Version: Next

9. Voting Contract

In this tutorial, we're going to deploy a contract that allows users to vote on multiple proposals that a voting administrator controls.

tip

Open the starter code for this tutorial in the Flow Playground:

https://play.onflow.org/d120f0a7-d411-4243-bc59-5125a84f99b3

The tutorial will be asking you to take various actions to interact with this code.

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Instructions that require you to take action are always included in a callout box like this one. These highlighted actions are all that you need to do to get your code running, but reading the rest is necessary to understand the language's design.

With the advent of blockchain technology and smart contracts, it has become popular to try to create decentralized voting mechanisms that allow large groups of users to vote completely on chain. This tutorial will provide a trivial example for how this might be achieved by using a resource-oriented programming model.

We'll take you through these steps to get comfortable with the Voting contract.

Deploy the contract to account 0x01
Create proposals for users to vote on
Use a transaction with multiple signers to directly transfer the Ballot resource to another account.
Record and cast your vote in the central Voting contract
Read the results of the vote

Before proceeding with this tutorial, we highly recommend following the instructions in Getting Started and Hello, World! to learn how to use the Playground tools and to learn the fundamentals of Cadence.

A Voting Contract in Cadence

In this contract, a Ballot is represented as a resource.

An administrator can give Ballots to other accounts, then those accounts mark which proposals they vote for and submit the Ballot to the central smart contract to have their votes recorded.

Using a resource type is logical for this application, because if a user wants to delegate their vote, they can send that Ballot to another account, and the use case of voting ballots benefits from the uniqueness and existence guarantees inherent to resources.

Deploy the Contract

Time to deploy the contract we'll be working with:

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Open Contract 1 - the ApprovalVoting contract.
In the bottom right deployment modal, press the arrow to expand and make sure account 0x01 is selected as the signer.
Click the Deploy button to deploy it to account 0x01

Deploy ApprovalVoting to account 0x01

The deployed contract should have the following contents:

ApprovalVoting.cdc

/*
*
*   In this example, we want to create a simple approval voting contract
*   where a polling place issues ballots to addresses.
*
*   The run a vote, the Admin deploys the smart contract,
*   then initializes the proposals
*   using the initialize_proposals.cdc transaction.
*   The array of proposals cannot be modified after it has been initialized.
*
*   Then they will give ballots to users by
*   using the issue_ballot.cdc transaction.
*
*   Every user with a ballot is allowed to approve any number of proposals.
*   A user can choose their votes and cast them
*   with the cast_vote.cdc transaction.
*
*/
access(all) contract ApprovalVoting {
    //list of proposals to be approved
    access(all) var proposals: [String]
    // number of votes per proposal
    access(all) let votes: {Int: Int}
    // This is the resource that is issued to users.
    // When a user gets a Ballot object, they call the `vote` function
    // to include their votes, and then cast it in the smart contract
    // using the `cast` function to have their vote included in the polling
    access(all) resource Ballot {
        // array of all the proposals
        access(all) let proposals: [String]
        // corresponds to an array index in proposals after a vote
        access(all) var choices: {Int: Bool}
        init() {
            self.proposals = ApprovalVoting.proposals
            self.choices = {}
            // Set each choice to false
            var i = 0
            while i < self.proposals.length {
                self.choices[i] = false
                i = i + 1
            }
        }
        // modifies the ballot
        // to indicate which proposals it is voting for
        access(all) fun vote(proposal: Int) {
            pre {
                self.proposals[proposal] != nil: "Cannot vote for a proposal that doesn't exist"
            }
            self.choices[proposal] = true
        }
    }
    // Resource that the Administrator of the vote controls to
    // initialize the proposals and to pass out ballot resources to voters
    access(all) resource Administrator {
        // function to initialize all the proposals for the voting
        access(all) fun initializeProposals(_ proposals: [String]) {
            pre {
                ApprovalVoting.proposals.length == 0: "Proposals can only be initialized once"
                proposals.length > 0: "Cannot initialize with no proposals"
            }
            ApprovalVoting.proposals = proposals
            // Set each tally of votes to zero
            var i = 0
            while i < proposals.length {
                ApprovalVoting.votes[i] = 0
                i = i + 1
            }
        }
        // The admin calls this function to create a new Ballot
        // that can be transferred to another user
        access(all) fun issueBallot(): @Ballot {
            return <-create Ballot()
        }
    }
    // A user moves their ballot to this function in the contract where
    // its votes are tallied and the ballot is destroyed
    access(all) fun cast(ballot: @Ballot) {
        var index = 0
        // look through the ballot
        while index < self.proposals.length {
            if ballot.choices[index]! {
                // tally the vote if it is approved
                self.votes[index] = self.votes[index]! + 1
            }
            index = index + 1;
        }
        // Destroy the ballot because it has been tallied
        destroy ballot
    }
    // initializes the contract by setting the proposals and votes to empty
    // and creating a new Admin resource to put in storage
    init() {
        self.proposals = []
        self.votes = {}
        self.account.save<@Administrator>(<-create Administrator(), to: /storage/VotingAdmin)
    }
}

