Million Ether Homepage

Storing Colors in a Pixel Grid

Million Ether Page Essential Smart Contract

Tip: Mining only when there are transactions

Tip: Using Remix IDE

Getting Started

Getting The Pixels and Ethereum Events

Building a DApp UI

Drawing in HTML Canvas

Using Websockets with Web3

Web3 and Events Over WebSockets

Drawing Pixels from the Blockchain

Viewing Pixels

Buying An Image -- Reading Pixels

Buying An Image -- Sending Transactions

Pixel bidding and structs

Payable Contracts, payable Functions

Contract Owner and require

Payable color pixel function

Buying Pixels

Sending funds from a contract

The safer way to send funds from a contract

The Pull Payment Pattern - Withdraw payments when outbid

Million Ether Homepage Summary

Auctioning Pixels

solidity

In this course, we'll build a DApp inspired by [The Million Dollar Homepage](https://en.wikipedia.org/wiki/The_Million_Dollar_Homepage). We'll build a 1 million pixel page, and each pixel can be colored by placing the highest bid.

In this course we'll walk through every step of writing a smart contract, deploying it, hosting auctions, and connecting it to the browser through web3. 

If you're looking to build real, full-stack ethereum Dapps, then this is a perfectly-sized example.

We'll build a DApp inspired by the Million Dollar Homepage, powered by an auction on the Ethereum blockchain

A common pattern in Solidity contracts is specifying an 

The idea is this: when we create the contract, in the contract's constructor, we'll store the address of the account that created this contract. That account becomes the owner. When the contract is destroyed, we'll send the funds back to that owner.

Here's how we modify our bank to have an owner:

pragma solidity ^0.4.11;

contract Bank {
  // first we'll add an address property to store the owner
  address public owner;

  event DepositMade(
    address sentBy,
    uint amount
  );

  // then we'll add a constructor function. Here we'll set owner to the msg.sender. That is, the address of the account that created this contract.
  function Bank() {
    owner = msg.sender;
  }

  function deposit() payable {
    DepositMade(msg.sender, msg.value);
  }

  // last we'll add a destroy function, and this function will call
  // `selfdestruct`, which is a special function that essentially "shuts down" our
  // contract. The argument to selfdestruct is the address where we want to send
  // all of the funds this contract controls.
  function destroy() {
    // warning -- anyone can call this...
    selfdestruct(owner);
  }
}

One problem with our destroy function is that 

 can call that function. We don't have any constraints on 

---
title: Contract Owner and require


slug: 'contract-owner-and-require'
thumbnail: https://embed-ssl.wistia.com/deliveries/86e20be8713a6a2a78e4bdd30945a4f777456518.jpg

privateVideoUrl: https://fullstack.wistia.com/medias/6rf1sdrmgu



---

### Adding an Owner

A common pattern in Solidity contracts is specifying an `owner`. 

The idea is this: when we create the contract, in the contract's constructor, we'll store the address of the account that created this contract. That account becomes the owner. When the contract is destroyed, we'll send the funds back to that owner.

Here's how we modify our bank to have an owner:

```javascript
pragma solidity ^0.4.11;

contract Bank {
  // first we'll add an address property to store the owner
  address public owner;

  event DepositMade(
    address sentBy,
    uint amount
  );

  // then we'll add a constructor function. Here we'll set owner to the msg.sender. That is, the address of the account that created this contract.
  function Bank() {
    owner = msg.sender;
  }

  function deposit() payable {
    DepositMade(msg.sender, msg.value);
  }

  // last we'll add a destroy function, and this function will call
  // `selfdestruct`, which is a special function that essentially "shuts down" our
  // contract. The argument to selfdestruct is the address where we want to send
  // all of the funds this contract controls.
  function destroy() {
    // warning -- anyone can call this...
    selfdestruct(owner);
  }
}
```

One problem with our destroy function is that _anyone_ can call that function. We don't have any constraints on _who_ can call `destroy` and selfdestruct our contract! 

Learn

The newline Guide to Building Your First GraphQL Server with Node and TypeScript

Teach

Amelia Wattenberger

Author of Fullstack D3

Community

Million Ether Homepage

WELCOMEIntroduction

Introduction:

Start Here

Module 1: Getting Started

Lesson 1: (3:23)

Million Ether Homepage

Lesson 2: (3:18)

Storing Colors in a Pixel Grid

Lesson 3: (4:29)

Million Ether Page Essential Smart Contract

Lesson 4: (1:56)

Tip: Mining only when there are transactions

Lesson 5: (7:08)

Tip: Using Remix IDE

Module 2: Viewing Pixels

Lesson 1: (6:08)

Getting The Pixels and Ethereum Events

Lesson 2: (7:15)

Gas

Lesson 3: (1:01)

Building a DApp UI

Lesson 4: (3:09)

Drawing in HTML Canvas

Lesson 5: (1:15)

Using Websockets with Web3

Lesson 6: (3:09)

Web3 and Events Over WebSockets

Lesson 7: (3:41)

Drawing Pixels from the Blockchain

Module 3: Buying Pixels

Lesson 1: (1:57)

Buying An Image -- Reading Pixels

Lesson 2: (4:51)

Buying An Image -- Sending Transactions

Lesson 3: (1:25)

Pixel bidding and structs

Lesson 4: (2:48)

Payable Contracts, payable Functions

Lesson 5: (3:35)

Contract Owner and require

Lesson 6: (2:38)

Payable color pixel function

Module 4: Auctioning Pixels

Lesson 1: (4:03)

Sending funds from a contract

Lesson 2: (3:26)

The safer way to send funds from a contract

Lesson 3: (4:37)

The Pull Payment Pattern - Withdraw payments when outbid

Lesson 4: (6:46)

Million Ether Homepage Summary

Contract Owner and require