scaffold-eth 挑战：实现锁定机制和资金撤回（Part2）

    // SPDX-License-Identifier: MIT      pragma solidity ^0.8.4;      import "hardhat/console.sol";      import "./ExampleExternalContract.sol";      /**     * @title Stacker Contract      * @author scaffold-eth      * @notice A contract that allow users to stack ETH      */      contract Staker {        // External contract that will old stacked funds        ExampleExternalContract public exampleExternalContract;        // 用户质押余额        mapping(address => uint256) public balances;        // Staking threshold        uint256 public constant threshold = 1 ether;        // Staking deadline        uint256 public deadline = block.timestamp   30 seconds;        // 合约事件        event Stake(address indexed sender, uint256 amount);        // Contract's Modifiers        /**       * @notice Modifier that require the deadline to be reached or not        * @param requireReached Check if the deadline has reached or not        */        modifier deadlineReached( bool requireReached ) {          uint256 timeRemaining = timeLeft();          if( requireReached ) {            require(timeRemaining == 0, "Deadline is not reached yet");          } else {            require(timeRemaining > 0, "Deadline is already reached");          }         _;        }        /**       * @notice Modifier that require the external contract to not be completed        */        modifier stakeNotCompleted() {          bool completed = exampleExternalContract.completed();          require(!completed, "staking process already completed");         _;        }        /**       * @notice Contract Constructor        * @param exampleExternalContractAddress Address of the external contract that will hold stacked funds        */        constructor(address exampleExternalContractAddress) {          exampleExternalContract = ExampleExternalContract(exampleExternalContractAddress);        }        function execute() public stakeNotCompleted deadlineReached(false) {          uint256 contractBalance = address(this).balance;          // check the contract has enough ETH to reach the treshold          require(contractBalance >= threshold, "Threshold not reached");          // Execute the external contract, transfer all the balance to the contract          // (bool sent, bytes memory data) = exampleExternalContract.complete{value: contractBalance}();          (bool sent,) = address(exampleExternalContract).call{value: contractBalance}(abi.encodeWithSignature("complete()"));          require(sent, "exampleExternalContract.complete failed");        }        /**       * @notice Stake method that update the user's balance        */        function stake() public payable deadlineReached(false) stakeNotCompleted {          // update the user's balance          balances[msg.sender]  = msg.value;          // emit the event to notify the blockchain that we have correctly Staked some fund for the user          emit Stake(msg.sender, msg.value);        }        /**       * @notice Allow users to withdraw their balance from the contract only if deadline is reached but the stake is not completed        */        function withdraw() public deadlineReached(true) stakeNotCompleted {          uint256 userBalance = balances[msg.sender];          // check if the user has balance to withdraw          require(userBalance > 0, "You don't have balance to withdraw");          // reset the balance of the user          balances[msg.sender] = 0;          // Transfer balance back to the user          (bool sent,) = msg.sender.call{value: userBalance}("");          require(sent, "Failed to send user balance back to the user");        }        /**       * @notice The number of seconds remaining until the deadline is reached        */        function timeLeft() public view returns (uint256 timeleft) {          if( block.timestamp >= deadline ) {            return 0;          } else {            return deadline - block.timestamp;          }        }      }