The concept of Meta-Classes is somewhat confusing, so we will go over a short example.
A meta-class is a class which is defined at run-time. A Contract is specified by an Application Binary Interface (ABI), which describes the methods and events it has. This description is passed the the Contract object at run-time, and it creates a new Class, adding all the methods defined in the ABI at run-time.
Deploying a Contract
Most often, any contract you will need to interact with will already be deployed, but for this example will will first deploy the contract.
const bytecode = "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";
// A Human-Readable ABI; we only need to specify relevant fragments,
// in the case of deployment this means the constructor
const abi = [
"constructor(uint totalSupply)"
];
const factory = new hethers.ContractFactory(abi, bytecode, signer);
// Deploy, setting total supply to 100 tokens (assigned to the deployer)
const contract = await factory.deploy(100, {gasLimit: 300000});
// The contract is not currentl live on the network yet, however
// its address is ready for us
contract.address
// '0x0000000000000000000000000000000001c50c95'
// Wait until the contract has been deployed before interacting
// with it; returns the receipt for the deployemnt transaction
await contract.deployTransaction.wait();
// {
// to: null,
// from: '0x0000000000000000000000000000000001c31552',
// timestamp: '1645019704.914711762',
// contractAddress: '0x0000000000000000000000000000000001c50c95',
// gasUsed: 240000,
// logsBloom: null,
// transactionId: '0.0.29562194-1645019693-973691488',
// transactionHash: '0xfa56c87dced6e18cdd1638d44a2dc121eca38fbac8d0bc35310ff741f1378078707077069447d7bee06dec1876be3e12',
// logs: [
// {
// timestamp: '1645019704.914711762',
// address: '0x0000000000000000000000000000000001c50c95',
// data: '0x0000000000000000000000000000000000000000000000000000000000000064',
// topics: [Array],
// transactionHash: '0xfa56c87dced6e18cdd1638d44a2dc121eca38fbac8d0bc35310ff741f1378078707077069447d7bee06dec1876be3e12',
// logIndex: 0,
// transactionIndex: 0
// },
// ],
// cumulativeGasUsed: 240000,
// type: 0,
// byzantium: true,
// status: 1,
// accountAddress: null,
// events: [
// {
// timestamp: '1645019704.914711762',
// address: '0x0000000000000000000000000000000001c50c95',
// data: '0x0000000000000000000000000000000000000000000000000000000000000064',
// topics: [Array],
// transactionHash: '0xfa56c87dced6e18cdd1638d44a2dc121eca38fbac8d0bc35310ff741f1378078707077069447d7bee06dec1876be3e12',
// logIndex: 0,
// transactionIndex: 0,
// removeListener: [Function (anonymous)],
// getTransaction: [Function (anonymous)],
// getTransactionReceipt: [Function (anonymous)]
// },
// ]
// }
new hethers.Contract( address , abi , providerOrSigner )
Creating a new instance of a Contract connects to an existing contract by specifying its address, and its abi (used to populate the class' methods) a providerOrSigner.
// A Human-Readable ABI; for interacting with the contract, we
// must include any fragment we wish to use
const abi = [
// Read-Only Functions
"function balanceOf(address owner) view returns (uint256)",
"function decimals() view returns (uint8)",
"function symbol() view returns (string)",
// Authenticated Functions
"function transfer(address to, uint amount) returns (bool)",
// Events
"event Transfer(address indexed from, address indexed to, uint amount)"
];
const address = "0x0000000000000000000000000000000001c50c95";
// Read-Only; By connecting to a Provider, allows:
// - Populating Unsigned Transactions for non-constant methods
// - Querying Filters
const erc20 = new hethers.Contract(address, abi, provider);
// Read-Write; By connecting to a Signer, allows:
// - Everything from Read-Only
// - Any constant function
// - Static Calling non-constant methods
// - Sending transactions for non-constant/static functions
const erc20_rw = new hethers.Contract(address, abi, signer);
Returns a new instance of the Contract attached to a new address. This is useful if there are multiple similar or identical copies of a Contract on the network and you wish to interact with each of them.
Return the number of listeners that are subscribed to event. If no event is provided, returns the total count of all events.
erc20.listeners( event ) ⇒ Array< Listener >
Return a list of listeners that are subscribed to event.
erc20.off( event , listener ) ⇒ this
Unsubscribe listener to event.
erc20.on( event , listener ) ⇒ this
Subscribe to event calling listener when the event occurs.
erc20.once( event , listener ) ⇒ this
Subscribe once to event calling listener when the event occurs.
erc20.removeAllListeners( [ event ] ) ⇒ this
Unsubscribe all listeners for event. If no event is provided, all events are unsubscribed.
Meta-Class Methods(added at Runtime)
Since the Contract is a Meta-Class, the methods available here depend on the ABI which was passed into the Contract.
erc20.decimals( [ overrides ] ) ⇒ Promise< number >
Returns the number of decimal places used by this ERC-20 token. This can be used with parseUnits when taking input from the user or formatUnits when displaying the token amounts in the UI.
Keep in mind that a signer must be connected in order to do Static calls
Transfers amount tokens to target from the current signer. The return value (a boolean) is inaccessible during a write operation using a transaction. Other techniques (such as events) are required if this value is required. On-chain contracts calling the transfer function have access to this result, which is why it is possible.