This contract implements a simple voting mechanism where an Administrator can initialize a vote with an array of proposals to vote on by using the initializeProposals function.

// function to initialize all the proposals for the voting
access(all) fun initializeProposals(_ proposals: [String]) {
    pre {
        ApprovalVoting.proposals.length == 0: "Proposals can only be initialized once"
        proposals.length > 0: "Cannot initialize with no proposals"
    }
    ApprovalVoting.proposals = proposals
    // Set each tally of votes to zero
    var i = 0
    while i < proposals.length {
        ApprovalVoting.votes[i] = 0
        i = i + 1
    }
}

Then they can give Ballot resources to other accounts. The other accounts can record their votes on their Ballot resource by calling the vote function.

access(all) fun vote(proposal: Int) {
    pre {
        self.proposals[proposal] != nil: "Cannot vote for a proposal that doesn't exist"
    }
    self.choices[proposal] = true
}

After a user has voted, they submit their vote to the central smart contract by calling the cast function, which records the votes in the Ballot and destroys the used Ballot.

// A user moves their ballot to this function in the contract where
// its votes are tallied and the ballot is destroyed
access(all) fun cast(ballot: @Ballot) {
    var index = 0
    // look through the ballot
    while index < self.proposals.length {
        if ballot.choices[index]! {
            // tally the vote if it is approved
            self.votes[index] = self.votes[index]! + 1
        }
        index = index + 1;
    }
    // Destroy the ballot because it has been tallied
    destroy ballot
}

When the voting time ends, the administrator can read the tallies for each proposal to see if a proposal has received the right number of votes.

Perform Voting

Performing the common actions in this voting contract only takes three types of transactions.

Initialize Proposals
Send Ballot to a voter
Cast Vote

We have a transaction for each step that we provide for you. With the ApprovalVoting contract to account 0x01:

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Open Transaction 1 which should have Transaction1.cdc
Submit the transaction with account 0x01 selected as the only signer.

Transaction1.cdc

import ApprovalVoting from 0x01
// This transaction allows the administrator of the Voting contract
// to create new proposals for voting and save them to the smart contract
transaction {
    prepare(admin: AuthAccount) {
        // borrow a reference to the admin Resource
        let adminRef = admin.borrow<&ApprovalVoting.Administrator>(from: /storage/VotingAdmin)!
        // Call the initializeProposals function
        // to create the proposals array as an array of strings
        adminRef.initializeProposals(
            ["Longer Shot Clock", "Trampolines instead of hardwood floors"]
        )
        log("Proposals Initialized!")
    }
    post {
        ApprovalVoting.proposals.length == 2
    }
}

This transaction allows the Administrator of the contract to create new proposals for voting and save them to the smart contract. They do this by calling the initializeProposals function on their stored Administrator resource, giving it two new proposals to vote on. We use the post block to ensure that there were two proposals created, like we wished for.

Next, the Administrator needs to hand out Ballots to the voters. There isn't an easy deposit function this time for them to send a Ballot to another account, so how would they do it?

This is where multi-signed transactions can come in handy!

Selecting multiple Accounts as Signers

A transaction has access to the private account objects of every account that signed it, so if both the admin and the voter sign a transaction, the admin can directly move a Ballot resource object to the other account's storage.

In the Flow playground, you can select multiple accounts to sign a transaction to be able to access the private account objects of both accounts.

