Creating your first Concordium dApp

This tutorial guides you through creating and deploying your first smart contract on Concordium. You’ll create a simple Hello World smart contract and deploy it on the testnet.

Before you start

Ensure you have set up your development environment by following this tutorial.

Creating your first smart contract

1. Initialize a new smart contract project:

   $ cargo concordium init
   

2. Select the default template when prompted, then choose an appropiate project name. The init command has now created a template smart contract and a corresponding local repository.

Write the contract

Modify src/lib.rs to create a simple message-storing contract:

1. Modify the existing state structure, the new state defines the message variable:

   /// The state of the smart contract.
   #[derive(Serialize, SchemaType)]
   pub struct State {
       message: String,
   }
   

2. Modify the init method which initialises the message variable:

   #[init(contract = "hello_world")]
   fn init(_ctx: &InitContext, _state_builder: &mut StateBuilder) -> InitResult<State> {
       Ok(State {
           message: "Hello World!".to_string(),
       })
   }
   

3. Modify the receive method by specifying String as the parameter type, changing it’s name to be more suggestive and assigning the input string to the state:

   #[receive(
       contract = "hello_world",
       name = "set_message",
       parameter = "String",
       error = "Error",
       mutable
   )]
   fn set_message(ctx: &ReceiveContext, host: &mut Host<State>) -> Result<(), Error> {
       let new_message: String = ctx.parameter_cursor().get()?;
       host.state_mut().message = new_message;
       Ok(())
   }
   

4. Modify the view function by specifying the String return value type and the message variable of the state

   #[receive(contract = "hello_world", name = "view", return_value = "String")]
   fn view(_ctx: &ReceiveContext, host: &Host<State>) -> ReceiveResult<String> {
       Ok(host.state().message.clone())
   }
   

5. Build the contract:

   $ cargo concordium build
   

   After the command runs successfully, a file called module.wasm.v1 will be generated in the concordium-out folder

Deploying your contract

We recommend deploying on the testnet, which can be done by following these steps:

1. Set up a wallet

2. Request testnet CCD using the Request option in the wallet

3. Access the Smart Contract Deploy Tool in order to deploy the generated module

4. Click on the button to connect to the Browser Wallet

5. Upload the module generated in the previous section in the Step 1 section of the dApp then click the button to deploy

6. Choose Derive from step 1 in Step 2, then click the button to initialise the smart contract, a Smart Contract Index will be shown

7. Input the index in the next step, then select the Derive From Smart Contract Index option

8. Choose view as the Entry Point Name, then click the Read Smart Contract button, “Hello World” will be displayed

Congratulations! You’ve now created and deployed your first smart contract on the Concordium blockchain.