdsale, AllowanceCrowdsale, HasNoTokens {

  event RateChange(uint256 amount);

  constructor (uint256 _rate, address _wallet, ERC20 _token, uint256 _cap) public Crowdsale(_rate, _wallet, _token) CappedCrowdsale(_cap) AllowanceCrowdsale(_wallet) {
  }

  function setRate(uint256 _rate) external onlyOwner {
    rate = _rate;
    emit RateChange(_rate);
  }

}

For Web3 frontend, I couldn't find a good completed example and eventually wrote my own one here.

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