To select multiple signers, you first need to include two arguments in the prepare block of your transaction:

prepare(acct1: AuthAccount, acct2: AuthAccount)

The playground will give you an error if the number of selected signers is different than the number of arguments to the prepare block. The playground also maps the accounts you select as signers to the arguments in the order that you select them. The first account you select will be the first argument, and the second account you select is the second argument.

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Open Transaction 2 which should have Transaction2.cdc.
Select account 0x01 as a signer first, then also select account 0x02.
Submit the transaction by clicking the Send button

Transaction2.cdc

import ApprovalVoting from 0x01
// This transaction allows the administrator of the Voting contract
// to create a new ballot and store it in a voter's account
// The voter and the administrator have to both sign the transaction
// so it can access their storage
transaction {
    prepare(admin: AuthAccount, voter: AuthAccount) {
        // borrow a reference to the admin Resource
        let adminRef = admin.borrow<&ApprovalVoting.Administrator>(from: /storage/VotingAdmin)!
        // create a new Ballot by calling the issueBallot
        // function of the admin Reference
        let ballot <- adminRef.issueBallot()
        // store that ballot in the voter's account storage
        voter.save<@ApprovalVoting.Ballot>(<-ballot, to: /storage/Ballot)
        log("Ballot transferred to voter")
    }
}

This transaction has two signers as prepare parameters, so it is able to access both of their private AuthAccount objects, and therefore their private account storage.

Because of this, we can perform a direct transfer of the Ballot by creating it with the admin's issueBallot function and then directly store it in the voter's storage by using the save function.

Account 0x02 should now have a Ballot resource object in its account storage. You can confirm this by selecting 0x02 from the lower-left sidebar and seeing Ballot resource listed under the Storage field.

Casting a Vote

Now that account 0x02 has a Ballot in their storage, they can cast their vote. To do this, they will call the vote method on their stored resource, then cast that Ballot by passing it to the cast function in the main smart contract.

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Open Transaction 3 which should contain Transaction3.cdc.
Select account 0x02 as the only transaction signer.
Click the send button to submit the transaction.

Transaction3.cdc

import ApprovalVoting from 0x01
// This transaction allows a voter to select the votes they would like to make
// and cast that vote by using the castVote function
// of the ApprovalVoting smart contract
transaction {
    prepare(voter: AuthAccount) {
        // take the voter's ballot our of storage
        let ballot <- voter.load<@ApprovalVoting.Ballot>(from: /storage/Ballot)!
        // Vote on the proposal
        ballot.vote(proposal: 1)
        // Cast the vote by submitting it to the smart contract
        ApprovalVoting.cast(ballot: <-ballot)
        log("Vote cast and tallied")
    }
}

In this transaction, the user votes for one of the proposals, and then moves their Ballot back to the smart contract via the cast() method where the vote is tallied.

Reading the result of the vote

At any time, anyone could read the current tally of votes by directly reading the fields of the contract. You can use a script to do that, since it does not need to modify storage.

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Open a Script 1 which should contain the code below.
Click the execute button to run the script.

Script1.cdc

import ApprovalVoting from 0x01
// This script allows anyone to read the tallied votes for each proposal
//
access(all) fun main() {
    // Access the public fields of the contract to log
    // the proposal names and vote counts
    log("Number of Votes for Proposal 1:")
    log(ApprovalVoting.proposals[0])
    log(ApprovalVoting.votes[0])
    log("Number of Votes for Proposal 2:")
    log(ApprovalVoting.proposals[1])
    log(ApprovalVoting.votes[1])
}

You should see something like this print:

"Number of Votes for Proposal 1:"
"Longer Shot Clock"
0
"Number of Votes for Proposal 2:"
"Trampolines instead of hardwood floors"
1

This shows that one vote was cast for proposal 1 and no votes were cast for proposal 2.

Other Voting possibilities

This contract was a very simple example of voting in Cadence. It clearly couldn't be used for a real-world voting situation, but hopefully you can see what kind of features could be added to it to ensure practicality and security.

A Voting Contract in Cadence​

Deploy the Contract​

Perform Voting​

Selecting multiple Accounts as Signers​

Casting a Vote​

Reading the result of the vote​

Other Voting possibilities​

A Voting Contract in Cadence

Deploy the Contract

Perform Voting

Selecting multiple Accounts as Signers

Casting a Vote

Reading the result of the vote

Other Voting possibilities