xapp-starter

xapp-starter

Starter kit for cross-domain apps (xApps).

Overview

Connext’s xcall is a single interface that can be used to send assets and arbitrary calldata from one chain to another.

In general, there are three types of information that can be bridged between chains.

• Asset transfers
• Unauthenticated calls
• Authenticated calls

This starter repo contains example contracts that demonstrate how these can be construted with xcall.

At a high level, this is the call flow between contracts:

Simple Bridge (asset transfer)

The SimpleBridge transfers tokens from a user on the origin chain to a specified address on the destination domain.

Since no calldata is involved, no target contract is needed.

Note that when sending tokens, the user will first have to call approve on the ERC20 to set a spending allowance for the SimpleBridge contract.

Tokens will move from the user’s wallet => SimpleBridge => Connext => recipient.

Contracts

• SimpleBridge.sol

Use cases for this pattern

• Send funds across chains

Greeter (asset transfer + unauthenticated call)

The DestinationGreeter contract on the destination chain has an updateGreeting function that changes a stored greeting variable. The SourceGreeter contract on the origin chain uses xcall to send encoded calldata for updateGreeting.

To demonstrate a combination of an asset transfer and an arbitrary call in a single xcall, the updateGreeting function will require a payment to update the greeting. For this example, the contract will be okay with any amount greater than 0.

updateGreeting is implemented as an unauthenticated call (there are no checks to determine who is calling the function). And so, this flow is essentially the same as the simple bridge except encoded calldata is also included in the xcall.

Contracts

• SourceGreeter.sol
• DestinationGreeter.sol

Use cases for this pattern

• Deposit funds into a liquidity pool on the destination chain
• Execute a token swap on the destination chain
• Connect DEX liquidity across chains in a single seamless transaction
• Zap into a vault from any chain

Greeter Authenticated (authenticated call)

The DestinationGreeterAuthenticated contract sets some permissioning constraints. It only allows greeting to be updated from SourceGreeterAuthenticated. In order to enforce this, the contract checks that the caller is the original sender from the origin domain.

Contracts

• SourceGreeterAuthenticated.sol
• DestinationGreeterAuthenticated.sol

Use cases for this pattern

• Hold a governance vote on Origin Chain and execute the outcome of it on the Destination Chain (and other DAO operations)
• Lock-and-mint or burn-and-mint token bridging
• Critical protocol operations such as replicating/syncing global constants (e.g. PCV) across chains
• Bringing UniV3 TWAPs to every chain without introducing oracles
• Chain-agnostic veToken governance
• Metaverse-to-metaverse interoperability

Ping Pong

An example of a nested xcall, when another xcall is executed in the destination receiver contract.

Contracts

• Ping.sol
• Pong.sol

Use cases for this pattern

• Implement JS-style “callbacks” to respond asynchronously between chains

Development

Getting Started

This project uses Foundry for testing, deploying, and interacting with contracts. Fully compatible hardhat support will be added in the near future.

• See the official Foundry installation instructions.
• Also, download make if you don’t already have it.
• Get some testnet tokens! The simplest method is to go to the testnet Bridge UI and mint yourself some TEST tokens. You can also call the mint() function directly in the TEST token contract.

Blueprint

src
├─ contract-examples
|  └─ simple-bridge
│    └─ SimpleBridge.sol
|  └─ greeter
│    └─ SourceGreeter.sol
│    └─ DestinationGreeter.sol
|  └─ greeter-authenticated
│    └─ SourceGreeterAuthenticated.sol
│    └─ DestinationGreeterAuthenticated.sol
|  └─ ping-pong
│    └─ Ping.sol
│    └─ Pong.sol
|  └─ test
│    └─ ...
├─ sdk-examples
│    └─ node-examples

Setup

make install
yarn
foundryup

Set up environment variables

Copy the .env.example into .env and fill in all the placeholders under the GENERAL section. Initial values are provided for Goerli as origin and Optimism-Goerli as destination.

Testing

There are some starter test cases in the src/tests directory for each of the examples.

