--- title: Smart contracts events _Follow along with this video:_ --- ### Prerequisites for picking a winner Going back to [lesson 1](https://updraft.cyfrin.io/courses/foundry/smart-contract-lottery/setup), we established that one of the Raffle contract goals is `...we should be able to automatically pick a winner out of the registered users.` What do we need to do that? 1. A random number 2. Use the random number to pick a winning player 3. Call `pickWinner` automatically For now, let's focus on points 1 and 2. But before diving straight into the randomness let's think a bit about the Raffle design. We don't have any problem with anyone calling `pickWinner`. As long as someone wants to pay the gas associated with that they are more than welcome to do it. But we need to make sure that a decent amount of time passed since the start of the raffle. We don't want to host a 10-second raffle where two people get to register and then someone calls the `pickWinner`. In that sense, we need to define a new state variable called `i_interval` which represents the duration of a raffle: ```solidity contract Raffle { error Raffle__NotEnoughEthSent(); uint256 private immutable i_entranceFee; // @dev Duration of the lottery in seconds uint256 private immutable i_interval; address payable[] private s_players; event EnteredRaffle(address indexed player); constructor(uint256 entranceFee, uint256 interval) { i_entranceFee = entranceFee; i_interval = interval; } } ``` Now that we have defined a raffle duration, we need to check it in `pickWinner`, but check it against what? We need to check it against the difference between the moment in time when the raffle started and the moment in time when the function `pickWinner` is called. But for that, we need to record the raffle starting time. Perform the following update: ```solidity contract Raffle{ error Raffle__NotEnoughEthSent(); uint256 private immutable i_entranceFee; // @dev Duration of the lottery in seconds uint256 private immutable i_interval; address payable[] private s_players; uint256 private s_lastTimeStamp; event EnteredRaffle(address indexed player); constructor(uint256 entranceFee, uint256 interval) { i_entranceFee = entranceFee; i_interval = interval; s_lastTimeStamp = block.timestmap; } } ``` And now we have all the prerequisites to perform the check: ```solidity // 1. Get a random number // 2. Use the random number to pick a player // 3. Automatically called function pickWinner() external { // check to see if enough time has passed if (block.timestamp - s_lastTimeStamp < interval) revert(); } ``` Don't worry! We will create a custom error for that in the next lesson. But before that let's talk randomness.