My first smart contract, done following a tutorial

escrow.sol

pragma solidity ^0.5.0;

// MIT License
// 
// Copyright (c) 2019 Érico Vieira Porto 
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

// Done following the live coding from BlockGeeks here
// https://www.youtube.com/watch?v=6RCtNrlzV28
// Video tutorial by Niloo Ravaei

contract Escrow {
    
    // using this and inState we provide basic state machine functionality
    enum State {
        eUninitiated, // initial state for the contract, before agreement
        eAwaitingPayment, // after both buyer and seller deposit money in the contract
        eAwaitingDelivery, // the buyer has paid
        eComplete // the buyer has received
    }
    State currentState;
    
    // the people involved on the contract transaction
    address payable buyer;
    address payable seller;

    // the price defined at contract initialization
    uint price;
    
    bool buyer_in;
    bool seller_in;
    
    // costless functions to view the variables from the contract
    function getBuyer() public view returns (address) { return buyer; }
    function getSeller() public view returns (address) { return seller; }
    function getPrice() public view returns (uint) { return price; }
    function getCurrentState() public view returns (State) {return currentState; }
    function getBuyerIn() public view returns (bool) { return buyer_in; }
    function getSellerIn() public view returns (bool) { return seller_in; }
      
    modifier inState(State expectedState) {
        // functions modified by this has to be at this state  
        require(currentState == expectedState); _;
    }
    
    modifier correctPrice() {
        // the correct price is defined when the contract is initialized
        require(msg.value == price); _;
    }
    
    modifier buyerOnly() {
        require(msg.sender == buyer); _;
    }
    
    constructor (address payable _buyer, address payable _seller, uint _price) public{
        // starts the contract, the state will be at eUnitiated at creation
        buyer = _buyer;
        seller = _seller;
        price = _price;
    }
    
    //this method expects ether
    //we want both buyer and seller to deposit some amount of money
    //for better incentives, it's the price
    function initiateContract() correctPrice inState(State.eUninitiated) payable public {
        if(msg.sender == buyer && buyer_in == false) {
            buyer_in = true;
        }
        if(msg.sender == seller && seller_in == false) {
            seller_in = true;
        }
        if (buyer_in && seller_in){
            currentState = State.eAwaitingPayment;
        }
    }
    
    // once buyer invokes this and pays the correct price, we can move to the next state
    function confirmPayment() buyerOnly correctPrice inState(State.eAwaitingPayment) payable public {
        currentState = State.eAwaitingDelivery;
    }
    
    // buyer has received the product and confirms delivery, we can return the escrow values.
    function confirmDelivery() buyerOnly inState(State.eAwaitingDelivery) public {
        seller.transfer(price * 2);
        buyer.transfer(price);
        
        currentState = State.eComplete;
    }
}