How can I build a custom FX tunnel? Is there a step-by-step guide available to help with the process?

The contracts in the following example consist of two main parts, one for the root chain (Goerli in this case), and one for the child chain (Mumbai). The Goerli contract is responsible for mapping tokens from the root chain to

the child chain and sending deposits to the child chain. The Mumbai contract is responsible for handling incoming messages from the Goerli contract, minting child tokens, and processing child-to-root withdrawals.

Here is an overview of the following contracts used:

• SafeToken (Goerli): This contract deploys a simple ERC20 token that can be used to test the FxPortal functionality.

• FxERC20 (Goerli): This contract template will create new child tokens on the Mumbai network. The contract is initialized with the address of the FxPortal contract and the address of the corresponding token on the root network. It contains a mint function that the root network contract to mint new child tokens can use.

• FxERC20 (Mumbai): This contract is a child token minted by the root network contract. It is initialized with the address of the FxPortal contract, the address of the corresponding token on the root network, and the token’s name, symbol, and decimals. It contains a burn function that can be used to burn child tokens.

• FxERC20RootTunnel (Goerli): This contract is responsible for mapping tokens from the root network to the child network and sending deposits to the child network. When a new token is mapped, the contract deploys a new child token contract using the FxERC20 template and then maps the root token to the child token. The contract sends a message to the child network containing the deposit details when a deposit is made.

• FxERC20ChildTunnel (Mumbai): This contract is responsible for processing messages from the root network and minting new child tokens, as well as handling child-to-root withdrawals. When a new token is mapped, the contract deploys a new child token contract using the FxERC20 template and then maps the root token to the child token. When a deposit message is received, the contract mints new child tokens and calls the onTokenTransfer function on the receiver (if it is a contract). When a withdrawal is made, the contract burns the child tokens and sends a message to the root network containing the withdrawal details.

Prerequisites:

• Goerli and Mumbai testnet accounts with some testnet ETH and MATIC tokens, respectively

• Metamask wallet

Steps:

1. Deploy the SafeToken contract on the Goerli testnet.

2. Deploy an FxERC20 contract on the Goerli network.

3. Deploy an FxERC20Child contract on the Mumbai network.

4. Deploy an FxERC20RootTunnel contract on the Goerli network.

   • Constructor arguments:

     • _checkpointManager : 0x2890bA17EfE978480615e330ecB65333b880928e

     • _fxRoot : 0x3d1d3E34f7fB6D26245E6640E1c50710eFFf15bA

     • _fxERC20Token : FxERC20 address (address of FxERC20 template created in step 2) 

5. Deploy an FxERC20ChildTunnel contract on the Mumbai network.

   • Constructor arguments:

     • _fxChild : 0xCf73231F28B7331BBe3124B907840A94851f9f11

     • _tokenTemplate : _fxERC20Token : FxERC20 address (address of FxERC20 template created in step 3) 

6. On the FxERC20ChildTunnel contract (step 5), set the setFxRootTunnel function. This should be done with the FxERC20RootTunnel contract address from step 4.

7. On the FxERC20RootTunnel contract (step 4), set the setFxChildTunnel function. This should be done with the FxERC20ChildTunnel contract address from step 5.

8. On the SafeToken contract from step 1, call the mint function to mint the desired amount of tokens.

9. On the SafeToken contract, call the approve function with the FxERC20RootTunnel contract address from step 4 and the mint amount from step 8.

10. On the FxERC20RootTunnel contract from step 4, call the deposit function with the SafeToken on the Goerli network from step 1, the user address, the mint amount from step 9, and the data parameter 0x00.

 Additional steps:

11. On the FxERC20ChildTunnel contract from step 5, call the balanceOf function to check that the child tokens have been minted.

12. To withdraw the child tokens back to the Goerli network, call the withdraw function on the FxERC20ChildTunnel contract with the amount of child tokens to be burned.

13. Wait for the transaction to be confirmed and the withdrawal to be processed. You can check the status of the withdrawal using the FxExplorer.

14. On the SafeToken contract from step 1, call the balanceOf function to check that the corresponding amount of tokens has been deposited on the Goerli network.

Congratulations, you have successfully tested the FxPortal functionality for transferring tokens between the Goerli and Mumbai networks!