How to make ethereum dapps
Today I'm going to show you how to build your first decentralized application, or dApp, on the Ethereum blockchain. I'll show you how to. Compile the contracts. At the truffle development command prompt. To connect your DApp to Ethereum, Optimism, and other networks, you'll need an Infura account. Sign up for an account here. Once you're signed in, create a. DRAWING FOR BEGINNERS WHERE TO START INVESTING
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Benefits of dapp development Zero downtime — Once the smart contract is deployed on the blockchain, the network as a whole will always be able to serve clients looking to interact with the contract. Malicious actors, therefore, cannot launch denial-of-service attacks targeted towards individual dapps. Resistance to censorship — No single entity on the network can block users from submitting transactions, deploying dapps, or reading data from the blockchain.
Complete data integrity — Data stored on the blockchain is immutable and indisputable, thanks to cryptographic primitives. Malicious actors cannot forge transactions or other data that has already been made public. This is not true in traditional models; for example, when we use online banking systems, we must trust that financial institutions will not misuse our financial data, tamper with records, or get hacked. Drawbacks of dapp development Maintenance — Dapps can be harder to maintain because the code and data published to the blockchain are harder to modify.
Performance overhead — There is a huge performance overhead, and scaling is really hard. To achieve the level of security, integrity, transparency, and reliability that Ethereum aspires to, every node runs and stores every transaction. On top of this, proof-of-stake consensus takes time as well. Network congestion — When one dapp uses too many computational resources, the entire network gets backed up. Currently, the network can only process about transactions per second; if transactions are being sent in faster than this, the pool of unconfirmed transactions can quickly balloon.
User experience — It may be harder to engineer user-friendly experiences because the average end-user might find it too difficult to set up a tool stack necessary to interact with the blockchain in a truly secure fashion. Centralization — User-friendly and developer-friendly solutions built on top of the base layer of Ethereum might end up looking like centralized services anyways. For example, such services may store keys or other sensitive information server-side, serve a frontend using a centralized server, or run important business logic on a centralized server before writing to the blockchain.
Simply declaring this struct won't actually give us a candidate. We need to instantiate it and assign it to a variable before we can write it to storage. The next thing we need is a place to store the candidates. We need a place to store one of the structure types that we've just created. We can do this with a Solidity mapping. A mapping in Solidity is like an associative array or a hash, that associates key-value pairs. This essentially gives us an id-based look up for each candidate.
Since this mapping is assigned to a state variable, we will write data to the blockchain anytime we assign new key-value pairs to it. Next, we set this mapping's visibility to public in order to get a getter function, just like we did with the candidate name in the smoke test. That's because any key in a mapping that hasn't been assigned a value yet will return a default value an empty candidate in this case.
For example, if we only had 2 candidates in this election, and we try to look up candidate 99, then the mapping will return an empty Candidate structure. This behavior makes it impossible to know how many candidates exist, and therefore we must use a counter cache. Inside the function, we increment the candidate counter cache to denote that a new candidate has been added.
Then we update the mapping with a new Candidate struct, using the current candidate count as the key. This Candidate struct is initialized with the candidate id from the current candidate count, the name from the function argument, and the initial vote count to 0. Note that this function's visibility is private because we only want to call it inside the contract. You can follow along with me as I demonstrate this in the video at I'll leave that to you as an exercise.
First, let me explain why testing is so important when you're developing smart contracts. We want to ensure that the contracts are bug free for a few reasons: 1. All of the code on the Ethereum blockchain is immutable; it cannot change. If the contract contains any bugs, we must disable it and deploy a new copy. This new copy will not have the same state as the old contract, and it will have a different address.
Deploying contracts costs gas because it creates a transaction and writes data to the blockchain. This costs Ether, and we want to minimize the amount of Ether we ever have to pay. If any of our contract functions that write to the blockchain contain bugs, the account who is calling this function could potentially waste Ether, and it might not behave the way they expect. Testing Now let's write some tests. Make sure you have Ganache running first. These come bundled with the Truffle framework.
