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 (access(all)), 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:
_32_32// BAD Practice: Do not hard code storage paths_32access(all)_32contract NamedFields {_32_32 access(all)_32 resource Test {}_32_32 init() {_32 // BAD: Hard-coded storage path_32 self.account.storage.save(<-create Test(), to: /storage/testStorage)_32 }_32}_32_32// GOOD practice: Instead, use a field_32//_32access(all)_32contract NamedFields {_32 _32 access(all)_32 resource Test {}_32_32 // GOOD: field storage path_32 access(all)_32 let TestStoragePath: StoragePath_32_32 init() {_32 // assign and access the field here and in transactions_32 self.TestStoragePath = /storage/testStorage_32 self.account.storage.save(<-create Test(), to: self.TestStoragePath)_32 }_32}
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.
This saves the time and fees required to read a property using a transaction.
Making the field or function access(all) 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:
_10// BAD: Field is private, so it cannot be read by the public_10access(self)_10let totalSupply: UFix64_10_10// GOOD: Field is public, so it can be read and used by anyone_10access(all)_10let 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​
_79access(all)_79contract AContract {_79_79 access(all)_79 let BResourceStoragePath: StoragePath_79_79 access(all)_79 let BResourcePublicPath: PublicPath_79_79 init() {_79 self.BResourceStoragePath = /storage/BResource_79 self.BResourcePublicPath = /public/BResource_79 }_79_79 // Resource definition_79 access(all)_79 resource BResource {_79_79 access(all)_79 var c: UInt64_79_79 access(all)_79 var d: String_79_79_79 // Generate a struct with the same fields_79 // to return when a script wants to see the fields of the resource_79 // without having to return the actual resource_79 access(all)_79 fun generateReport(): BReportStruct {_79 return BReportStruct(c: self.c, d: self.d)_79 }_79_79 init(c: UInt64, d: String) {_79 self.c = c_79 self.d = d_79 }_79 }_79_79 // Define a struct with the same fields as the resource_79 access(all)_79 struct BReportStruct {_79_79 access(all)_79 var c: UInt64_79_79 access(all)_79 var d: String_79_79 init(c: UInt64, d: String) {_79 self.c = c_79 self.d = d_79 }_79_79 }_79}_79..._79// Transaction_79import AContract from 0xAContract_79_79transaction {_79 prepare(acct: auth(IssueStorageCapabilityController, PublishCapability) &Account) {_79 //..._79 let cap = acct.capabilities.storage.issue<&AContract.BResource>(AContract.BResourceStoragePath)_79 acct.capabilities.publish(cap, at: AContract.BResourcePublicPath)_79 }_79}_79// Script_79import AContract from 0xAContract_79_79// Return the struct with a script_79access(all)_79fun main(account: Address): AContract.BReportStruct {_79 // borrow the resource_79 let b = getAccount(account).capabilities_79 .borrow<&AContract.BResource>(AContract.BResourcePublicPath)_79 // return the struct_79 return b.generateReport()_79}
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 initializer
and deliver them to an address or &Account specified as an argument.
See how this is done in the LockedTokens contract initializer:
and in the transaction that is used to deploy it:
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​
_37// BAD: Unclear naming_37//_37access(all)_37contract Tax {_37 _37 // Do not use abbreviations unless absolutely necessary_37 access(all)_37 var pcnt: UFix64_37_37 // Not clear what the function is calculating or what the parameter should be_37 access(all)_37 fun calculate(num: UFix64): UFix64 {_37 // What total is this referring to?_37 let total = num + (num * self.pcnt)_37_37 return total_37 }_37}_37_37// GOOD: Clear naming_37//_37access(all)_37contract TaxUtilities {_37_37 // Clearly states what the field is for_37 access(all)_37 var taxPercentage: UFix64_37_37 // Clearly states that this function calculates the_37 // total cost after tax_37 access(all)_37 fun calculateTotalCostPlusTax(preTaxCost: UFix64): UFix64 {_37 let postTaxCost = preTaxCost + (preTaxCost * self.taxPercentage)_37_37 return postTaxCost_37 }_37}
Plural names for arrays and maps are preferable​
For example, use 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.
_22// Pseudo-code_22_22transaction {_22_22 access(all)_22 let buyerCollectionRef: &NonFungibleToken.Collection_22_22 prepare(acct: auth(BorrowValue) &Account) {_22_22 // Get tokens to buy and a collection to deposit the bought NFT to_22 let temporaryVault <- vaultRef.withdraw(amount: 10.0)_22 let self.buyerCollectionRef = acct.storage.borrow(from: /storage/Collection)_22_22 // purchase, supplying the buyers collection reference_22 saleRef.purchase(tokenID: 1, recipient: self.buyerCollectionRef, buyTokens: <-temporaryVault)_22_22 }_22 post {_22 // verify that the buyer now owns the NFT_22 self.buyerCollectionRef.idExists(1) == true: "Bought NFT ID was not deposited into the buyers collection"_22 }_22}
Avoid unnecessary load and save storage operations, prefer in-place mutations​
Problem​
When modifying data in account storage, load() and save() are costly operations:
All data is unnecessarily moved out of the account, then moved back into the account.
This can quickly cause your transaction to reach its limits.
This also applies to nested, stored in fields, arrays, and dictionaries: Moving objects out of containers and moving them back into the container, just to modify the object, 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: the dictionary can be updated in-place, which is easier and more efficient. 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.
Fields and value in containers, such as in arrays and dictionaries,
can be borrowed using a reference expression (&v as &T).
Example​
_36// BAD: Loads and stores a resource to use it_36//_36transaction {_36_36 prepare(acct: auth(LoadValue, SaveValue) &Account) {_36_36 // Removes the vault from storage, a costly operation_36 let vault <- acct.storage.load<@ExampleToken.Vault>(from: /storage/exampleToken)_36_36 // Withdraws tokens_36 let burnVault <- vault.withdraw(amount: 10)_36_36 destroy burnVault_36_36 // Saves the used vault back to storage, another costly operation_36 acct.storage.save(to: /storage/exampleToken)_36_36 }_36}_36_36// GOOD: Uses borrow instead to avoid costly operations_36//_36transaction {_36_36 prepare(acct: auth(BorrowValue) &Account) {_36_36 // Borrows a reference to the stored vault, much less costly operation_36 let vault <- acct.storage.borrow<&ExampleToken.Vault>(from: /storage/exampleToken)_36_36 let burnVault <- vault.withdraw(amount: 10)_36_36 destroy burnVault_36_36 // No `save` required because we only used a reference_36 }_36}
Capabilities
Capability bootstrapping​
Problem​
An account must be given a capability to an object stored in another account. To create (issue) 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 authorized in order to only give recipients access to the functionality that the provider is willing to allow them to copy.
Check for existing capability before publishing new one​
Problem​
When publishing a capability, a capability might be already be published at the specified path.
Solution​
Check if a capability is already published at the given path.
Example​
_13transaction {_13 prepare(signer: auth(Capabilities) &Account) {_13 let capability = signer.capabilities.storage_13 .issue<&ExampleToken.Vault>(/storage/exampleTokenVault)_13_13 let publicPath = /public/exampleTokenReceiver_13_13 if signer.capabilities.exits(publicPath) {_13 signer.capabilities.unpublish(publicPath)_13 }_13 signer.capabilities.publish(capability, at: publicPath)_13 }_13}