# Get & Set Value 3.0 |  Web3.0 dApp Dev 0x06

By [Web3dAppDevCamp](https://paragraph.com/@apecoder) · 2021-12-12

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In PurposeHandler version 2.0, we learned to add permission control to functions via the `require` statement.

Now we can add a real economy mechanism to it, so that the value of `purpose` is controlled by a real bidding auction mechanism!

### 0x01 Add Owner

To get the owner of a contract `owner` is a must-learn operation for newbies.

The easiest way is to set a public `owner` variable and pass the `_owner` parameter in the constructor.

    pragma solidity >=0.8.0 <0.9.0;
    //SPDX-License-Identifier: MIT
    
    contract PurposeHandler {
    
      string public purpose = "Building Unstoppable Apps";
      address public owner = owner;
      // fill in your own address (start with 0x) here
    
      constructor(address _owner) {
        owner = _owner;
      }
    
      function setPurpose(string memory newPurpose) public {
          // about msg.sender:
          // https://cryptozombies.io/en/lesson/2/chapter/3
          // about require:
          // https://cryptozombies.io/en/lesson/2/chapter/4
          require( msg.sender == owner, "NOT THE OWNER!");
    
          purpose = newPurpose;
          console.log(msg.sender,"set purpose to",purpose);
      }
    }
    

In production, this is generally achieved by importing `ownable.sol`.

`ownable.sol` in OpenZepplin.

> [https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/access/Ownable.sol](https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/access/Ownable.sol)

`ownable.sol`:

    pragma solidity ^0.4.25;
    
      /**
      * @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;
        }
      }
    

### 0x02 Create `auction mechanism` by a require rule

The rule is that if the bid is higher than the current price, then the value of `set-purpose` can be modified; if it is not as high as the current price, then an error is thrown.

    pragma solidity >=0.8.0 <0.9.0;
    //SPDX-License-Identifier: MIT
    
    contract PurposeHandler {
    
      string public purpose = "Building Unstoppable Apps";
      address public owner = owner;
      uint256 public price = 0.001 ether;
      // fill in your own address (start with 0x) here
    
      constructor(address _owner) {
        owner = _owner;
      }
    
      function setPurpose(string memory newPurpose) payable public {
          require( msg.value > price, "NOT ENOUGH!");
          purpose = newPurpose;
          // update price when guy set
          price = msg.value;
          console.log(msg.sender,"set purpose to",purpose);
      }
    
    }
    

### 0x03 Withdrawal mechanism

The function `getBalance()` allows the `owner` to get the balance in the contract, and the function `getMyMoney` allows the balance in the contract to be withdrawn by.

    pragma solidity >=0.8.0 <0.9.0;
    //SPDX-License-Identifier: MIT
    
    contract PurposeHandler {
    
      string public purpose = "Building Unstoppable Apps";
      address public owner;
      uint256 public price = 0.001 ether;
    
      constructor(address _owner) {
        owner = _owner;
      }
    
      function setPurpose(string memory newPurpose) payable public {
          require( msg.value > price, "NOT ENOUGH!");
          purpose = newPurpose;
          // update price when guy set
          price = msg.value;
      }
    
      function getBalance() view public returns(uint256) {
          return address(this).balance;
      }
    
      function getMyMoney(address _to, uint256 amount) public {
              require(msg.sender==owner);
              address payable receiver = payable(_to);
            receiver.transfer(amount);
      }
    }
    

### 0x04 Record logs of set purpose by Event

What is an event?

> A blockchain is a list of blocks - they are basically made up of transactions. Each transaction has a receipt attached to it, which contains zero or more log entries. These log entries represent the results generated when an event is triggered in a smart contract.
> 
> In Solidity source code, to define an `event`, you need to mark it by prefixing it with the event keyword (similar to the usage of the `function` keyword). You can then call or trigger the event from within any function that you wish to generate the event. You can trigger events from any function using the `emit` keyword.
> 
> Someone may want to add "listen" for events in the DAPP. It uses the filtering functionality of Web 3.0 ([filtering functionality of Web 3.0](https://github.com/ethcore/parity#eth_newfilter)).
> 
> \--[https://learnblockchain.cn/question/29](https://learnblockchain.cn/question/29)

We can add `event` to the setPurpose function to record the history of set purpose.

Translated with [www.DeepL.com/Translator](http://www.DeepL.com/Translator) (free version)

    pragma solidity >=0.8.0 <0.9.0;
    //SPDX-License-Identifier: MIT
    
    contract PurposeHandler {
    
      event SetPurpose(address sender, string purpose);
    
      string public purpose = "Building Unstoppable Apps";
      address public owner;
      uint256 public price = 0.001 ether;
    
      constructor(address _owner) {
        owner = _owner;
      }
    
      function setPurpose(string memory newPurpose) payable public {
          require( msg.value > price, "NOT ENOUGH!");
          purpose = newPurpose;
          // update price when guy set
          price = msg.value;
          emit SetPurpose(msg.sender, purpose);
      }
    
      function getBalance() view public returns(uint256) {
          return address(this).balance;
      }
    
      function getMyMoney(address _to, uint256 amount) public {
              require(msg.sender==owner);
              address payable receiver = payable(_to);
            receiver.transfer(amount);
      }
    }
    

### Summary

That's it for Get & Set value series. Here are all the articles for Get & Set value series.

> *   [Eth.build Quickstart | Web3.0 dApp Dev 0x01](https://twitter.com/Web3dAppCamp/status/1450618786436943874)
>     
> *   [Web3.0 dApp Developer Growth Path | Web3.0 dApp Dev 0x02](https://twitter.com/Web3dAppCamp/status/1452046900983570434)
>     
> *   [Scaffold-Eth Quickstart | web3.0 dApp Dev 0x03](https://twitter.com/Web3dAppCamp/status/1453378592050585604)
>     
> *   [Get & Set Value 1.0 | Web3.0 dApp Dev 0x04](https://twitter.com/Web3dAppCamp/status/1457505517606035459)
>     
> *   [Get & Set Value 2.0 | Web3.0 dApp Dev 0x05](https://twitter.com/Web3dAppCamp/status/1462976622152220673)
>     
> *   Get & Set Value 3.0 | Web3.0 dApp Dev 0x06
>     

In the next post, we move on to the next chapter - the design and implementation of the NFT dApp!

**Authors:**

leeduckgo

msfew

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*Originally published on [Web3dAppDevCamp](https://paragraph.com/@apecoder/get-set-value-3-0-web3-0-dapp-dev-0x06)*