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• Introduction
• Preparation
• Smart Contract
• Call Contract
• sponsorship function
• Front-end Project
• Summary

We will use Conflux Studio to develop a simple token application - Coin, under the Oceanus network in this tutorial.

With this tutorial, users can learn how to write and call Conflux smart contracts, configure smart contract sponsors, and how to use web front-end projects to interact with smart contracts to develop a complete DApp that includes the front-end and smart contracts.

If there are any problems while reading the tutorial, please report it on Github Issues.

Please download Conflux Studio from the download page of GitHub. Currently Conflux Studio supports macOS, Linux and Windows systems, please download the corresponding version according to your computer system.

After installing Conflux Studio correctly and starting it for the first time, users will see a welcome page. Please follow the prompts to complete the download, installation and startup of Docker, Conflux Node and Conflux Truffle.

After installation, first users need to create a keypair to continue the following contract deployment and calling.

Users can open the keypair manager by clicking the key icon at the bottom left on any interface of Conflux Studio. Click the Create button to open the New Keypair pop-up window, enter the name of the keypair and click the Save button. And then users will see the address of the keypair generated in the keypair manager. A keypair consists of a private key and a public key. The public key is also called address in smart contracts.

To export the private key, users can click the eye-shaped button behind each address to open the private key pop-up window, which displays the address and the private key. In following steps, users will need to export private keys through the manager.

Three keypairs are needed to finish this tutorial:

• minter_key: used for the signature of Coin contract deployment and is the most frequently used keypair in this tutorial
• receiver_key: used to receive transfers in the Coin contract, and will be used when transferring
• sponsor_key: used for the Coin contract sponsorship function, which will be used in the sponsorship function sponsorship sponsorship function

Contract deployment and calling in this tutorial will be based on the Oceanus network. Click the inverted triangle of the Network label at the top to open the drop-down menu. Select the Oceanus network.

And then users will see on the main page that the current network is oceanus. The left side of the page displays the node URL, Chain ID, and TPS information of the current network, and the right side shows the block information of the current network.

Click the Explorer tab at the top to open the block explorer, and paste the keypair address in the address bar, then users can see the CFX balance of the pasted address on the left.

In the blockchain world, the method of applying for testnet tokens is called faucet. Currently, the total of tokens applied by each faucet under the Oceanus network is 100 CFX.

There are two ways to get CFX:

• After entering the address, click the faucet-shaped icon on the right side of the address bar, and then Conflux Studio will automatically apply for CFX for the address.
• Users can also directly enter https://wallet.confluxscan.io/faucet/dev/ask?address={address} in a browser to apply for CFX.

Please use the above mentioned methods to apply for CFX Tokens for minter_key and sponsor_key in Conflux Studio. After application, the balances on the two accounts will be updated from 0 CFX to 100 CFX.

The current balance is:

• minter_key: 100 CFX
• receiver_key: 0 CFX
• sponsor_key: 100 CFX

Click the Project tab on the top left, switch to the project list page, click the New button on the right,the Create a New Project pop-up window will jump out. Enter Project Name and choose coin Template, and then click Create Project.

The Coin contract is a simple token contract in which:

• The mint method can be used to issue additional tokens.
• The send method can be used to transfer a certain amount of tokens to others , and the transfer information will be recorded in the event.
• The balanceOf method can be used to query the token balance of a given address.
• The add_privilege method can be used to add addresses to the sponsor whitelist.
• The remove_privilege method is used to remove addresses from the sponsor whitelist.

The Conflux smart contract is developed using the Solidity language. Open the contracts/Coin.sol file in the directory. This is the core code of the project:

// Specify the version of Solidity, and tell the compiler that the compatible version of this code is 0.5.0 to 0.7.0 through Pragmas (https://solidity.readthedocs.io/en/latest/layout-of-source-files.html#pragmas)
pragma solidity >=0.5.0 <0.7.0;

// import the SponsorWhitelistControl contract
import "./SponsorWhitelistControl.sol";

// import the SponsorWhitelistControl contract
contract Coin {
    // two State Variables are defined (https://solidity.readthedocs.io/en/latest/structure-of-a-contract.html#state-variables)
    address public minter;
    mapping (address => uint) private balances;

    // Use the SponsorWhitelistControl contract to connect to the system contract
    SponsorWhitelistControl constant private SPONSOR = SponsorWhitelistControl(address(0x0888000000000000000000000000000000000001));

    // define the event of `Sent` and the from / to / amount column
    event Sent(address from, address to, uint amount);

