Open the starter code for this tutorial in the Flow Playground. It is the same code that was in the previous tutorial:
The tutorial will ask you to take various actions to interact with this code.
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.
This tutorial builds on the previous
Before beginning this tutorial, you should have an idea of how accounts,transactions,resources, and signers work with basic field types.
This tutorial will build on your understanding of accounts and resources.
You'll learn how to interact with resources using capabilities
In Cadence, resources are a composite type like a struct or a class, but with some special rules:
- Each instance of a resource can only exist in exactly one location and cannot be copied.
- Resources must be explicitly moved from one location to another when accessed.
- Resources also cannot go out of scope at the end of function execution, they must be explicitly stored somewhere or destroyed.
Use-Cases for Capabilities and Scripts
Let's look at why you would want to use capabilities to expand access to resources in a real-world context.
A real user's account will contain functions and fields that need varying levels of access scope and privacy. For example, if you're working on an app that allows users to exchange tokens. While you definitely want to make a feature like withdrawing tokens from an account only accessible by the owner of the tokens, your app should allow anybody to deposit tokens.
Capabilities are what allows for this detailed control of owned assets. They allow a user to indicate which of the functionality of their account should be accessible to themselves, their trusted friends, and the public.
For example, a user might want to allow a friend of theirs to use some of their money to spend, in this case, they could create a capability that gives the friend access to only this part of their account, instead of having to give full control over.
Or if a user authenticates a trading app for the first time, they could ask the user for a capability object that allows the app to access the trading functionality of a user's account so that the app doesn't need to ask the user for a signature every time.
In this tutorial, you will:
- Interact with the resource we created using transactions
- Create capabilities to extend the resource access scope
- Execute a script that interacts with the resource
Accessing Resources with Capabilities
Before following this tutorial, you should have the
HelloWorld contract deployed in account
just like in the previous
Resource contract tutorial.
Open the Account
0x01 tab with file named
HelloWorldResource.cdc should contain the following code:
Deploy this code to account
0x01 using the
Click on the
Create Hello transaction and send it with
0x01 as the signer.
The contract and transaction above creates and stores the resource we'll be using in this tutorial. For a more detailed breakdown of the contract, have a look at the previous tutorial.
Creating Capabilities and References to Stored Resources
You need explicit permission from the owner of an account to access its storage. Capabilities allow an account owner to grant access to specific fields and functions stored in their accounts. (Explained more below)
In this transaction, you create a new capability,
then use the
link function to create a public link to your
HelloAsset resource object.
Next you use that link to borrow a reference
to the underlying object and call the
A detailed explanation of what is happening in this transaction
is below the transaction code so, if you feel lost, keep reading!
Open the transaction named
Create Link should contain the following code:
0x01 is still selected as a transaction signer.
Send button to send the transaction.
In this transaction, we use the prepare phase to:
- Create a capability with the
linkmethod to the stored object
HelloWorld.HelloAssetfrom the account path
- Store the capability in the account path
- Use the
borrowmethod to create a reference to the object we linked to called
- Call the
hello()function using the reference we created,
You should see
"Hello, World" show up in the console again.
You might be confused that we were able to call a method on the
without actually being directly in control of it!
It is also stored in the
/storage/ domain of the account, which should be private.
This is because we created a capability for the
Capabilities are kind of like pointers in other languages, but which much more fine-grained control.
Capability Based Access Control
Capabilities allow the owners of objects to specify what functionality of their private objects is available to others. Think of it kind of like an account's API, if you're familiar with the concept. The account owner has private objects stored in their storage, like their collectibles or their money, but they might still want others to be able to see what collectibles they have in their account, or they want to allow anyone to deposit more money of a certain currency in their account. Since these objects are stored in private storage by default, the owner has to do something to open up access to these while still retaining full control. We create capabilities to accomplish this.
In our example, the owner of
HelloAsset might still want to let other people call the
This is what capabilities are for. They represent a link to an object in an account's storage that has the type specified when the link is created.
It is important to remember that someone else who has this capability cannot move or destroy the object that the capability is linked to!
They can only access fields that the owner has explicitly declared in the type specification of the
link method (described below).
Capabilities do not have any meaningful functionality on their own, but every capability has a
which creates a reference to the object that the capability is linked to.
This reference is used to read fields or call methods on the object they reference
as if the owner of the reference had the actual object.
Note that this only allows access to fields and methods. It does not allow copying, moving, or modifying the original object directly.
