Prepare your first contract

As you learned in Blockchain basics decentralized applications are most often written as smart contracts. Although Substrate is primarily a framework and toolkit for building custom blockchains, it can also provide a platform for smart contracts. This tutorial demonstrates how to build a basic smart contract to run on a Substrate-based chain. In this tutorial, you'll explore using ink! as a programming language for writing Rust-based smart contracts.

Before you begin

Before you begin, verify the following:

You have good internet connection and access to a shell terminal on your local computer.
You are generally familiar with software development and using command-line interfaces.
You are generally familiar with blockchains and smart contract platforms.
You have installed Rust and set up your development environment as described in Install.

Tutorial objectives

By completing this tutorial, you will accomplish the following objectives:

Learn how to create a smart contract project.
Build and test a smart contract using the ink! smart contract language.
Deploy a smart contract on a local Substrate node.
Interact with a smart contract through a browser.

Update your Rust environment

For this tutorial, you need to add some Rust source code to your Substrate development environment.

To update your development environment:

Open a terminal shell on your computer.
Update your Rust environment by running the following command:
```
rustup component add rust-src --toolchain nightly
```
Verify that you have the WebAssembly target installed by running the following command:
```
rustup target add wasm32-unknown-unknown --toolchain nightly
```
If the target is installed and up-to-date, the command displays output similar to the following:
```
info: component 'rust-std' for target 'wasm32-unknown-unknown' is up to date
```

Install the Substrate contracts node

To simplify this tutorial, you can download a precompiled Substrate node for Linux or macOS. The precompiled binary includes the FRAME pallet for smart contracts by default.

To install the contracts node on macOS or Linux:

Open the Releases page.
Download the appropriate compressed archive for your local computer.
Open the downloaded file and extract the contents to a working directory.

Alternatively, you can build the preconfigured contracts-node manually by running cargo install contracts-node on your local computer. If you can't download the precompiled node, you can compile it locally with a command similar to the following:

cargo install contracts-node --git https://github.com/paritytech/substrate-contracts-node.git --tag <latest-tag> --force --locked

You can find the latest tag to use on the Tags page.

Install additional packages

After downloading or compiling the contracts-node package, you need to install two additional packages:

The WebAssembly binaryen package for your operating system to optimize the WebAssembly bytecode for the contract.
The cargo-contract command line interface you'll use to set up smart contract projects.

Install the WebAssembly optimizer

To install the binaryen package:

Open a terminal shell on your computer.
Use the appropriate package manager for your operating system to install the package.

For example, on Ubuntu or Debian, run the following command:
```
sudo apt install binaryen
```
On macOS, run the following command:
```
brew install binaryen
```
For other operating systems, you can download the binaryen release directly from WebAssebly releases.

Install the cargo-contract package

After you've installed the WebAssembly binaryen package, you can install the cargo-contract package. The cargo-contract package provides a command-line interface for working with smart contracts using the ink! language.

Open a terminal shell on your computer.
Install cargo-dylint to check ink! contracts and warn you about issues that might lead to security vulnerabilities.
```
cargo install cargo-dylint
```
Install cargo-contract by running the following command:
```
cargo install cargo-contract --force
```
Verify the installation and explore the commands available by running the following command:
```
cargo contract --help
```

Create a new smart contract project

You are now ready to start developing a new smart contract project.

To generate the files for a smart contract project:

Open a terminal shell on your computer.
Create a new project folder named flipper by running the following command:
```
cargo contract new flipper
```
Change to the new project folder by running the following command:
```
cd flipper/
```
List all of the contents of the directory by running the following command:
```
ls -al
```
You should see that the directory contains the following files:
```
-rwxr-xr-x   1 dev-doc  staff   285 Mar  4 14:49 .gitignore
-rwxr-xr-x   1 dev-doc  staff  1023 Mar  4 14:49 Cargo.toml
-rwxr-xr-x   1 dev-doc  staff  2262 Mar  4 14:49 lib.rs
```
Like other Rust projects, the Cargo.toml file is used to provide package dependencies and configuration information. The lib.rs file is used for the smart contract business logic.

Explore the default project files

By default, creating a new smart contract project generates some template source code for a very simple contract that has one function—flip()—that changes a Boolean variable from true to false and a second function—get—that gets the current value of the Boolean. The lib.rs file also contains two functions for testing that the contract works as expected.

