This post is meant for solidity noobs (like me) who have recently started dabbling with ethereum DApp development & the blockchain technology.
I’ve had a hard time following Solidity tutorials and its official documentation. Some snippets are simply not up-to-date, especially the web3 API. Those few things can be really frustrating to figure out on your own, and I’ve taken great pains to figure out the actual web3 API usage rather than what’s described in the doc, and I’d like to share them. This is the first post in the series and also my first blog, hopefully you will not be so disappointed =)
ps- you can also read the same on my repository.
Events help clients keep track of a particular occurrence — the closing of an Auction, for example. So let’s explore how to create events, emit them and establish a mechanism that notifies us upon firing of the particular event.
We’ll start with a very basic Auction Contract. The end result will load an HTML document which notifies that the call to close the auction will be made after 3 seconds. When the auction is closed, an event is triggered, notifying web3 of the closing, which in turn notifies our listener, whose code changes the text to “Auction closed”.
Below are the 3 truffle files (the entire truffle project will soon be available under the “project” directory):
index.html
<!DOCTYPE html>
<html>
<head>
<title>MetaCoin - Default Truffle App</title>
<link href='https://fonts.googleapis.com/css?family=Open+Sans:400,700' rel='stylesheet' type='text/css'>
<link href="./app.css" rel='stylesheet' type='text/css'>
<script src="./app.js"></script>
</head>
<body>
<div id = "status">Closing Auction in 3 seconds...</div>
</body>
</html>Auction.sol
contract Auction {
event AuctionClosed (uint highestBid); //declare eventto be triggered when Auction closes
address public creator; function Auction () { //Auction Constructor to register the creator of the contract
creator = msg.sender;
} function closeAuction (uint someRandomBid) {
if (msg.sender == creator) { //make sure that auction is being ended by the creator themselves
AuctionClosed (someRandomBid); //trigger the event to notify the listeneres that the auction has ended
return;
}
throw;
}
}app.js
window.onload = function () {
let accounts = web3.eth.accounts; //create local variable for easy access
let maxBid = Math.ceil (Math.random () * 1000); //the maximum bid placed by the end of the auction
let status = document.getElementById ('status'); Auction.new ({from: accounts [0]}) //create new contract object
.then ( (contract) => {
contract.AuctionClosed ().watch ( (err, response) => { //set up listener for the AuctionClosed Event
//once the event has been detected, take actions as desired
status.innerHTML = 'The auction has ended! Highest Bid is ' + response.args.highestBid;
}); setTimeout ( () => { //simulate an auction for 3 seconds, after which the creator closes the auction
contract.closeAuction (maxBid, {from: accounts [0]});
}, 3000);
})
.catch ( (err) => {
status.innerHTML = 'Some error occured. I guess shit happens =(';
});
};The main thing in our Solidity contract is the declaration and triggering of the AuctionClosed event. The main thing in app.js is the right way to use the web3 API to listen for events and act upon them as and when they occur.Now, fire up 2 terminals. In the first one, launch the testrpc utility to simulate the ethereum blockchain:
testrpcIn the second terminal, navigate to the root directory of this project, then:
truffle compile
truffle serveThen open up your browser and launch http://localhost:8080/ (if you haven’t messed around with any of the default settings), wait for 3 seconds and BAM! You will see the highest bid (randomly generated, duh).
NOTE: There is no gas involved in this because testrpc is basically a simulation.That’s all folks. Would love to hear your feedback on this =)
If you liked it, share it with your friends so they too can learn something new. If you hated it, share it with your friends so they too can suffer a bit!