    // the constructor of the Coin contract, specify the address of the minter in the constructor
    constructor() public {
        // msg.sender is the address of the account signed when deploying the contract, assign this address to minter
        minter = msg.sender;
    }

    // define the mint method, through which tokens can be issued
    function mint(address receiver, uint amount) public {
        require(msg.sender == minter);
        require(amount < 1e60);
        balances[receiver] += amount;
    }

    // define the send method, through which tokens can be transferred to other accounts
    function send(address receiver, uint amount) public {
        require(amount <= balances[msg.sender], "Insufficient balance.");
        balances[msg.sender] -= amount;
        balances[receiver] += amount;
        // trigger the Sent event by the emit method to record the transfer information
        emit Sent(msg.sender, receiver, amount);
    }

    // define the balanceOf method, which is a view type method for querying the account balance
    function balanceOf(address tokenOwner) public view returns(uint balance){
      return balances[tokenOwner];
    }

    // define the add_privilege method, call the system contract method add_privilege to add the address to the contract sponsor whitelist
    function add_privilege(address account) public payable {
        address[] memory a = new address[](1);
        a[0] = account;
        SPONSOR.add_privilege(a);
    }

    // define the remove_privilege method, call the system contract method remove_privilege to remove the address from the contract sponsor whitelist
    function remove_privilege(address account) public payable {
        address[] memory a = new address[](1);
        a[0] = account;
        SPONSOR.remove_privilege(a);
    }
}

Click the Build button (hammer-shaped) on the toolbar to compile the contract. The compilation result will be saved in the build/Coin.json file.

Click the deploy button in the toolbar to open the Deploy modal. You can enter constructor parameters, signer, gas limit and gas price for the deployment. In the constructor of the contract code, minter is assigned the value msg.sender, which is the address of signer.

After deployment, the result will be shown in the json file under the deploys folder. In this file, users can find the address of the current contract deployment in contractCreated. In the rest of this tutorial, we will use contract_addr to represent the contract address.

Click the Contract tab at the top to switch to the contract page. Enter the contract_addr address in the address bar and load the contract.

The contract page consists of three parts:

• On the left is the contract call area
• The middle is the contract data query area
• On the right is the event query area

Choose the mint method from the drop-down menu in the contract call area, and input the information in the parameter area below:

• receiver: The address to receive tokens. Enter the minter_key address.
• amount: The total number of tokens issued. Enter the integer 1000.
• Value: optional. For details, please refer to the explanation of Value. Enter 0 or leave it blank.
• Signer: The signature address of this transaction. If the sponsorship function is not enabled, the transaction fee will be deducted from this address. The address is obtained through msg.sender in the contract code. Enter the minter_key address.

After inputting all the information, click the execute button, and then Conflux Studio will automatically construct the transaction and push it to the network. After successful execution, users can see the transaction in the Result area below.

Click the drop-down menu in the contract data query area and select the balanceOf method. It is the query method defined in the code. Fill in the minter_key address in the tokenOwner below and click Execute, and then users can see in the Result area below that the Coin token balance of the minter_key account is 1,000. With the same method, users can see that the token balance of the receiver_key account is 0.

Click the drop-down menu in the contract call are, select the send method, and fill in the Parameters:

• receiver: The receiving address. Enter the receiver_key address.
• amount: The amount of tokens transferred. Enter the integer 200.
• Signer: The signature address of this transaction, and the amount of tokens transferred out will be deducted from this account. Enter the minter_key address.

Click Execute to complete the transfer and users can see that there are only 800 tokens left in the minter_key account, and the receiver_key account balance has changed from 0 to 200 tokens.

Each method called by the Conflux smart contract may carry the Value parameter. It is optional. If users input the Value parameter, the smart contract will transfer a certain amount of CFX tokens specified in it to the receiver account in addition to the tokens transferred through the above mentioned method. The additional transfer amount is the value filled in the Value parameter. The Value parameter is necessary for some smart contract methods to complete calling, but it is not mandatory for the Coin contract.

It will be used in the sponsorship function we will discuss later.

Select the Sent method in the event query area and click Execute. Users can see the transfer records in the Event Logs below. The columns of the Sent events are defined by the parameters of the Sent event in the code (epoch is the time when the event occurred, and it is default data of the system). The parameters defining the Sent method are from, to and amount, which correspond to the address of the originator, address of the receiver and the amount of the transferred tokens.

