Example #1 for "How to make a user friendly Ethereum dApp"

TransactionDelegate.sol

// Transaction Delegation is a generic way to enable transactions to be signed by one ethereum account
// and be sent by a different account. Enabling improved UX patterns where users of an application
// do not need to pay for transaction gas and do not necessarily need to have any ether at all to
// interact with an application.
//
// To delegate a transaction, a user A creates a transaction and signs the hash keccak256(calldata, nonce).
// The calldata and signature v, r, s is then communicated to user B who will submit the transaction on 
// their behalf. User B submits the transaction by calling `TransactionDelegate.delegatedTxn`.
//
// This contract requires elevated privledges to the target contract allowing it to submit transactions
// on anothers behalf
//
// The pattern is inspired by https://gist.github.com/backus/55b874940fe46f1e9847ff1b763d0424
//
// THIS HAS NOT BEEN TESTED AT ALL


// https://github.com/aragon/aragonOS/blob/dev/contracts/common/DelegateProxy.sol
import "@aragon/os/contracts/common/DelegateProxy.sol";

contract TransactionDelegate is DelegateProxy {

  address public target;
  // track nonces to prevent replay and ensure txn ordering
  mapping (address => uint64) nonces;

  constructor(address _target) public {
    target = _target;
  }

  function delegatedTxn(bytes calldata, uint8 nonce, uint8 v, bytes32 r, bytes32 s) public {
    // Recreate the hash the user signed
    bytes32 digest = keccak256(calldata, nonce);
    
    address signer = ecrecovery(digest, v, r, s);

    // First 20 bytes of calldata should always be address transaction is being sent on behalf of
    // functions should follow pattern like: doActionFor(address _for, ...)
    address sender = calldata[:20]; // pseudo code

    require(sender == signer);
    require(nonce == nonces[sender]);
    
    nonces[sender] = nonces[sender] + 1;
    
    delegatedFwd(target, calldata);
  }
}