文件列表:

.idea (0, 2018-02-11)
.idea\.name (7, 2018-02-11)
.idea\JediBet.iml (281, 2018-02-11)
.idea\encodings.xml (159, 2018-02-11)
.idea\misc.xml (545, 2018-02-11)
.idea\modules.xml (266, 2018-02-11)
LICENSE (1210, 2018-02-11)
contracts (0, 2018-02-11)
contracts\Bet.sol (4857, 2018-02-11)
contracts\Migrations.sol (515, 2018-02-11)
migrations (0, 2018-02-11)
migrations\1_initial_migration.js (129, 2018-02-11)
migrations\2_deploy_contracts.js (107, 2018-02-11)
test (0, 2018-02-11)
test\testBet.js (9260, 2018-02-11)
truffle-config.js (135, 2018-02-11)
truffle.js (135, 2018-02-11)

# A Jedi’s Guide to Ethereum — Part 3 Master Yoda and Obi-Wan love to gamble. If you can recall in [Part 1](https://medium.com/@cipherz/super-simple-ethereum-part-1-7e363dbc1c65) and [Part 2](https://medium.com/@cipherz/super-simple-ethereum-part-2-6611721a391) of this series, Yoda and Obi-Wan created a private Ethereum network in order to transfer some Eth between each other to settle a bet. They have now decided to create a smart contract to settle their bets in a trustless way. They want to do the following * Deploy a smart contract to hold the Eth and bets of each Jedi * Check the results of the bet * Use the deployed smart contract to pay out the winnings The Masters are into a really simple game called *over/under *— where they guess a number between 1 and 10. The rules are as follows * The number is **randomly** generated(by the smart contract in this case) * The player **closest** to the number without going over it wins * If the players guess the **same** number the winnings are split * If both players guess **above** the number the winnings are split * If there is an **error** in the contract execution, the bets are returned to the players #### Setup A contract is a bit of code that performs some logical operation based on given inputs. The contract is stored and executed in the blockchain. Ethereum’s preferred language for interacting with its blockchain is called *Solidity*. The syntax and usage of Solidity is beyond the scope of this article, but we will comment on the key parts of the code. Before we get into the code itself, let install a helpful development tool called, *truffle. *Truffle is a node.js CLI tool that will greatly simplify our Solidity development, testing, and deployment. Install *truffle* $ npm install -g truffle Create project $ mkdir ~/ethereum/JediBet && cd ~/ethereum/JediBet $ truffle init You should see the following structure $ ls build migrations truffle-config.js contracts test truffle.js Create a file named *Bet.sol *with the following content pragma solidity ^0.4.8; contract Bet { } Create a file named *2_deploy_contracts.js *with the following content Bet = artifacts.require("./Bet.sol"); module.exports = (deployer) { deployer.deploy(Bet); }; Run the *truffle *development environment to simulate an ethereum network. We will use our private network later, but this should be good enough to develop on right now. Notice how the ethereum testnet starts up with a command line as well. Pretty nice. $ truffle development ... truffle(develop)> Compile the empty contract, *Bet.sol* truffle(develop)> compile Compiling ./contracts/Bet.sol... Writing artifacts to ./build/contracts Migrate the contract(Publish it on the blockchain). Note that this publishes the contract to a local test network that truffle maintains. You can view the output from this network by running the following from the project directory. Also, note that this test network does not require you to mine the transactions as a part of your development and testing. In the real world you would have to mine any transaction or contract you send to the blockchain. truffle development --log truffle(develop)> migrate Compiling ./contracts/Bet.sol... Writing artifacts to ./build/contracts Using network 'develop'. Running migration: 1_initial_migration.js Replacing Migrations... ... 0x7bc67e72eca3ffc723a8279d2e00194a19d65f4f42d87b4c51b6e8a2245a5fe6 Migrations: 0x8cdaf0cd25***87258bc13a92c0a6da926******4c0 Saving successful migration to network... ... 0xd7bc86d31bee32fa3***8f1c1eabce403a1b5d570340a3a9cdba53a472ee8c956 Saving artifacts... Running migration: 2_deploy_contracts.js Replacing Bet... ... 0x5c1dfbe5827ed7380063290a48f2357ce2969f57ed590***a1c9222e6***00ab56 Bet: 0x345ca3e014aaf5dca488057592ee47305d9b3e10 Saving successful migration to network... ... 