Conflux Studio supports the sponsorship function provided by the Conflux system contract.sponsorship function

Through the system contract, the sponsorship function can be set for other contracts by four methods:

• add_privilege: add addresses to the contract sponsor whitelist. When an address in the sponsor whitelist calls the method of this contract, the handling fee will not be paid by the calling address but by the sponsor account. Adding the special address 0x0000000000000000000000000000000000000000 means that the caller wants to pay for all addresses that call this contract.
• remove_privilege: remove addresses from the contract sponsor whitelist.
• set_sponsor_for_collateral: set the sponsor account and amount of the collateral for storage.
• set_sponsor_for_gas: set the gas fee sponsor account, the amount and the upper limit of the amount for each transaction

Activating the sponsorship function of a contract requires to set up a sponsor account, amount, and a sponsor whitelist. In this tutorial, we will use Conflux Studio to set the sponsor account and amount through the system contract, and add addresses to the whitelist through the Coin contract. After settings, the minter_key account will not be deducted the handling fee when calling the methods of the Coin contract, and the fee will be paid by the sponsor_key account.

Visit the system contract address 0x0888000000000000000000000000000000000001 in Conflux Studio, and users can see the four methods to set sponsors in the contract call area mentioned previously.

Choose the set_sponsor_for_collateral method. It has three parameters:

• contract_addr: Set the contract address for sponsor. Enter contract_addr.
• Value: set the sponsor amount. Enter the integer 40.
• Signer: sponsor account address. Enter the sponsor_key address.

Enter the above parameters and execute. The system contract will set up the sponsor of the collateral for storage of the Coin contract, and the sponsor_key account will be deducted 40 CFX.

Choose the set_sponsor_for_gas method. It has four parameters:

• contract_addr: Set the contract address for sponsor. Enter contract_addr.
• upper_bound: set the upper limit of the sponsorship for each transaction. Enter 1000000000000.
• Value: set the sponsor amount. Enter the integer 40.
• Signer: sponsor account address. Enter the sponsor_key address.

Enter the above parameters and execute. The system contract will set up the sponsor account for the transaction fees of the Coin contract. And now the sponsor_key account will be deducted again by 40 CFX.

After the call of these two methods, the Coin contract sponsor account is set up, and the sponsor_key account will pay 40 CFX for the handling fee and 40 for the collateral for storage of the Coin contract. Since there is no account address in the current sponsor whitelist, it is necessary to add addresses to the whitelist to complete the sponsor setting.

The method of setting the sponsor whitelist is integrated in the Coin contract. By calling this method, users can add/delete addresses to/from the sponsor whitelist.

Access the contract_addr contract in Conflux Studio and select the add_privilege method:

• account: add the address to the whitelist. Enter the minter_key address.
• Value: leave it blank.
• Signer: The signature address of this transaction. Enter the minter_key address.

Execute, and then the sponsor whitelist and the sponsorship function of the Coin contract will be ready.

Before testing the sponsorship function, please check and record the CFX balance of the minter_key account. For example, in this tutorial, the initial balance of minter_key is 97.6210937497093952 CFX.

Go back to the contract call page of the Coin contract, call the mint method and use the minter_key address to issue 1,000 additional tokens. And then query the balance of the minter_key address again, which is still 97.6210937497093952 CFX.Issue Additional Tokens

Users can see that in the transaction of additional token issuance, the handling fee that should have been paid by the minter_key account is paid by the sponsor_key account.

Users can find the source code of the front-end project in the Conflux front-end repository.

• Download the front-end project: git clone https://github.com/ObsidianLabs/conflux-frontend-react
• Use npm install or yarn to install project dependencies.

Conflux Portal is a browser plug-in provided by Conflux. Currently, Conflux Portal is supported by Chrome and Firefox. Users can use Conflux Portal to manage private keys and sign transactions.

Go to Conflux Portal GitHub to download and install. The source code of the project can be found in GitHub.

Here users need to import the addresses generated in Conflux Studio into Conflux Portal. After installation, click Import, paste the private key of minter_key in Conflux Studio (how to export the private key is introduced in the Create Wallet part previously), and click the Import button to import the private key.

Before running the project, users need to modify some default environment variables.

After deploying the contract , contractCreated will be generated, which is the address of the smart contract deployed in the network. Open the root directory of the project and find the .env file that includes the environment variables of the project. Change the value of REACT_APP_CONFLUX_COIN_ADDRESS to contract_addr.

Use yarn start to start the project. When the development server runs, the front-end page will be opened in the browser (if not, please visit http://localhost:3000 in the browser).

