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Cadence Design Patterns

This is a selection of software design patterns developed by core Flow developers while writing Cadence code for deployment to Flow Mainnet.

Many of these design patters apply to most other programming languages, but some are specific to Cadence.

Design patterns are building blocks for software development. They may provide a solution to a problem that you encounter when writing smart contracts in Cadence. If they do not clearly fit, these patterns may not be the right solution for a given situation or problem. They are not meant to be rules to be followed strictly, especially where a better solution presents itself.

General

These are general patterns to follow when writing smart contracts.

Use named value fields for constants instead of hard-coding

Problem

Your contracts, resources, and scripts all have to refer to the same value. A number, a string, a storage path, etc. Entering these values manually in transactions and scripts is a potential source of error. See Wikipedia's page on magic numbers

Solution

Add a public (pub), constant (let) field, e.g. a Path , to the contract responsible for the value, and set it in the contract's initializer. Refer to that value via this public field rather than specifying it manually.

Example Snippet:

// BAD Practice: Do not hard code storage paths
pub contract NamedFields {
    pub resource Test {}
    init() {
        // BAD: Hard-coded storage path
        self.account.save(<-create Test(), to: /storage/testStorage)
    }
}
// GOOD practice: Instead, use a field
//
pub contract NamedFields {
    pub resource Test {}
    // GOOD: field storage path
    pub let TestStoragePath: StoragePath
    init() {
        // assign and access the field here and in transactions
        self.TestStoragePath = /storage/testStorage
        self.account.save(<-create Test(), to: self.TestStoragePath)
    }
}

Example Code

Script-Accessible public field/function

Data availability is important in a blockchain environment. It is useful to publicize information about your smart contract and the assets it controls so other smart contracts and apps can easily query it.

Problem

Your contract, resource or struct has a field or resource that will need to be read and used on or off-chain, often in bulk.

Solution

Make sure that the field can be accessed from a script (using a PublicAccount) rather than requiring a transaction (using an AuthAccount). This saves the time and fees required to read a property using a transaction. Making the field or function pub and exposing it via a /public/ capability will allow this.

Be careful not to expose any data or functionality that should be kept private when doing so.

Example:

// BAD: Field is private, so it cannot be read by the public
access(self) let totalSupply: UFix64
// GOOD: Field is public, so it can be read and used by anyone
pub let totalSupply: UFix64

Script-Accessible report

Problem

Your contract has a resource that you wish to access fields of. Resources are often stored in private places and are hard to access. Additionally, scripts cannot return resources to the external context, so a struct must be used to hold the data.

Solution

Return a reference to a resource if the data from a single resource is all that is needed. Otherwise, declare a struct to hold the data that you wish to return from the script. Write a function that fills out the fields of this struct with the data from the resource that you wish to access. Then call this on the resource that you wish to access the fields of in a script, and return the struct from the script.

See Script-Accessible public field/function, above, for how best to expose this capability.

Example Code

pub contract AContract {
    pub let BResourceStoragePath: StoragePath
    pub let BResourcePublicPath: PublicPath
    init() {
        self.BResourceStoragePath = /storage/BResource
        self.BResourcePublicPath = /public/BResource
    }
    // Resource definition
    pub resource BResource {
        pub var c: UInt64
        pub var d: String
        // Generate a struct with the same fields
        // to return when a script wants to see the fields of the resource
        // without having to return the actual resource
        pub fun generateReport(): BReportStruct {
            return BReportStruct(c: self.c, d: self.d)
        }
        init(c: UInt64, d: String) {
            self.c = c
            self.d = d
        }
    }
    // Define a struct with the same fields as the resource
    pub struct BReportStruct {
        pub var c: UInt64
        pub var d: String
        init(c: UInt64, d: String) {
            self.c = c
            self.d = d
        }
    }
}
...
// Transaction
import AContract from 0xAContract
transaction {
        prepare(acct: AuthAccount) {
            //...
            acct.link<&AContract.BResource>(AContract.BResourcePublicPath, target: AContract.BResourceStoragePath)
        }
}
// Script
import AContract from 0xAContract
// Return the struct with a script
pub fun main(account: Address): AContract.BReportStruct {
    // borrow the resource
    let b = getAccount(account)
        .getCapability(AContract.BResourcePublicPath)
        .borrow<&AContract.BResource>()
    // return the struct
    return b.generateReport()
}

Init Singleton

Problem

An admin resource must be created and delivered to a specified account. There should not be a function to do this, as that would allow anyone to create an admin resource.

Solution

Create any one-off resources in the contract's init() function and deliver them to an address or AuthAccount specified as an argument.

See how this is done in the LockedTokens contract init function:

LockedTokens.cdc

and in the transaction that is used to deploy it:

admin_deploy_contract.cdc

Use descriptive names for fields, paths, functions and variables

Problem

Smart contracts often are vitally important pieces of a project and often have many other smart contracts and applications that rely on them. Therefore, they need to be clearly written and easy to understand.

