DApps are decentralized applications that utilize blockchains to store a history of operations (e.g. transactions). They can also consist of functional code, called smart contracts, that are published to the blockchain. DApps interfaces can be graphical, console-based, or APIs.
Popular categories for DApps include finance, exchanges, and gambling, but social applications and games also exist.
One of the primary use cases for a DApp is sending cryptocurrency between two accounts. For example, when Alice wishes to send some MTR and MTRG to Bob's account. Application developers can create methods for this on the Meter blockchain using meterify, an extended version of Web3 (AKA the Ethereum JavaScript API library).
The final project files can be found here:
Prerequisites
Meterify requires that Node.js version 10.15.1 or above is , as well as npm.
$ node --version
v10.16.3
$ npm --version
6.9.0
Example DApp
Step 1 - Project Setup
Create a directory (e.g. meter-dapp) and initialize a new project using npm. Then install the app prerequisites meterify and web3, plus the JavaScript bindings for the Solidity compiler (solc).
If the code runs without any errors the connection was successful.
Step 3 - Creating an Account-Generating Function
meterify.eth.accounts.create()
The meterify.eth.accounts package contains functions for generating accounts and signing transactions and data. Create an object, using Alice's and Bob's names as keys. Perform a loop on the object, calling the create function. This will generate new local accounts that contain both a private key and a public key. Add each account to the object as is created.
var accounts = createAccounts({"alice":{},"bob":{}});
function createAccounts(accounts){
for(key in accounts){
accounts[key] = meterify.eth.accounts.create();
}
return accounts;
}
Step 4 - Add Accounts to a Wallet.
meterify.eth.accounts.wallet;
meterify.eth.accounts also contains an in memory wallet to store multiple accounts. Loop through the object returned by the createAccounts function, and add Alice's and Bob's accounts to the wallet, using each account's private key.
function addAccountsToWallet(accounts){
for(var key in accounts){
meterify.eth.accounts.wallet.add(accounts[key].privateKey);
}
}
Step 5 - Send MTR to an Account
Use the eth package's sendTransaction method to create a function for sending some MTR and MTRG from Alice to Bob.
/* Note: Alice's account should already contain some MTR and MTRG
* for this example code to work.
*/
sendCrypto(accounts.alice.address, accounts.bob.address, '0000000000', '1000000000000000000');
sendCrypto(accounts.alice.address, accounts.bob.address, '0000000001', '1000000000000000000');
function sendCrypto(fromAddress, toAddress, code, amount){
meterify.eth.sendTransaction(
{
from: fromAddress,
to: toAddress,
value: amount,
data: code
}
).then(
receipt => {}
).then(
data => {
console.log("MTR sent: "+JSON.stringify(data));
next();
}
).catch(function(error){
console.log("Error: "+error);
});
}
Step 6 - Run the application.
$ node index.js
Load, Deploy, and Test a Smart Contract
The following modifications to the example demonstrate the use of a sample smart contract on the Meter blockchain. Again, follow the steps to add code snippets to the existing index.js file.
Step 1 - Load a Smart Contract.
function loadContract(file){
console.log("Loading contract: "+file);
const contractFile = fs.readFileSync(file).toString();
const solc = require('solc');
const compiledCode = solc.compile(contractFile);
var data = {};
data.token_abiDefinition = JSON.parse(compiledCode.contracts[':SAMPLEToken'].interface)
let token_byteCode = compiledCode.contracts[':SAMPLEToken'].bytecode
data.token_byteCode = "0x" + token_byteCode;
console.log("Contract Loaded.");
return data;
}
Units in meterify are Wei, where 1 MTR = 10e18 Wei. Note that the identifier code for MTR is 0000000000, while MTRG is 0000000001. sendTransaction returns a that is considered resolved once the receipt becomes available.