Next, we call the "contract" function, and write all our tests within the callback function. This callback function provides an "accounts" variable that represents all the accounts on our blockchain, provided by Ganache. The first test checks that the contract was initialized with the correct number of candidates by checking the candidates count is equal to 2.
The next test inspects the values of each candidate in the election, ensuring that each candidate has the correct id, name, and vote count. Client-Side Application Now let's start building out the client-side application that will talk to our smart contract. We'll use this existing code to get started. Let's also take note of a few other things that came with the Truffle Pet Shop box like the Bootstrap framework that will keep us from having to write any CSS in this tutorial.
Go ahead and replace all of the content of your "index. We configure web3 inside the "initWeb3" function. Initialize contracts: We fetch the deployed instance of the smart contract inside this function and assign some values that will allow us to interact with it. Render function: The render function lays out all the content on the page with data from the smart contract.
For now, we list the candidates we created inside the smart contract. We do this by looping through each candidate in the mapping, and rendering it to the table. We also fetch the current account that is connected to the blockchain inside this function and display it on the page. Now let's view the client-side application in the browser. Notice that your application says "Loading That's because we're not logged in to the blockchain yet! In order to connect to the blockchain, we need to import one of the accounts from Ganache into Metamask.
You can watch me set up Metamask in the video at Once you're connected with Metamask, you should see all of the contract and account data loaded. Cast Votes - Step 3 The accompanying video footage for this portion of the tutorial begins at Now let's add the ability to cast votes in the election. Let's look at a few other things that it does: It accepts one argument. This is an unsigned integer with the candidate's id. Its visibility is public because we want an external account to call it.
It adds the account that voted to the voters mapping that we just created. This will allow us to keep track that the voter has voted in the election. We access the account that's calling this function with the global variable "msg. It implements require statements that will stop execution if the conditions are not met. First require that the voter hasn't voted before. We do this by reading the account address with "msg.
If it's there, the account has already voted. Next, it requires that the candidate id is valid. The candidate id must be greater than zero and less than or equal to the total candidate count. Test that the voter is added to the mapping whenever they vote. Next we can write a few test for our function's requirements. We can dig into this error message to ensure that the error message contains the "revert" substring.
Then we can ensure that our contract's state was unaltered by ensuring that the candidates did not receive any votes. Then we'll cast a vote on their behalf. Then we'll try to vote again. We'll assert that an error has occurred here. We can inspect the error message, and ensure that no candidates received votes, just like the previous test. We will populate the select options with the candidates provided by our smart contract in our "app.
The form has an "onSubmit" handler that will call the "castVote" function. We will define this in our "app. Now let's update our app. First we list all the candidates from the smart contract inside the form's select element. Then we'll hide the form on the page once the account has voted. When we call the vote function from our smart contract, we pass in this id, and we provide the current account with the function's "from" metadata.
This will be an asynchronous call. When it is finished, we'll show the loader and hide the page content. Whenever the vote is recorded, we'll do the opposite, showing the content to the user again. Now your front-end application should look like this: Go ahead and try the voting function. Once you do, you should see a Metamask confirmation pop up like this: Once you click submit, you've successfully casted a vote!
You'll still see a loading screen. For now, you'll have to refresh the page to see the votes recorded. We'll implement the functionality update the loader automatically in the next section. If you got stuck, you can reference the full client-side code at this point in the tutorial here. Watch Events - Step 4 The accompanying video footage for this portion of the tutorial begins at The very last step in this tutorial is to trigger an event whenever a vote is cast.
This will allow us to update our client-side application when an account has voted. Fortunately, this is quite easy. These logs contain the event that was triggered. We check that the event is the correct type, and that it has the correct candidate id. Now let's update the client-side application to listen for the voted event and fire a page refresh any time that it is triggered.
First, we subscribe to the voted event by calling the "votedEvent" function. We pass in some metadata that tells us to listen to all events on the blockchain. Then we "watch" this event. Inside here, we log to the console anytime a "votedEvent" is triggered.
We also re-render all the content on the page. This will get rid of the loader after the vote has been recorded, and show the updated vote count on the table. Be patient, it might take a few seconds for the event to trigger.
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