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Overview

The Incrementor contract demonstrates cross-chain messaging by incrementing a counter on a destination chain from a source chain. It showcases the BridgeAppBase pattern for building cross-chain applications. Contract Location: contracts/examples/Incrementor.sol

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How It Works

1

Deploy on Both Chains

Deploy the Incrementor contract on both source and destination chains.
2

Configure Cross-Chain Addresses

Tell each contract about its counterpart on the other chain.
3

Send Message

Call send() on the source chain to trigger an increment on the destination chain.
4

Message Bridged

The message is validated by oracles and executed on the destination chain.
5

Counter Incremented

The claimedTimes counter is incremented on the destination chain.

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Contract Code

Incrementor.sol
contract Incrementor is BridgeAppBase {
    uint256 public claimedTimes;
    
    function send(bytes memory _data) external payable {
        // Send message to destination chain
        bytes32 submissionId = debridgeGate.sendMessage{value: msg.value}(
            chainIdTo,
            abi.encodePacked(addressTo),
            abi.encodeWithSelector(this.onBridgedMessage.selector, _data)
        );
        
        emit Sent(submissionId, msg.sender);
    }
    
    function onBridgedMessage(bytes calldata _data) 
        external 
        onlyControllingAddress 
        whenNotPaused 
        returns (bool) 
    {
        claimedTimes++;
        emit Claimed(_data);
        return true;
    }
}

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Deployment Steps

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1. Deploy Contracts

Deploy on Testnet
# Deploy on BSC Testnet
yarn hardhat run --network bsctest examples/src/incrementorScripts/deploy.ts

# Deploy on Kovan (or another testnet)
yarn hardhat run --network kovan examples/src/incrementorScripts/deploy.ts
Save the proxy addresses printed in the console.

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2. Configure Addresses

Update examples/src/incrementorScripts/constants.ts with deployed addresses:
constants.ts
export const INCREMENTOR_ADDRESS_ON_BSC_TEST = "0x...";
export const INCREMENTOR_ADDRESS_ON_KOVAN = "0x...";
Link Contracts
# Tell BSC contract about Kovan contract
yarn hardhat run --network bsctest examples/src/incrementorScripts/setContractAddressOnChainId.ts

# Tell Kovan contract about BSC contract
yarn hardhat run --network kovan examples/src/incrementorScripts/addControllingAddress.ts

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Sending a Message

Send Cross-Chain Message
yarn hardhat run --network bsctest examples/src/incrementorScripts/send.ts
This will:
  1. Call send() on BSC testnet
  2. Emit a Sent event with submissionId
  3. Oracles observe and sign the message
  4. Message is executed on Kovan
  5. claimedTimes is incremented

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Tracking the Transaction

Use the deBridge explorer to track your cross-chain transaction: Enter the transaction hash or submissionId to see the status.

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Verifying the Result

Check Counter Value
yarn hardhat run --network kovan examples/src/incrementorScripts/getClaimedTimesOnReceivingChain.ts
You should see the claimedTimes value has increased.

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Key Concepts

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BridgeAppBase Pattern

The Incrementor extends BridgeAppBase, which provides:
  • Access Control: Only authorized cross-chain addresses can call functions
  • Chain Mapping: Track which addresses on which chains are allowed
  • Pausability: Emergency pause functionality

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onlyControllingAddress Modifier

Security Pattern
modifier onlyControllingAddress() {
    // Verify caller is CallProxy
    require(msg.sender == address(callProxy), "Only CallProxy");
    
    // Get cross-chain context
    uint256 chainIdFrom = callProxy.submissionChainIdFrom();
    bytes memory sender = callProxy.submissionNativeSender();
    
    // Verify sender is authorized
    require(isControllingAddress(chainIdFrom, sender), "Unauthorized");
    _;
}
This ensures only the authorized contract on the source chain can trigger functions.

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Using the Live Example

There’s a deployed Incrementor contract on BSC testnet: You can call the send() function directly from BSCscan to see cross-chain messaging in action.
Include an execution fee in msg.value so keepers automatically claim your transaction on the destination chain.

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Common Issues

Possible causes:
  • Insufficient execution fee
  • Contracts not properly linked
  • Destination chain not supported
Solution:
  • Check submissionId in explorer
  • Verify contract configuration with getContractAddressOnChainId()
  • Manually claim if execution fee was 0
Possible causes:
  • onBridgedMessage not public/external
  • Missing onlyControllingAddress modifier
  • Insufficient gas limit
Solution:
  • Review contract implementation
  • Check function visibility
  • Increase execution fee for more gas

Sending ETH

Transfer native tokens cross-chain

Cross-Chain Calls

Advanced cross-chain patterns

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