Solution

All fields, functions, types, variables, etc., need to have names that clearly describe what they are used for.

account / accounts is better than array / element.

providerAccount / tokenRecipientAccount is better than acct1 / acct2.

/storage/bestPracticesDocsCollectionPath is better than /storage/collection

Example

// BAD: Unclear naming
//
pub contract Tax {
    // Do not use abbreviations unless absolutely necessary
    pub var pcnt: UFix64
    // Not clear what the function is calculating or what the parameter should be
    pub fun calculate(num: UFix64): UFix64 {
        // What total is this referring to?
        let total = num + (num * self.pcnt)
        return total
    }
}
// GOOD: Clear naming
//
pub contract TaxUtilities {
    // Clearly states what the field is for
    pub var taxPercentage: UFix64
    // Clearly states that this function calculates the
    // total cost after tax
    pub fun calculateTotalCostPlusTax(preTaxCost: UFix64): UFix64 {
        let postTaxCost = preTaxCost + (preTaxCost * self.taxPercentage)
        return postTaxCost
    }
}

Include concrete types in type constraints, especially "Any" types

Problem

When specifying type constraints for capabilities or borrows, concrete types often do not get specified, making it unclear if the developer actually intended it to be unspecified or not. Paths also use a shared namespace between contracts, so an account may have stored a different object in a path that you would expect to be used for something else. Therefore, it is important to be explicit when getting objects or references to resources.

Solution

A good example of when the code should specify the type being restricted is checking the FLOW balance: The code must borrow &FlowToken.Vault{FungibleToken.Balance}, in order to ensure that it gets a FLOW token balance, and not just &{FungibleToken.Balance}, any balance – the user could store another object that conforms to the balance interface and return whatever value as the amount.

When the developer does not care what the concrete type is, they should explicitly indicate that by using &AnyResource{Receiver} instead of &{Receiver}. In the latter case, AnyResource is implicit, but not as clear as the former case.

Plural names for arrays and maps are preferable

e.g. accounts rather than account for an array of accounts.

This signals that the field or variable is not scalar. It also makes it easier to use the singular form for a variable name during iteration.

Use transaction post-conditions when applicable

Problem

Transactions can contain any amount of valid Cadence code and access many contracts and accounts. The power of resources and capabilities means that there may be some behaviors of programs that are not expected.

Solution

It is usually safe to include post-conditions in transactions to verify the intended outcome.

Example

This could be used when purchasing an NFT to verify that the NFT was deposited in your account's collection.

// Psuedo-code
transaction {
    pub let buyerCollectionRef: &NonFungibleToken.Collection
    prepare(acct: AuthAccount) {
        // Get tokens to buy and a collection to deposit the bought NFT to
        let temporaryVault <- vaultRef.withdraw(amount: 10.0)
        let self.buyerCollectionRef = acct.borrow(from: /storage/Collection)
        // purchase, supplying the buyers collection reference
        saleRef.purchase(tokenID: 1, recipient: self.buyerCollectionRef, buyTokens: <-temporaryVault)
    }
    post {
        // verify that the buyer now owns the NFT
        self.buyerCollectionRef.idExists(1) == true: "Bought NFT ID was not deposited into the buyers collection"
    }
}

Avoid excessive load and save storage operations (prefer in-place mutations)

Problem

When modifying data in account storage, load() and save() are costly operations. This can quickly cause your transaction to reach the gas limit or slow down the network.

This also applies to contract objects and their fields (which are implicitly stored in storage, i.e. read from/written to), or nested resources. Loading them from their fields just to modify them and save them back is just as costly.

For example, a collection contains a dictionary of NFTs. There is no need to move the whole dictionary out of the field, update the dictionary on the stack (e.g. adding or removing an NFT), and then move the whole dictionary back to the field, it can be updated in-place. The same goes for a more complex data structure like a dictionary of nested resources: Each resource can be updated in-place by taking a reference to the nested object instead of loading and saving.

Solution

For making modifications to values in storage or accessing stored objects, borrow() should always be used to access them instead of load or save unless absolutely necessary. borrow() returns a reference to the object at the storage path instead of having to load the entire object. This reference can be assigned to or can be used to access fields or call methods on stored objects.

Example

// BAD: Loads and stores a resource to use it
//
transaction {
    prepare(acct: AuthAccount) {
        // Removes the vault from storage, a costly operation
        let vault <- acct.load<@ExampleToken.Vault>(from: /storage/exampleToken)
        // Withdraws tokens
        let burnVault <- vault.withdraw(amount: 10)
        destroy burnVault
        // Saves the used vault back to storage, another costly operation
        acct.save(to: /storage/exampleToken)
    }
}
// GOOD: Uses borrow instead to avoid costly operations
//
transaction {
    prepare(acct: AuthAccount) {
        // Borrows a reference to the stored vault, much less costly operation
        let vault <- acct.borrow<&ExampleToken.Vault>(from: /storage/exampleToken)
        let burnVault <- vault.withdraw(amount: 10)
        destroy burnVault
        // No `save` required because we only used a reference
    }
}

Capabilities

Capability Bootstrapping

Problem

An account must be given a capability to a resource or contract in another account. To create, i.e. link the capability, the transaction must be signed by a key which has access to the target account.