As you progress through the tutorial, you'll modify different parts of the starter code. By the end of the tutorial, you'll have a more advanced smart contract that looks like the Flipper example.

To explore the default project files:

Open a terminal shell on your computer, if needed.
Change to project folder for the flipper smart contract, if needed:
Open the Cargo.toml file in a text editor and review the dependencies for the contract.

In the [dependencies] section, modify the scale and scale-info settings, if necessary.

scale = { package = "parity-scale-codec", version = "3", default-features = false, features = ["derive"] }
scale-info = { version = "2", default-features = false, features = ["derive"], optional = true }

Save any changes to the Cargo.toml file, then close the file.
Open the lib.rs file in a text editor and review the macros, constructors, and functions defined for the contract.
- The storage macro defines a structure to stores a single boolean value for the contract.
- The new and default functions initialize the boolean value to false.
- There's a message macro with a flip function to change the state of the data stored for the contract.
- There's a message macro with a get function to get the current state of the data stored for the contract.

Test the default contract

At the bottom of the lib.rs source code file, there are simple test cases to verify the functionality of the contract. You can test whether this code is functioning as expected using the offchain test environment.

To test the contract:

Open a terminal shell on your computer, if needed.
Verify that you are in the flipper project folder, if needed.
Use the test subcommand to execute the default tests for the flipper contract by running the following command:
```
cargo test
```
The command should compile the program and display output similar to the following to indicate successful test completion:
```
running 2 tests
test flipper::tests::it_works ... ok
test flipper::tests::default_works ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
```

Build the contract

After testing the default contract, you are ready to compile this project to WebAssembly.

To build the WebAssembly for this smart contract:

Open a terminal shell on your computer, if needed.
Verify that you are in the flipper project folder.
Compile the flipper smart contract using the nightly toolchain by running the following command:
```
cargo +nightly contract build
```
This command builds a WebAssembly binary for the flipper project, a metadata file that contains the contract Application Binary Interface (ABI), and a .contract file that you use to deploy the contract. For example, you should see output similar to the following:
```
Original wasm size: 47.8K, Optimized: 22.4K

The contract was built in DEBUG mode.

Your contract artifacts are ready. You can find them in:
/Users/dev-doc/flipper/target/ink

- flipper.contract (code + metadata)
- flipper.wasm (the contract's code)
- metadata.json (the contract's metadata)
```
The .contract file that includes both the business logic and metadata is the file you use to deploy the contract on a chain.

The metadata.json file in the target/ink directory describes all the interfaces that you can use to interact with this contract. This file contains several important sections:
- The spec section includes information about the functions—like constructors and messages—that can be called, the events that are emitted, and any documentation that can be displayed. This section also includes a selector field that contains a 4-byte hash of the function name and is used to route contract calls to the correct functions.
- The storage section defines all the storage items managed by the contract and how to access them.
- The types section provides the custom data types used throughout the rest of the JSON.

Start the Substrate smart contracts node

If you have successfully installed substrate-contracts-node, you can start a local blockchain node for your smart contract.

To start the preconfigured contracts-node:

Open a new terminal shell on your computer, if needed.
Change to the root directory that contains the substrate-contracts-node binary.

For example, if you downloaded the precompiled binary on a macOS computer, you can run the following command:
```
cd artifacts/substrate-contracts-node-mac
```

Start the contracts node in local development mode by running the following command:

substrate-contracts-node --dev

You should see output in the terminal similar to the following:

2022-03-07 14:46:25 Substrate Contracts Node
2022-03-07 14:46:25 ✌️  version 0.8.0-382b446-x86_64-macos
2022-03-07 14:46:25 ❤️  by Parity Technologies <admin@parity.io>, 2021-2022
2022-03-07 14:46:25 📋 Chain specification: Development
2022-03-07 14:46:25 🏷  Node name: possible-plants-8517
2022-03-07 14:46:25 👤 Role: AUTHORITY
2022-03-07 14:46:25 💾 Database: RocksDb at /var/folders/2_/g86ns85j5l7fdnl621ptzn500000gn/T/substrateEdrJW9/chains/dev/db/full
2022-03-07 14:46:25 ⛓  Native runtime: substrate-contracts-node-100 (substrate-contracts-node-1.tx1.au1)
2022-03-07 14:46:25 🔨 Initializing Genesis block/state (state: 0xe9f1…4b89, header-hash: 0xa1b6…0194)
2022-03-07 14:46:25 👴 Loading GRANDPA authority set from genesis on what appears to be first startup.
2022-03-07 14:46:26 🏷  Local node identity is: 12D3KooWQ3P8BH7Z1C1ZoNSXhdGPCiPR7irRSeQCQMFg5k3W9uVd
2022-03-07 14:46:26 📦 Highest known block at #0

After a few seconds, you should see blocks being finalized.

