Getting Started with Contract Standards

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.20;

// @dev Import the 'MessagingFee' and 'MessagingReceipt' so it's exposed to OApp implementers
import { OAppSender, MessagingFee, MessagingReceipt } from "./OAppSender.sol";
// @dev Import the 'Origin' so it's exposed to OApp implementers
import { OAppReceiver, Origin } from "./OAppReceiver.sol";
import { OAppCore } from "./OAppCore.sol";

/**
 * @title OApp
 * @dev Abstract contract serving as the base for OApp implementation, combining OAppSender and OAppReceiver functionality.
 */
// highlight-next-line
abstract contract OApp is OAppSender, OAppReceiver {}

// LayerZero/V2/oapp/contracts/oapp/OAppReceiver.sol
// @dev must-have configurations for standard OApps
function setPeer(uint32 _eid, bytes32 _peer) public virtual onlyOwner {
    peers[_eid] = _peer; // Array of peer addresses by destination.
    emit PeerSet(_eid, _peer); // Event emitted each time a peer is set.
}

// LayerZero/V2/oapp/contracts/oapp/OAppCore.sol

/**
 * @dev Entry point for receiving messages or packets from the endpoint.
 * @param _origin The origin information containing the source endpoint and sender address.
 *  - srcEid: The source chain endpoint ID.
 *  - sender: The sender address on the src chain.
 *  - nonce: The nonce of the message.
 * @param _guid The unique identifier for the received LayerZero message.
 * @param _message The payload of the received message.
 * @param _executor The address of the executor for the received message.
 * @param _extraData Additional arbitrary data provided by the corresponding executor.
 *
 * @dev Entry point for receiving msg/packet from the LayerZero endpoint.
 */
function lzReceive(
    Origin calldata _origin,
    bytes32 _guid,
    bytes calldata _message,
    address _executor,
    bytes calldata _extraData
) public payable virtual {
    // Ensures that only the endpoint can attempt to lzReceive() messages to this OApp.
    if (address(endpoint) != msg.sender) revert OnlyEndpoint(msg.sender);

    // Ensure that the sender matches the expected peer for the source endpoint.
    if (_getPeerOrRevert(_origin.srcEid) != _origin.sender) revert OnlyPeer(_origin.srcEid, _origin.sender);

    // Call the internal OApp implementation of lzReceive.
    _lzReceive(_origin, _guid, _message, _executor, _extraData);
}