0xf36163615f41ef7ed8f4a8f192149a0bf633fe1a23***ce001bf44c43dc7bdda0 Saving artifacts... truffle(develop)> #### What? We have just published an empty ethereum contract, *Bet.sol*, to the truffle testnet. We can’t do anything with it because it has no logic in it yet, but we will add that in the next section. #### The Code Like good citizens of the Republic, we are creating tests for our code using truffle’s integration with the *Mocha *test framework. The *Empire *has been blocking online content through it’s galactic firewalls, but Yoda has managed to find some good Solidity resources through the decentralized hosting network, [Substratum](https://substratum.net/). Solidity code and tests are outside of the scope of this article but they are presented below for your reference. #### The Contract If you recall, the purpose of the contract is to facilitate the betting and payout of Yoda and Obi-Wan’s favorite betting game, ‘Over-Under’. It is basically guessing a number between 1 and 10 without going over it, but don’t tell them that. The code below supports the following functions * Originate Bet — The *originator* can create a bet with an amount and the value guessed * Take Bet — The *taker *can accept the bet by verifying the amount of the bet and passing in a value for their guess. * Payout Bet — Either gambler* *can request the contract to payout the bet upon its conclusion. A couple helpful points to consider while reviewing the contract * A *contract *can do pretty much everything an *account *can do(Receive, Transfer, Send, Transact). In this case, our gamblers are sending their ether to the smart contract where it is stored within the contract’s account. It is not an IOU, it is literally maintaining the pot for the gamblers and providing a trustless escrow for their outcome. * Any transaction with the Ethereum blockchain requires *gas *— and this is no exception. The truffle tests do not specify a gas amount, but the *gas* is being taken from the amount sent. Note that *gas* is paid by the **Sender **of a transaction, so the originator, taker, and contract all pay *gas* to make this trustless system work. Typically, the *gas *amounts are very small, however, it is up to the contract developer to make sure that the minimum amount of *gas* is used while still ensuring the safety of the contract. * Disclaimer — The contract below does not handle error conditions or prevent a bad actor from messing with our bet. We will improve our contract in the next article where we add a simple web ui to facilitate bets more easily. #### Bet.sol The contract above is testable with the mocha tests with truffle. Create a *testBet.js* file in the *test* root project directory. Use the content below #### testBet.js Run the test as follows truffle(develop)> test './test/testBet.js' Using network 'develop'. Contract: Bet We should be able to start a bet by setting a guess and sending the bet amount that the contract was initialized with (38ms) The originating bet amount in the contract should match the passed in values The originating bet guess in the contract should match the passed in values The originator balance should be less the bet amount and gas (160ms) We should be able to take a bet by setting a guess and sending the bet amount that the contract was initialized with Taking the bet should fail if the bet amount does not equal the bet amount that the contract was initialized with The taker bet guess in the contract should match the passed in values The taker balance should be less the bet amount and gas (142ms) The contract balance should reflect the originator and taker bets The taker or originator should be able to call the payout to transfer winnings Originator and Taker balances should reflect bet outcome (272ms) ONLY the taker or originator should be able to call the payout function ONLY the taker or originator should be able to call the getBetAmount function ONLY the taker or originator should be able to call the getOriginatorGuess function ONLY the taker or originator should be able to call the getTakerGuess function ONLY the taker or originator should be able to call the getPot function ONLY the taker or originator should be able to call the getBetAmount function 17 passing (929ms) truffle(develop)> #### Summary We learned a lot in this portion of our series * Setting up a development environment with truffle * Creating and deploying our first Solidity smart contract * Test-Driven-Development of our Solidity smart contract In our [next article](https://medium.com/@cipherz/a-jedis-guide-to-ethereum-part-3b-remix-1b8d***d909d4) we will deploy this smart contract to our local ethereum network using the Remix IDE and learn how to use this powerful resource. The updated code for this example is here: [https://github.com/cipherzzz/JediBet](https://github.com/cipherzzz/JediBet) * [Ethereum](https://medium.com/tag/ethereum?source=post) * [Solidity](https://medium.com/tag/solidity?source=post) * [Truffle](https://medium.com/tag/truffle?source=post) * [Smart Contra](https://medium.com/tag/smart-contra?source=post) ### [CipherZ](https://medium.com/@cipherz)

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