Building apps on ethereum
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- 09.01.2021
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While there's fairly good documentation out there for each of these tools individually, there's not a lot that helps you put them all together and understand how they work with each other.
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| Building apps on ethereum | Welcome to the temporary home of learning materials relating to development on ethereum. In my future tutorials and guides, I'll be diving into more complex smart contract development and also how to deploy them as subgraphs to expose a GraphQL API on top of them and implement things like pagination and full text search. These migrations are similar to other web development frameworks that require migrations to change the state of a database. This is that reference. Importing account into metamask: Step 1: In you running ganache app pick one of the account building apps on ethereum wish to import and click on the key at the right most side. |
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You can only re-deploy a new copy. We write all our tests in Javascript inside this file with the Mocha testing framework and the Chai assertion library. These come bundled with the Truffle framework. We'll write all these tests in JavaScript to simulate client-side interaction with our smart contract, much like we did in the console.
This test does two things: Checks that the smart contract has an address, i. Checks that the name was set when it was deployed. You can watch the video for a more in-depth explanation of this testing file.
Great job! If you got stuck, feel free to re-watch this portion of the video for further clarification. You can also find a copy of all the code here. Part 2: Sell Products The accompanying video footage for this portion of the tutorial begins at You can also find the code here. Now let's continue building out the marketplace smart contract.
We'll create the first feature, which will allow a user to list an item for sale in the marketplace. That's exactly what we've done by creating a Product struct. It stores all the attributes of a product that we'll need, like id, name, price, owner, and purchased. Next, we need a place to store this products on the blockchain. Mappings have unique keys that return unique values.
In our case, we will use an id as a key, and the value will be a Product struct. This will essentially allow us to look up a product by id, like a database. You can't check the "length" or "size" of a mapping in Solidiy. So if we want to fetch all the product, we must read them out individually. We'll use the counter cache to determine how may times to do this.
The product price is expressed in Wei, Ether's smallest subdivision see video for full explanation. Next, we add some requirements that must be satisfied before the function continues execution. We check that the name is present, and that the price is greater than 0. Then we create a new product and add it to the mapping. Note that msg. Finally, we trigger an event to let everyone know that the product was created sucesfully.
Now let's add the event definition so that it can be triggered: event ProductCreated uint id, string name, uint price, address owner, bool purchased ; External subscribers can listen for this event to verify that a product was created on the blockchain.
We'll check for this event inside the smart contract tests momentarily. First, we add some extra tools to our test suite like this. We already installed these in our package. This sets up the test example with a before hook, which creates a product before each test runs. Then, we check the smart contract event logs to see that the product was created with the correct values. We dig into the event logs like this: result.
We check that all of these values are correct, like id, name, price, etc Finally, we check failure cases. For example, we make sure the function call fails if there is no name, or if the price is less than or equal to 0. Great work! Part 3: Buy Products The accompanying video footage for this portion of the tutorial begins at Now let's create a function to purchase products. Whenver someone calls this function, they will submit the id of the product that they want to purchase this will be handled by our client side application.
Additionally, they will send Ethereum cryptocurrency from their wallet to purchase the product when they call this function. We'll see this in action momentarily. First, we fetch the product from the mapping and create a new copy of it in memory. Then we store the current owner to a variable. We will transfer the ownership of the product inside this function, so we need to know who the original owner was.
Next, we add a few requirements. We check for a valid id, that there is enough Etherum cryptocurrency in the transaction, that the buyer is not the seller, and that the product has not been purchased already. Then we facilitate the transaction.
We transfer the ownership to the buyer, mark the product as purchased, and add the product back to the mapping. After this, we send the cryptocurrency payment to the seller. Note, that the Ether cryptocurrency amount is tracked in Solidity with the function's metadata. We can access this value with the global variable msg. Again, this value is expressed in Wei see video for further explanation.
Lastly, we trigger an event to declare that the product has been created successfully. Now let's create a new event for product sales. Finally, let's deploy the smart contract to the network so that we can start building the client side application to interact with it in the next section.
We can re-run our migrations with the --reset flag to migrate the smart contracts to the network. Part 4: Marketplace Website Setup Front End The accompanying video footage for this portion of the tutorial begins at Now let's start building the client side application for the Marketplace. We'll use Metamask for this, which we installed in the dependencies section of the tutorial.
To do this we'll need to do two things: Connect Metamask to our Ganache personal blockchain instance Import some accounts from Ganache into Metamask so that we can act on their behalf as users of our marketplace application You can watch me set up metamask at this point in the video.
Next, let me briefly explain why we're using React. We need a way to manage all of the client side behavior of our application, and store data from the blockchain. Instead of doing this all by hand, React. You can watch my full explanation of React. We'll start by importing web3 into our main App.
Inside of here, we'll call a function that instantiates web3. You should consider trying MetaMask! Don't worry if you don't understand everything inside this function. That's okay! It is taken directly from the instructions that Metamask provides.
I'm going to go ahead and create a new component for the Navbar while we're here. I do this at the end of the video, so it's a little out of order. However, I think this is the best time to do it. First, create a new component called Navbar. This uses React's props object, which is available to all React components. We'll need to pass these props down to this component when we render it inside of App.
Let's do that next. Now import the Navbar component at the top of App. Now go visit your app in the web browser and see the account listed on the page! Next, let's add the connection to the marketplace smart contract. First, we'll import the smart contract ABI at the top of App.
Contract Marketplace. We'll use this network ID to connect to the smart contract deployed to the Ganache network, rather than the main Ethereum network for example. Next, we instantiate the smart contract with Web3. We need 2 pieces of information in order to do this: the smart contract ABI, and the address. We fetch both of those from the file we just imported. Finally, if we can't find the smart contract on the network, we alert the user.
You can test this out by switching to the main Ethereum network in Metamask just don't forget to switch back. We covered a lot of ground very quickly in this section. Don't worry if there is a lot of information. Feel free to re-watch this portion of the video for further clarification. Part 5: Sell Products Front End The accompanying video footage for this portion of the tutorial begins at Now let's create a way to sell products from our marketplace website.
We'll do these tasks: Create a react component that holds the scaffold for our code, including a form that allows users to list new products, and a table that shows products for sale. We'll wire up the form so that users can actually list their product for sale on the blockchain. First, let's add some more data to our loadBlockchainData function. Then we call the smart contract function with Web3.
This calls the function and tells Web3 that the current account is the user that's calling it. Finally, once the transaction receipt has been received, we remove the app from "loading" state so that the user knows the function call is complete. But first we must create it! Let's create a new file in the current directory called Main. It also creates a form that will add the product to the blockchain by calling the createProduct function. Feel free to see a full explanation of this code in the video.
But this component won't work just yet! We need to do a few things first. Go back to App. If it is loading, then it will show the loader! Ethereum's decentralized finance DeFi system never sleeps or discriminates. With just an internet connection, you can send, receive, borrow, earn interest, and even stream funds anywhere in the world. Explore DeFi The internet of assets Ethereum isn't just for digital money.
Anything you can own can be represented, traded and put to use as non-fungible tokens NFTs. You can tokenise your art and get royalties automatically every time it's re-sold. Or use a token for something you own to take out a loan. The possibilities are growing all the time.
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