Create Smart Contract and use ABI functions - ethereum

Question

For a recent test it's been requested me to generate a Smart Contract with Ethernum and use some functions provided through ABI json to extract some info.
I'm using https://ropsten.etherscan.io as suggested.
I studied Ethernum for the last 2 days and I tried with Solidity Remix to extract these info but I don't understand how to use the ABI functions with Solidity Remix.
All I have is an Address contract and an ABI contract.
Is there someone available to provide me some info?
Thanks

Answer 1

I would recommend you to programmatically do it using a library like web3js, web3js allows you to interact with the Ethereum network (accounts, smart contracts) through the RPC webservice.
In the following example, I deployed a contract called SimpleStorage on a local blockchain with Truffle and Ganache (tools and framework for ethereum).
pragma solidity ^0.4.2;
contract SimpleStorage {
uint public value;
function SimpleStorage() {
value = 1;
}
function setValue(uint val) {
value = val;
}
function getValue() returns(uint) {
return value;
}
}
Each contract deployed on the Ethereum Blockchain has a ABI (Application Binary Interface) sort of Swagger for your Smart Contract. Programs use the ABI to interact with the Smart Contract via RPC.
Each contract is deployed at a unique address like 0x3450226a2fccb0d3668e7c3a730c43ef50ec8a06
1. Initiate a nodeJS project and add web3js library
$ npm init
$ npm install web3#0.20.6 -s
2. Create a JavaScrit file index.js
Inject the dependency
const Web3 = require('web3');
Declare the rpc endpoint of your node. I'm using a local blockchain, but you could easily connect to a Ropsten public node with Infura for example (depends on which network you contract is deployed)
const RPC_ENDPOINT = "http://localhost:8545" //https://ropsten.infura.io
Connect to a Ethereum node
var web3 = new Web3(new Web3.providers.HttpProvider(RPC_ENDPOINT));
Set default account
web3.eth.defaultAccount = web3.eth.accounts[0]
Put your ABI here and the address where the smart contract is deployed
var abi = [...];
var address = "0x3450226a2fccb0d3668e7c3a730c43ef50ec8a06";
Load the contract schema from the abi
var SimpleStorageContract = web3.eth.contract(abi);
Instantiate the contract by address
var simpleStorageContractInstance = SimpleStorageContract.at(address);
Call one of the ABI function
var value = simpleStorageContractInstance.getValue.call();
console.log("value="+value);
Result:
When I invoke the function getValue of my SimpleStorage contract instance, the function returns 1.
value=1
Full code:
const Web3 = require('web3');
const RPC_ENDPOINT = "http://localhost:8545"
// Connection to a Ethereum node
var web3 = new Web3(new Web3.providers.HttpProvider(RPC_ENDPOINT));
// Set default account
web3.eth.defaultAccount = web3.eth.accounts[0]
// ABI describes a smart contract interface developped in Solidity
var abi = [
{
"constant": true,
"inputs": [],
"name": "value",
"outputs": [
{
"name": "",
"type": "uint256"
}
],
"payable": false,
"stateMutability": "view",
"type": "function"
},
{
"inputs": [],
"payable": false,
"stateMutability": "nonpayable",
"type": "constructor"
},
{
"constant": false,
"inputs": [
{
"name": "val",
"type": "uint256"
}
],
"name": "setValue",
"outputs": [],
"payable": false,
"stateMutability": "nonpayable",
"type": "function"
},
{
"constant": false,
"inputs": [],
"name": "getValue",
"outputs": [
{
"name": "",
"type": "uint256"
}
],
"payable": false,
"stateMutability": "nonpayable",
"type": "function"
}
];
// Address where the smart contract is deployed
var address = "0x3450226a2fccb0d3668e7c3a730c43ef50ec8a06";
// Load the contract schema from the abi
var SimpleStorageContract = web3.eth.contract(abi);
// Instanciate by address
var simpleStorageContractInstance = SimpleStorageContract.at(address);
// Call one of the ABI function
var value = simpleStorageContractInstance.getValue.call();
console.log("value="+value);
GitHub of the project:
https://github.com/gjeanmart/stackexchange/tree/master/51809356-create-smart-contract-and-use-abi-functions
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