Develop a Smart Contract

Develop a smart contract that increments a value.

In Prepare your first contract, you learned the basic steps for building and deploying a smart contract on a Substrate-based blockchain using a default first project.

For this tutorial, you'll develop a new smart contract that increments a counter value each time you execute a function call.

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.

• You have completed Prepare your first contract and have the Substrate contracts node installed locally.

Tutorial objectives

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

• Learn how to use a smart contract template.

• Store simple values using a smart contract.

• Increment and retrieve stored values using a smart contract.

• Add public and private functions to a smart contract.

Smart Contracts and ink!

In Prepare your first contract, you installed the cargo-contract package for command-line access to the ink! programming language.

The ink! language is an embedded domain specific language.

This language enables you to write WebAssembly-based smart contracts using the Rust programming language.

The language uses standard Rust patterns with specialized #[ink(...)] attribute macros.

These attribute macros describe what different parts of your smart contract represent so they can be transformed into Substrate-compatible WebAssembly bytecode.

Create a new smart contract project

Smart Contracts that run on Substrate start as projects. You create projects using cargo contract commands.

For this tutorial, you'll create a new project for the incrementer smart contract.

Creating a new project adds a new project directory and default starter files — also called template files — to the project directory.

You will modify these starter template files to build the smart contract logic for the incrementer project.

To create a new project for your smart contract:

1. Open a terminal shell on your local computer, if you don’t already have one open.

2. Create a new project named incrementer by running the following command:

   cargo contract new incrementer

3. Change to the new project directory by running the following command:

   cd incrementer/

4. Open the lib.rs file in a text editor.

   By default, the template lib.rs file contains the source code for the flipper smart contract with instances of the flipper contract name renamed incrementer.

5. Replace the default template source code with new incrementer source code
6. Save the changes to the lib.rs file, then close the file.

7. Verify that the program compiles and passes the trivial test by running the following command:

   cargo test

   You can ignore any warnings because this template code is simply a skeleton. The command should display output similar to the following to indicate successful test completion:

   running 1 test
   test incrementer::tests::default_works ... ok
   
   test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

8. Verify that you can build the WebAssembly for the contract by running the following command:

   cargo contract build

   If the program compiles successfully, you are ready to start programming.

Storing Simple Values

Now that you have some starter source code for the incrementer smart contract, you can introduce some new functionality.

For example, this smart contract requires storage of simple values.

The following code in this section is intended to illustrate the functionality of the ink! language. The code you will be using in the rest of this tutorial begins in the next section, Update your smart contract.

You can store simple values for a contract using the #[ink(storage)] attribute macro:

#[ink(storage)]
pub struct MyContract {
  // Store a bool
  my_bool: bool,
  // Store a number
  my_number: u32,
}

Supported types

ink! smart contracts support most Rust common data types, including booleans, unsigned and signed integers, strings, tuples, and arrays.

These data types are encoded and decoded using the Parity scale codec for efficient transmission over the network.

In addition to common Rust types that can be encoded and decoded using the scale codec, the ink! language supports Substrate-specific types — like AccountId, Balance, and Hash — as if they were primitive types.

The following code illustrates how to store an AccountId and Balance for this contract:

#[ink::contract]
mod MyContract {

  // Our struct will use those default ink! types
  #[ink(storage)]
  pub struct MyContract {
    // Store some AccountId
    my_account: AccountId,
    // Store some Balance
    my_balance: Balance,
  }
/* --snip-- */
}

Constructors

Every ink! smart contract must have at least one constructor that runs when the contract is created. However, a smart contract can have multiple constructors, if needed.

The following code illustrates using multiple constructors:

#[ink::contract]
mod my_contract {

    #[ink(storage)]
    pub struct MyContract {
        number: u32,
    }

    impl MyContract {
        /// Constructor that initializes the `u32` value to the given `init_value`.
        #[ink(constructor)]
        pub fn new(init_value: u32) -> Self {
            Self {
                number: init_value,
            }
        }

        /// Constructor that initializes the `u32` value to the `u32` default (0).
        ///
        /// Constructors can delegate to other constructors.
        #[ink(constructor)]
        pub fn default() -> Self {
            Self {
                number: Default::default(),
            }
        }
    /* --snip-- */
    }
}

Update your smart contract

Now that you have learned about storing simple values, declaring data types, and using constructors, you can update your smart contract source code to implement the following:

• Create a storage value called value with a data type of i32.

• Create a new Incrementer constructor and set its value to init_value.

• Create a second constructor function named default that has no input, and creates a new Incrementer with its value set to 0.

To update the smart contract:

1. Open the lib.rs file in a text editor.

2. Replace the Storage Declaration comment by declaring the storage item named value with the data type of i32.

   #[ink(storage)]
   pub struct Incrementer {
       value: i32,
   }

3. Modify the Incrementer constructor to set its value to init_value.

   impl Incrementer {
        #[ink(constructor)]
        pub fn new(init_value: i32) -> Self {
            Self { value: init_value }
        }
   }

4. Add a second constructor function named default that creates a new Incrementer with its value set to 0.

   #[ink(constructor)]
   pub fn default() -> Self {
       Self {
           value: 0,
       }
   }

5. Save your changes and close the file.

6. Try running the test subcommand again and you will see that the tests are now failing. This is because we need to update the get function and modify the tests to match the changes we implemented. We will do that in the next section.

Add a function to get a storage value

Now that you have created and initialized a storage value, you can interact with it using public and private functions. For this tutorial, you add a public function (a.k.a message) to get a storage value.

Note that all public functions must use the #[ink(message)] attribute macro.