Unit Tests

make test-unit-simple-bridge
make test-unit-destination-greeter
make test-unit-destination-greeter-auth
make test-unit-ping
make test-unit-pong

Integration Tests

This uses forge’s --forked mode. Make sure you have GOERLI_RPC_URL defined in your .env file as these tests currently fork Goerli.

make test-forked-simple-bridge
make test-forked-source-greeter
make test-forked-source-greeter-auth

Deployment

Deploy contracts in this repository using the RPC provider of your choice (make sure all the variables under GENERAL are set in .env).

Deployment order for Simple Bridge

```bash
make deploy-simple-bridge
```

Deployment order for SourceGreeter + DestinationGreeter

```bash
make deploy-source-greeter
```

```bash
make deploy-destination-greeter
```

Deployment order for SourceGreeterAuthenticated + DestinationGreeterAuthenticated

```h
make deploy-source-greeter-auth
```

Use the origin domain and address of `SourceGreeterAuthenticated` contract address as values for `ORIGIN_DOMAIN` and `SOURCE_CONTRACT` in `.env` before deploying `DestinationGreeterAuthenticated`.

```bash
make deploy-destination-greeter-auth
```

Deployment order for Ping + Pong

```bash
make deploy-ping
```

```bash
make deploy-pong
```

Verification

It’s much easier to read contract values after they’re verified! We use another forge command to do this.

For example, to verify DestinationGreeter.sol:

forge verify-contract --chain 1735356532 <CONTRACT_ADDRESS> src/contract-examples/greeter/DestinationGreeter.sol:DestinationGreeter <ETHERSCAN_KEY>

Live Testnet Testing

The core set of Connext contracts have already been deployed to testnet. For the most up-to-date contracts, please reference the Connext deployments.

• Execute transfer on SimpleBridge

  After deploying SimpleBridge, set the SIMPLE_BRIDGE and RECIPIENT variables in .env and run:

  With Forge:

  make transfer
  

  With Hardhat:

  yarn hardhat transfer
  

• Execute updateGreeting on SourceGreeter

  After deploying SourceGreeter and DestinationGreeter, set the SOURCE_GREETER, DESTINATION_GREETER, DESTINATION_TOKEN, and NEW_GREETING variables in .env and run:

  With Forge:

  make update-greeting
  

  With Hardhat:

  yarn hardhat update-greeting
  

• Execute updateGreeting on SourceGreeterAuthenticated

  After deploying SourceGreeterAuthenticated and DestinationGreeterAuthenticated, set the SOURCE_GREETER_AUTHENTICATED and DESTINATION_GREETER_AUTHENTICATED variables in .env and run:

  With Forge:

  make update-greeting-auth
  

  With Hardhat:

  yarn hardhat update-greeting-auth
  

• Execute startPingPong on Ping

  After deploying Ping and Pong, set the PING and PONG variables in .env and run:

  With Forge:

  make start-ping-pong
  

  With Hardhat:

  yarn hardhat start-ping-pong
  

Check Execution Results

You can just check your wallet balance in the Simple Bridge example to see if the funds arrived at the destination address. To check calldata results, you can read the updated variables on the target contract on Etherscan or use tools like Foundry’s cast command.

SDK Examples

There is a simple NodeJS example of using the SDK in /src/sdk-examples/. This example demonstrates how to configure the SDK, construct the various params (like estimating relayer fee), and call xcall.

The script fires off a cross-chain transfer that sends funds from your wallet on the source domain to the same address on the destination domain.

For a more detailed step-by-step, check out the SDK Guide.

Getting Started

• Make sure dependencies are installed.

  yarn install
  

• Get some testnet tokens! The simplest method is to go to the testnet Bridge UI and mint yourself some TEST tokens. You can also call the mint() function directly in the TEST token contract.

• Make sure you set your private key in .env

  PRIVATE_KEY = <PRIVATE_KEY>
  

  (Optionanal) The example uses sane defaults for a Goerli -> Optimism-Goerli transfer but feel free to change these (especially RPCs as they currently use public defaults):

  ORIGIN_RPC_URL
  GOERLI_RPC_URL
  OPTIMISM_GOERLI_RPC_URL
  ORIGIN_TOKEN
  AMOUNT
  SLIPPAGE
  

Run the Example

yarn xtransfer