To interact with the blockchain, you need to connect to this node. You can connect to the node through a browser by opening the Contracts UI.

Navigate to the Contracts UI in a web browser, then click Yes allow this application access.
Select Local Node.

Deploy the contract

At this point, you have completed the following steps:

Installed the packages for local development.
Generated the WebAssembly binary for the flipper smart contract.
Started the local node in development mode.
Connected to a local node through the Contracts UI front-end.

The next step is to deploy the flipper contract on your Substrate chain.

However, deploying a smart contract on Substrate is a little different than deploying on traditional smart contract platforms. For most smart contract platforms, you must deploy a completely new blob of the smart contract source code each time you make a change. For example, the standard ERC20 token has been deployed to Ethereum thousands of times. Even if a change is minimal or only affects some initial configuration setting, each change requires a full redeployment of the code. Each smart contract instance consumes blockchain resources equivalent to the full contract source code, even if no code was actually changed.

In Substrate, the contract deployment process is split into two steps:

Upload the contract code to the blockchain.
Create an instance of the contract.

With this pattern, you can store the code for a smart contract like the ERC20 standard on the blockchain once, then instantiate it any number of times. You don't need to reload the same source code repeatedly, so your smart contract doesn't consume unnecessary resources on the blockchain.

Upload the contract code

For this tutorial, you use the Contracts UI front-end to deploy the flipper contract on the Substrate chain.

To upload the smart contract source code:

Open to the Contracts UI in a web browser.
Verify that you are connected to the Local Node.
Click Add New Contract.
Click Upload New Contract Code.
Select an Account to use to create a contract instance.

You can select any existing account, including a predefined account such as alice.
Type a descriptive Name for the smart contract, for example, Flipper Contract.
Browse and select or drag and drop the flipper.contract file that contains the bundled Wasm blob and metadata into the upload section.
Click Next to continue.

Create an instance on the blockchain

Smart contracts exist as an extension of the account system on the Substrate blockchain. When you create an instance of this smart contract, Substrate creates a new AccountId to store any balance managed by the smart contract and to allow you to interact with the contract.

After you upload the smart contract and click Next, the Contracts UI displays information about the content of the smart contract.

To create the instance:

Review and accept the default Deployment Constructor options for the initial version of the smart contract.
Review and accept the default Max Gas Allowed.
Click Next.

The transaction is now queued. If you needed to make changes, you could click Go Back to modify the input.
Click Upload and Instantiate.

Depending on the account you used, you might be prompted for the account password. If you used a predefined account, you won't need to provide a password.

Call the smart contract

Now that your contract has been deployed on the blockchain, you can interact with it. The default flipper smart contract has two functions—flip() and get()—and you can use the Contracts UI to try them out.

get() function

You set the initial value of the flipper contract value to false when you instantiated the contract. You can use the get() function to verify the current value is false.

To test the get() function:

Select any account from the Account list.

This contract doesn't place restrictions on who is allowed to send the get() request.
Select get(): bool from the Message to Send list.
Click Read.
Verify that the value false is returned in the Call Results.

flip() function

The flip() function changes the value from false to true.

To test the flip() function:

Select any predefined account from the Account list.

The flip() function is a transaction that changes the chain state and requires an account with funds to be used to execute the call. Therefore, you should select an account that has a predefined account balance, such as the alice account.
Select flip() from the Message to Send list.
Click Call.
Verify that the transaction is successful in the Call Results.
Select get(): bool from the Message to Send list.
Click Read.
Verify the new value is true in the Call Results.

Next steps

Congratulations!

In this tutorial, you learned:

How to create a new smart contract project using the ink! smart contract language.
How to test and build a WebAssembly binary for a simple default smart contract.
How to start a working Substrate-based blockchain node using the contracts node.
How to deploy a smart contract by connecting to a local node and uploading and instantiating the contract.
How to interact with a smart contract using the Contracts UI browser client.

Additional smart contract tutorials build on what you learned in this tutorial and lead you deeper into different stages of contract development.

You can learn more about smart contract development in the following topics: