pragma solidity ^0.4.10;

contract Safebuy{

    uint public price;

    address public seller;

    address public buyer;

    constructor() public payable{

        seller = msg.sender;

        //uint value = msg.value / 2 ;

        //require(value *2 ==msg.value);

        price = msg.value;

    }

    enum State{Create,Block,Release}

    State public state;

    //根据函数的执行,更改条件状态--对函数方法的执行规定一定的顺序

    modifier StateStatus(State _state){

        require(state == _state);

        _;

    }

    modifier OnlyBuyer(){

        require(buyer == msg.sender);

        _;

    }

    modifier OnlySeller(){

        require(seller == msg.sender);

        _;

    }

    event Abort();

    event Deal();

    event EndDeal();

    function getBalance() view public  returns(uint){

        return address(this).balance;

        //向该合约内共放入的以太币

        //刚部署合约时:address(this).balance = msg.value

        //买家交易达成时:address(this).balance = msg.value *2

    }

    //state = State.Release 其他函数都无法执行,该合约被终止

    //abort()在其他2个函数执行之前才能调用

    function abort() OnlySeller StateStatus(State.Create) public{

        emit Abort();

        state = State.Release;

        seller.transfer(address(this).balance);

    }

    //deal() 确定购买:在其他2个函数执行之前才能调用,先到先得,只执行一次

    function deal()  StateStatus(State.Create) payable public{

        emit Deal();

        buyer = msg.sender;

        require(msg.value == price);

        state = State.Block;

    }

    //endDeal 确认付款:在deal()执行之后,才能够执行该函数

    function endDeal() OnlyBuyer StateStatus(State.Block)payable public{

        emit EndDeal();

        state = State.Release;

        // 注意: 这实际上允许买方和卖方阻止退款 - 应该使用取回模式

        //buyer.transfer(msg.value);在执行deal函数时,已经扣掉了ether

        seller.transfer(address(this).balance);

    }

}

学到的知识点:

    1.enum:枚举根据改变枚举的状态,能够确定函数的执行顺序

    2.address(this).balance:往该合约内注入的以太币

    3.在函数执行之后,修改状态变量的值