transferFrom()

• Moves amount tokens from sender to recipient using the allowance mechanism. amount is then deducted from the caller’s allowance.

• Returns a boolean value indicating whether the operation succeeded.

• Emits a Transfer event.

If A has an allowance granted by B with yourToken.sol, A has the liberty to transfer the tokens to destination address of choice. A is not limited to transferring its allowance only to its' own address.

• ---

• userA calls transferFrom() on service contract

• spender = caller of _msgSender() -> spender = service contract

• _spendAllowance(from, servicecontract, amt)

  • servicecontract's allowance is decremented

• _transfer(from, to, amt).

---

• userA calls sellTokens() - function of Service contract

  • within the scope of sellTokens(), msg.sender = userA

• msg.sender = userA, gets passed as parameter into .transferFrom()

• inside transferFrom(), Service contract calls _msgSender() - function of YourToken.sol

  • within internal scope, Service contract calls Token contract

  • msgSender() = Service contract

  • spender = Service Contract