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Last active July 6, 2021 06:43
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An illustration of sending data from Ethereum root chain to Matic child chain
// File: contracts/child/ChildToken/ChildERC20.sol
pragma solidity 0.6.6;
contract ChildERC20 is
bytes32 public constant DEPOSITOR_ROLE = keccak256("DEPOSITOR_ROLE");
string memory name_,
string memory symbol_,
uint8 decimals_,
address childChainManager
) public ERC20(name_, symbol_) {
_setupRole(DEFAULT_ADMIN_ROLE, _msgSender());
_setupRole(DEPOSITOR_ROLE, childChainManager);
_initializeEIP712(name_, ERC712_VERSION);
// This is to support Native meta transactions
// never use msg.sender directly, use _msgSender() instead
function _msgSender()
returns (address payable sender)
return ContextMixin.msgSender();
* @notice called when token is deposited on root chain
* @dev Should be callable only by ChildChainManager
* Should handle deposit by minting the required amount for user
* Make sure minting is done only by this function
* @param user user address for whom deposit is being done
* @param depositData abi encoded amount
function deposit(address user, bytes calldata depositData)
uint256 amount = abi.decode(depositData, (uint256));
// tokens minted on child chain for `user`
_mint(user, amount);
// this is not the only way of doing it, using low level call here
// this might be little overengineering for this given scenario
(bool success, bytes memory result) = address(this).call(abi.encodeWithSignature("callback(address,uint256)", user, amount));
// or as we know function to be called in this contract, we can do another thing
// given `callback` function is predefined for child chain contract
// where we can also emit some event which can be caught, offchain
callback(user, amount);
function callback(address user, uint256 amount) internal {
// doing nothing useful here, as of now
// but you can always implement custom logic here, which is to be executed
// whenever any token deposit is performed from root to child
* @notice called when user wants to withdraw tokens back to root chain
* @dev Should burn user's tokens. This transaction will be verified when exiting on root chain
* @param amount amount of tokens to withdraw
function withdraw(uint256 amount) external {
_burn(_msgSender(), amount);
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What is the use of that function _msgSender() in line 30??
Can someone please explain the working of that function??

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