Cairo Smart Contracts and Starknet Intro Workshop

Workshop by JohnnyTime and Smart Contract Programmer

The following is a demo of an exercise in the Cairo Smart Contract Hacking Course by JohnnyTime. A Live workshop was held at the Blockchain Security Academy, where we went through the exercise and explained the solution.

YouTube video of the workshop recording: TBD.

Prerequisites

1. Install asdf
2. Using asdf, add scrab 2.6.3 as a plugin: asdf plugin add scarb
3. Using asdf, install scarb 2.6.3 - asdf install scarb 2.6.3
4. Using asdf, set as a global var version - asdf global scarb 2.6.3
5. Install Starknet-Foundry 0.24.0
   • Run - curl -L https://raw.githubusercontent.com/foundry-rs/starknet-foundry/master/scripts/install.sh | sh
   • Install version 0.23.0 | snfoundryup -v 0.23.0
6. Install Universal Sierra Compiler
   • Run - curl -L https://raw.githubusercontent.com/software-mansion/universal-sierra-compiler/master/scripts/install.sh | sh

In the following exercise, your goal is to create a simple smart contract with Storage, and Events, and the test is using Starknet-Foundry.

If you are not sure about the syntax, you can always refer to the Lecture video or to the Cheatsheet File.

Tasks

Cairo Contract Implementation

In the file src\lib.cairo:

1. Define an interface IMyFirstCairoContract with 2 functions:
   1. set_number() - receives a u256 number and returns nothing.
   2. get_number() - received nothing and returns a u256.
2. Create a new cairo smart contract MyFirstCairoContract:
   1. Define the contract storage with one u256 variable named number.
   2. Define an event that is called NumberChanged, which will be emitted anytime the number is being changed in the storage, the event should be emitted with the old and new number
   3. Create a constructor function that receives an initial_value u256 and writes it to storage, don't forget to emit a NumberChanged event.
   4. Implement the IMyFirstCairoContract interface, and both set_number() and get_number() accordingly so they will set and get the number from the contract's storage, and emi$$ $$t an event in case the number is changed.

Testing Our Contract

In the file tests\test_contract.cairo:

1. Import all the relevant libraries:
   1. Starknet Contract Address - starknet::ContractAddress;
   2. Starknet Foundry - snforge_std::{declare, ContractClassTrait, start_prank, stop_prank, CheatTarget, start_warp};
   3. Your Cairo contract Dispatcher and Dispatcher Trait.
2. Inside the test first_cairo_contract_tests:
   1. Declare the contract class.
   2. Prepare the constructor call data using Serde.
   3. Deploy the contract and create a Dispatcher.
   4. Check the initial value in the contract storage, and make sure it's correct (use assert).
   5. Update the number in storage to 1337.
   6. Check the new value in the contract storage, and make sure it's correct (use assert).

Bonus: In your test file, check that the right events were emitted after the number was changed.

Checking the Exercise

To run all tests, use the command snforge test.

Useful links

Anatomy of a Simple contract

Contract Storage

Contract Functions

Contract Events

Running Tests snfoge