Payable Functions

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract PiggyBank {
    address public owner;
    mapping(address => uint256) public deposits;

    event Deposited(address indexed user, uint256 amount);
    event Withdrawn(address indexed owner, uint256 amount);

    constructor() { owner = msg.sender; }

    // payable — accepts ETH
    function deposit() public payable {
        require(msg.value > 0, "Send some ETH");
        deposits[msg.sender] += msg.value;
        emit Deposited(msg.sender, msg.value);
    }

    function withdraw() public {
        require(msg.sender == owner, "Not owner");
        uint256 amount = address(this).balance;
        (bool ok, ) = owner.call{value: amount}("");
        require(ok, "Transfer failed");
        emit Withdrawn(owner, amount);
    }
}

contract FallbackDemo {
    event Received(address sender, uint256 amount);
    event FallbackCalled(address sender, bytes data);

    // Called when ETH is sent with no data
    receive() external payable {
        emit Received(msg.sender, msg.value);
    }

    // Called when no function matches, or as backup
    fallback() external payable {
        emit FallbackCalled(msg.sender, msg.data);
    }

    function getBalance() public view returns (uint256) {
        return address(this).balance;
    }
}

contract FundedAtDeploy {
    address public owner;

    // Accepts ETH at deployment
    constructor() payable {
        owner = msg.sender;
        require(msg.value >= 0.1 ether, "Need at least 0.1 ETH");
    }

    function getBalance() public view returns (uint256) {
        return address(this).balance;
    }
}