_Follow along with this video:_ --- ### Testing and refactoring the performUpkeep Let's give some love to `performUpkeep`, starting with some tests. Starting light, open the `RaffleTest.t.sol` and paste the following: ```solidity function testPerformUpkeepCanOnlyRunIfCheckUpkeepIsTrue() public { // Arrange vm.prank(PLAYER); raffle.enterRaffle{value: entranceFee}(); vm.warp(block.timestamp + interval + 1); vm.roll(block.number + 1); // Act / Assert // It doesnt revert raffle.performUpkeep(""); } ``` We prank the `PLAYER` address, then use it to call `enterRaffle` with the correct `entranceFee`. We use the `warp` and `roll` to set `block.timestamp` into the future. Lastly, we call `performUpkeep`. As you've figured out, we are not running any asserts here. But that is ok because if `performUpkeep` had a reason to fail, then it would have reverted and our `forge test` would have caught it. Run the test using: `forge test --mt testPerformUpkeepCanOnlyRunIfCheckUpkeepIsTrue` It passes, amazing! Keep going! Let's test if `performUpkeep` reverts in case `checkUpkeep` is false: ```solidity function testPerformUpkeepRevertsIfCheckUpkeepIsFalse() public { // Arrange uint256 currentBalance = 0; uint256 numPlayers = 0; Raffle.RaffleState rState = raffle.getRaffleState(); // Act / Assert vm.expectRevert( abi.encodeWithSelector( Raffle.Raffle__UpkeepNotNeeded.selector, currentBalance, numPlayers, rState ) ); raffle.performUpkeep(""); } ``` This can be understood easier if we start from the end. We want to call `performUpkeep` and we expect it to revert. For that, we use the `vm.expectRevert` to indicate that we expect the next call to revert. If we access [this link](https://book.getfoundry.sh/cheatcodes/expect-revert) we can see that in case we use a custom error with parameters we can specify them as follows: ```solidity vm.expectRevert( abi.encodeWithSelector(CustomError.selector, 1, 2) ); ``` In our case the custom error has 3 parameters: ```solidity error Raffle__UpkeepNotNeeded( uint256 currentBalance, uint256 numPlayers, uint256 raffleState ); ``` First parameter: `Raffle.Raffle__UpkeepNotNeeded.selector`; Second parameter: `currentBalance`; Third parameter: `numPlayers`; Fourth parameter: `raffleState`; Out of all of them, the only one available is the first. We define a `currentBalance` and `numPlayers` and assign them both 0. To get the `raffleState` we can use the `getRaffleState` view function. Run the test using: `forge test --mt testPerformUpkeepRevertsIfCheckUpkeepIsFalse` Everything passes, great! I know some concepts haven't been explained. I'm referring to `encodeWithSelector` and the general concept of function selectors. These will be introduced in the next sections. Great work! Now let's further explore events.