Call a contract on chain B from chain A

To call a contract on chain B from chain A, the user needs to call callContract on the gateway of chain A, specifying:

• The destination chain, which must be an EVM chain from Chain names.
• The destination contract address, which must inherit from QPExecutable defined in QPExecutable.sol.
• The payload bytes to pass to the destination contract.

If you want to try this out directly on the blockchain, you can use Remix to try out Quantum Portal’s GMP.

function callContract(
    string memory destinationChain,
    string memory contractAddress,
    bytes memory payload
) external;

QPExecutable has an execute function that will be triggered by the Quantum Portal network on the destination chain after the callContract function has been executed on the source chain. This will validate the contract call and then invoke your _execute method, where you should write any custom logic.

function _execute(
    string memory sourceChain,
    string memory sourceAddress,
    bytes calldata payload
) internal virtual {}

The payload passed to callContract (and ultimately to _execute and _executeWithToken) has type bytes. Use the ABI encoder/decoder convert your data to bytes.

Example of payload encoding in JavaScript (using ethers.js):

const { ethers } = require("ethers");

// encoding a string
const payload = ethers.utils.defaultAbiCoder.encode(
  ["string"],
  ["Hello from contract A"]
);

Example of payload decoding in Solidity:

function _execute(
    string memory sourceChain,
    string memory sourceAddress,
    bytes calldata payload
) internal override {
    // decoding a string
    string memory _message = abi.decode(payload, (string));
}

Learn More: For a detailed tutorial on building a full-stack interchain decentralized application with Next.js, Solidity, and Quantum Portal’s General Message Passing callContract to send messages between blockchains, check out our comprehensive guide: [Build a Full-Stack Interchain Application With Next.js, Solidity & Quantum Portal](https://QP.network/blog/how-to-build-interchain-dapp-with-next.js-solidity-and-Quantum Portal).