When the project starts, the interface will display four modules:

• Conflux network information module in the upper left
• Conflux Portal module in the upper right
• Coin contract module in the lower left
• SponsorWhitelistControl contract module in the lower right

Click the Connect to Conflux Portal button in the Conflux Portal module to open Conflux Portal, enter the password and select the account to connect. After connection, you will see the currently connected account address and the CFX balance in the account below the button.

Users can use the token issuance and token transfer functions with the Coin contract module in the lower left.

• Additional token issuance: select the mint method and enter the minter_key address (the token issuance address) in the receiver bar and 100 (the number of tokens) in the amount bar, click Push Transaction, and click the Confirm button in the pop-up ConfluxPortal Notification window to confirm the transaction.

• Additional token issuance: select the mint method and enter the minter_key address (the token issuance address) in the receiver bar and 100 (the number of tokens) in the amount bar, click Push Transaction, and click the Confirm button in the pop-up ConfluxPortal Notification window to confirm the transaction.

Select the balanceOf method and enter the query address in the tokenOwner box, and click the Query Data button to view the account balance.

Select the Sent event and click Query Data to see the record of transfer events triggered by transfer operations.

The project uses React for development. It is mainly composed of three parts: view components, js-conflux-sdk and Conflux Portal.

The .env environment variable under the project root directory defines two environment variables:

• REACT_APP_CONFLUX_NODE_RPC: Conflux network node address. Currently the default is the address of Oceanus network
• REACT_APP_CONFLUX_COIN_ADDRESS: the address of the deployed Coin smart contract.

The view component is in the src/components of the project, where App.js is the main entrance of the page. It is used for page layout and contract information reading.

Used for rendering Conflux network information. The value of Node URL is the value set by REACT_APP_CONFLUX_NODE_RPC in the .env environment variable file (the default is Oceanus network).

Used for rendering the connection information of Conflux Portal and provides interactive buttons for connecting to Conflux Portal.

• connectConfluxPortal: used to call the enable method of Conflux Portal to start Conflux (the conflux portal instance is injected into windows.portal by the browser plug-in). After starting, call the getAccount method to get the account in the Portal.
• refreshBalance: used to call the getBalance method of the Conflux SDK to update the account balance information.
• renderPortalButton: render the button to connect to the Portal according to the current state

Used for rendering Conflux contract information. Two contracts, Coin and SponsorWhitelistControl, are provided in this project.

ConfluxContract.js consists of three components, namely:

• ConfluxContract: render the information of the contract according to the incoming contract abi, including the contract address, contract method and event, the interactive logic submitted by the contract and the results after execution.
• ContractMethods: render the information of the contract according to the incoming contract abi, including the contract address, contract method and event, the interactive logic submitted by the contract and the results after execution.
• ConfluxForm: render the input form according to the abi of the method or event.

lib is in the src/lib of the project, where the files are mainly to provide services for the view component, including connecting to the network, constructing transactions, obtaining accounts, and reading contracts.

conflux.js is the package of js-conflux-sdk. It is the JavaScript SDK provided by Conflux. In this front-end project, we use SDK to connect to the Conflux network, interact with the contract and construct instances in the contract.

conflux-portal.js is the package of Conflux Portal. In this project, we use the browser plug-in Conflux Portal to finish the transaction signature. Calling the enable method provided by Conflux Portal can start the connection between the project and Conflux Portal (users need to check whether they have correctly installed the plug-in in their browser by checking whether window.conflux is empty in the constructor). conflux-portal.js provides the two main methods to get accounts and send transactions: getAccount and sendTransaction.

Two json files are provided under the lib/abi folder, namely Coin.json and SponsorWhitelistControl.json. These two are the abi files needed to construct the contract.

In this development tutorial, we introduced how to use Conflux Studio to develop a complete Coin DApp, including

• Use the keypair manager to create an account and export the account private key
• Switch to Oceanus network and view network information
• Apply for CFX Tokens in the account
• Create, compile and deploy project
• Analyze the Coin contract code, learn how to write the reading and writing methods and events of the contract
• Use the contract explorer to call the token issuance, transfer, balance query and event query methods of the Coin contract
• Set up and use the sponsorship function of the smart contract
• Import the private key into Conflux Portal and connect to the front-end project
• Call the additional token issuance, transfer, balance query and event query methods of the Coin contract in the front-end project
• Analyze the front-end project code, learn how to connect to the network and implement transactions through Conflux Portal and Conflux JavaScript SDK