To add the public function to the smart contract:

1. Open the lib.rs file in a text editor.

2. Update the get public function to return the data for the value storage item that has the i32 data type.

   #[ink(message)]
   pub fn get(&self) -> i32 {
       self.value
   }

   Because this function only reads from the contract storage, it uses the &self parameter to access the contract functions and storage items.

   This function does not allow changes to the state of the value storage item.

   If the last expression in a function does not have a semicolon (;), Rust treats it as the return value.

3. Replace the Test Your Contract comment in the private default_works function with code to test the get function.

   #[ink::test]
   fn default_works() {
       let contract = Incrementer::default();
       assert_eq!(contract.get(), 0);
   }

4. Save your changes and close the file.

5. Check your work using the test subcommand, and you will see that it is still failing, because we need to update the it_works test and add a new public function to increment the value storage item.

   cargo test

Add a function to modify the storage value

At this point, the smart contract does not allow users modify the storage. To enable users to modify storage items, you must explicitly mark value as a mutable variable.

To add a function for incrementing the stored value:

1. Open the lib.rs file in a text editor.

2. Add a new inc public function to increment the value stored using the by parameter that has data type of i32.

   #[ink(message)]
   pub fn inc(&mut self, by: i32) {
       self.value += by;
   }

3. Add a new test to the source code to verify this function.

   #[ink::test]
   fn it_works() {
       let mut contract = Incrementer::new(42);
       assert_eq!(contract.get(), 42);
       contract.inc(5);
       assert_eq!(contract.get(), 47);
       contract.inc(-50);
       assert_eq!(contract.get(), -3);
   }

4. Save your changes and close the file.

5. Check your work using the test subcommand:

   cargo test

   The command should display output similar to the following to indicate successful test completion:

   running 2 tests
   test incrementer::tests::it_works ... ok
   test incrementer::tests::default_works ... ok
   
   test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Build the WebAssembly for the contract

After you test the incrementer contract, you are ready to compile this project to WebAssembly.

To build the WebAssembly for this smart contract:

1. Open a terminal shell on your computer, if needed.

2. Verify that you are in the incrementer project folder.

3. Compile the incrementer smart contract by running the following command:

   cargo contract build

   The command displays output similar to the following:

   Your contract artifacts are ready. You can find them in:
   /Users/dev-docs/incrementer/target/ink
   
   - incrementer.contract (code + metadata)
   - incrementer.wasm (the contract's code)
   - incrementer.json (the contract's metadata)

Deploy and test the smart contract

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

Then you can use cargo-contract to deploy and test the smart contract.

To deploy on the local node:

1. Open a terminal shell on your computer, if needed.

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

   substrate-contracts-node --log info,runtime::contracts=debug 2>&1

3. Upload and instantiate the contract

   cargo contract instantiate --constructor default --suri //Alice --salt $(date +%s)

   Dry-running default (skip with --skip-dry-run)
       Success! Gas required estimated at Weight(ref_time: 321759143, proof_size: 0)
   Confirm transaction details: (skip with --skip-confirm)
    Constructor default
           Args
      Gas limit Weight(ref_time: 321759143, proof_size: 0)
   Submit? (Y/n):
      Events
       Event Balances ➜ Withdraw
         who: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
         amount: 2.953956313mUNIT
       ... snip ...
       Event System ➜ ExtrinsicSuccess
         dispatch_info: DispatchInfo { weight: Weight { ref_time: 2772097885, proof_size: 0 }, class: Normal, pays_fee: Yes }
   
   Code hash 0x71ddef2422fdb8358b503d5ef122c088a2dc6486dd460c37b01d672a8d319959
   Contract 5Cf6wFEyZnqvNJaKVxnWswefo7uT4jVsgzWKh8b78GLDV6kN

4. Increment the value

   cargo contract call --contract $INSTANTIATED_CONTRACT_ADDRESS --message inc --args 42 --suri //Alice

 Dry-running inc (skip with --skip-dry-run)
  Success! Gas required estimated at Weight(ref_time: 8013742080, proof_size: 262144)
 Confirm transaction details: (skip with --skip-confirm)
      Message inc
         Args 42
    Gas limit Weight(ref_time: 8013742080, proof_size: 262144)
 Submit? (Y/n):
    Events
     Event Balances ➜ Withdraw
       who: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
       amount: 98.97416μUNIT
     Event Contracts ➜ Called
       caller: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
       contract: 5Cf6wFEyZnqvNJaKVxnWswefo7uT4jVsgzWKh8b78GLDV6kN
     Event TransactionPayment ➜ TransactionFeePaid
       who: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
       actual_fee: 98.97416μUNIT
       tip: 0UNIT
     Event System ➜ ExtrinsicSuccess
       dispatch_info: DispatchInfo { weight: Weight { ref_time: 1383927346, proof_size: 13255 }, class: Normal, pays_fee: Yes }

1. Get the current value

   cargo contract call --contract 5Cf6wFEyZnqvNJaKVxnWswefo7uT4jVsgzWKh8b78GLDV6kN --message get --suri //Alice --dry-run

   Result Success!
   Reverted false
     Data Tuple(Tuple { ident: Some("Ok"), values: [Int(42)] })

As you can see the value being read from the contract is 42, which matches our previous step!

Next steps

In this tutorial, you learned some basic techniques for creating smart contracts using the ink! programming language and attribute macros.

For example, this tutorial illustrated:

• How to add storage items, specify data types, and implement constructors in a new smart contract project.

• How to add functions to a smart contract.

• How to add a test to a smart contract.

• How to upload and instantiate the contract using cargo-contract.

You can find an example of the final code for this tutorial in the assets for the smart-contracts.

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

• Use maps for storing values
• Build an ERC20 token contract
• Troubleshoot smart contracts