Create ounter.sol

ounter.sol

@@ -0,0 +1,44 @@
+
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import {IGmpReceiver} from "contracts/IGmpReceiver.sol";
+
+contract Counter is IGmpReceiver {
+    address private immutable _gateway;
+    uint256 public number;
+
+    constructor(address gateway) {
+        _gateway = gateway;
+    }
+
+    function onGmpReceived(bytes32, uint128, bytes32, bytes calldata) external payable returns (bytes32) {
+        require(msg.sender == _gateway, "unauthorized");
+        number++;
+        return bytes32(number);
+    }
+}
+
+
+
+IGmpReceiver.sol
+
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+interface IGmpReceiver {
+    /**
+     * @dev Handles the receipt of a single GMP message.
+     * The contract must verify the msg.sender, it must be the Gateway Contract address.
+     *
+     * @param id The EIP-712 hash of the message payload, used as GMP unique identifier
+     * @param network The chain_id of the source chain that send the message
+     * @param source The pubkey/address which sent the GMP message
+     * @param payload The message payload with no specified format
+     * @return 32-byte result, which will be stored together with the GMP message
+     */
+    function onGmpReceived(bytes32 id, uint128 network, bytes32 source, bytes calldata payload)
+        external
+        payable
+        returns (bytes32);
+}