To transfer / deliver the capability to the other account, the transaction also needs write access to that one. It is not as easy to produce a single transaction which is authorized by two accounts as it is to produce a typical transaction which is authorized by one account.

This prevents a single transaction from fetching the capability from one account and delivering it to the other.

Solution

The solution to the bootstrapping problem in Cadence is provided by the Inbox API

Account A (which we will call the provider) creates the capability they wish to send to B (which we will call the recipient), and stores this capability on their account in a place where the recipient can access it using the Inbox.publish function on their account. They choose a name for the capability that the recipient can later use to identify it, and specify the recipient's address when calling publish. This call to publish will emit an InboxValuePublished event that the recipient can listen for off-chain to know that the Capability is ready for them to claim.

The recipient then later can use the Inbox.claim function to securely grab the capability from the provider's account. They must provide the name and type with which the capability was published, as well as the address of the provider's account (all of this information is available in the InboxValuePublished event emitted on publish). This will remove the capability from the provider's account and emit an InboxValueClaimed event that the provider can listen for off-chain.

One important caveat to this is that the published capability is stored on the provider's account until the recipient claims it, so the provider can also use the Inbox.unpublish function to remove the capability from their account if they no longer wish to pay for storage for it. This also requires the name and type which the capability was published, and emits an InboxValueUnpublished event that the recipient can listen for off-chain.

It is also important to note that the recipient becomes the owner of the capability object once they have claimed it, and can thus store it or copy it anywhere they have access to. This means providers should only publish capabilities to recipients they trust to use them properly, or limit the type with which the capability is restricted in order to only give recipients access to the functionality that the provider is willing to allow them to copy.

Capability Revocation

Problem

A capability provided by one account to a second account must able to be revoked by the first account without the co-operation of the second.

See the Capability Controller FLIP for a proposal to improve this in the future.

Solution

The first account should create the capability as a link to a capability in /private/, which then links to a resource in /storage/. That second-order link is then handed to the second account as the capability for them to use.

Account 1: /private/capability → /storage/resource

/private/revokableLink -> /private/capability

Account 2: /storage/capability -> (Capability(→Account 1: /private/revokableLink))

If the first account wants to revoke access to the resource in storage, they should delete the /private/ link that the second account's capability refers to. Capabilities use paths rather than resource identifiers, so this will break the capability.

The first account should be careful not to create another link at the same location in its private storage once the capability has been revoked, otherwise this will restore the second account's capability.

Check for existing links before creating new ones

When linking a capability, the link might be already present. In that case, Cadence will not panic with a runtime error but the link function will return nil. The documentation states that: The link function does not check if the target path is valid/exists at the time the capability is created and does not check if the target value conforms to the given type. In that sense, it is a good practice to check if the link does already exist with AuthAccount.getLinkTarget before creating it with AuthAccount.link(). AuthAccount.getLinkTarget will return nil if the link does not exist.

Example

transaction {
    prepare(signer: AuthAccount) {
        // Create a public capability to the Vault that only exposes
        // the deposit function through the Receiver interface
        //
        // Check to see if there is a link already and unlink it if there is
        if signer.getLinkTarget(/public/exampleTokenReceiver) != nil {
            signer.unlink(/public/exampleTokenReceiver)
        }
        signer.link<&ExampleToken.Vault{FungibleToken.Receiver}>(
            /public/exampleTokenReceiver,
            target: /storage/exampleTokenVault
        )
    }
}

General

Use named value fields for constants instead of hard-coding​

Problem​

Solution​

Script-Accessible public field/function​

Problem​

Solution​

Script-Accessible report​

Problem​

Solution​

Example Code​

Init Singleton​

Problem​

Solution​

Use descriptive names for fields, paths, functions and variables​

Problem​

Solution​

Example​

Include concrete types in type constraints, especially "Any" types​

Problem​

Solution​

Plural names for arrays and maps are preferable​

Use transaction post-conditions when applicable​

Problem​

Solution​

Example​

Avoid excessive load and save storage operations (prefer in-place mutations)​

Problem​

Solution​

Example​

Capabilities

Capability Bootstrapping​

Problem​

Solution​

Capability Revocation​

Problem​

Solution​

Check for existing links before creating new ones​

Example​

Use named value fields for constants instead of hard-coding

Problem

Solution

Script-Accessible public field/function

Problem

Solution

Script-Accessible report

Problem

Solution

Example Code

Init Singleton

Problem

Solution

Use descriptive names for fields, paths, functions and variables

Problem

Solution

Example

Include concrete types in type constraints, especially "Any" types

Problem

Solution

Plural names for arrays and maps are preferable

Use transaction post-conditions when applicable

Problem

Solution

Example

Avoid excessive load and save storage operations (prefer in-place mutations)

Problem

Solution

Example

Capability Bootstrapping

Problem

Solution

Capability Revocation

Problem

Solution

Check for existing links before creating new ones

Example