Simple flashloan example for the Uniswap V4 singleton.
All pools are contained in the same contract and there is no flash fee, making it a suitable flash lender.
The abstract implementation is relatively simplified and not gas optimized for the sake of readability.
The function that must be implemented is as follows:
function _handleFlashloan(address token) internal override returns (bytes memory) {
// ...
}Where token is the ERC20 (or Ether if the address is zero) can be handled.
The returned bytes of the function has no restrictions, it may be arbitrary data, but it will be
propagated back up from the PoolManager's lock function.
The flashloan contract MUST contain at least the balance of the token at the start of the internal
function call. Taking and settling the flashloan is abstracted.
To start the execution, call initiate(address token).
This DOES NOT implement authorization guards. While the lack of a flash fee removes the griefing vulnerability of bleeding fees from unauthorized parties, if the contract holds a nonzero token or ether balance at the end of the transaction, implementors must perform authorization checks to avoid exploitation.