Let's break down what is happening in this transaction.
First, we create a public link to the private
HelloAsset object in
_10let capability = account.link<&HelloWorld.HelloAsset>(/public/Hello, target: /storage/Hello)
link method returns a capability that can be used to access this link.
HelloAsset object is stored in
/storage/HelloAssetTutorial, which only the account owner can access.
They want any user in the network to be able to call the
hello() method. So they make a public capability in
To create a capability, we use the
AuthAccount.link method to link a new capability to an object in storage.
The type contained in
<> is the reference type that the capability represents.
The capability says that whoever borrows a reference from this capability can only have access to the fields and methods
that are specified by the type in
The specified type has to be a subtype of the type of the object being linked to,
meaning that it cannot contain any fields or functions that the linked object doesn't have.
A reference is referred to by the
& symbol. Here, the capability references the
so we specify
<&HelloWorld.HelloAsset> as the type, which gives access to everything in the
The first argument to the
link function is the path where you want to store the link for the capability
target argument is the path to the object in storage that is to be linked to.
We always store links for capabilities in the
- We choose
/private/if we only want to allow one or a small number of users to access it
- We choose
/public/if we want any user in the network to be able to access it.
Capabilities always link to objects in the
To borrow a reference to an object from the capability, we use the capability's
_10let helloReference = capability.borrow()_10?? panic("Could not borrow a reference to the hello capability")
This method creates the reference as the type we specified in
<> in the
While borrowing the reference, we use
because the borrowing of the reference could fail.
The reference could be
nil if the targeted storage slot is empty, is already borrowed,
or if the requested type exceeds what is allowed by the capability.
We panic with a descriptive error message so the caller can know better what went wrong.
We separate this process into capabilities and references to protect against reentrancy attacks. A reentrancy attack is where a malicious actor could call into an object multiple times. These attacks have plagued other smart contract languages. Only one reference to an object can exist at a time, so this type of vulnerability isn't possible for objects in storage when you use Cadence.
Additionally, the owner of an object can effectively revoke capabilities they have created by moving the underlying object or destroying the link with the
If the referenced object is moved or the link is destroyed, capabilities that have been created from that link are invalidated.
You can find more detailed documentation about capabilities in the language reference.
Now, anyone can call the
hello() method on your
HelloAsset object by borrowing a reference with your public capability in
(Covered in the next section)
Lastly, we call the
hello() method with our borrowed reference:
_10// Call the hello function using the reference to the HelloAsset resource_10log(helloReference.hello())
At the end of the transaction execution, the
helloReference value is lost,
but that is ok because while it references a resource, it isn't the actual resource itself, so it is ok to lose it.
In the next section, we look at how capabilities can expand the access a script has to an account.
A script is a very simple transaction type in Cadence that cannot perform any writes to the blockchain and can only read the state of an account or contract.
To execute a script, write a function called
access(all) fun main().
You can click the execute script button to run the script.
The result of the script will be printed to the console output.
Open the file
Script1.cdc should look like the following:
Here's what this script does:
- It gets an
getAccountand assigns it to the variable
- Borrows a reference using the
borrowmethod for the capability from the
Create Linktransaction and assigns it to
- Logs the result of the
helloReferenceto the console.
_10let helloAccount = getAccount(0x01)
Account reference is available to anyone in the network for every account,
but only has access to a small subset of functions that can be read from the
/public/ domain in an account.
Then, the script borrows the capability that was created in
_10// Borrow the public capability from the public path of the owner's account_10let helloReference = helloAccount.capabilities_10.borrow<&HelloWorld.HelloAsset>(/public/HelloAssetTutorial)_10?? panic("Could not borrow a reference to the hello capability")
To borrow a capability that is stored in an account, use the
borrow() returns a reference to the storage object that the capability targets.
The borrow will fail if the capability does not exist,
the capabilities target storage path does not store a value,
or the value cannot be borrowed with the given type.
Then, the script uses the reference to call the
hello() function and prints the result.
Let's execute the script to see it run correctly.
Execute button in the playground.
You should see something like this print:
_10> "Hello, World"_10> Result > "void"
Good work! Your script ran successfully.
One other really cool feature of scripts is that since they can't actually change anything on chain, they can access any accounts' private storage and objects. This allows scripts even more power to understand the full state of the chain and it is safe because they can't actually make any changes. Also, everything on-chain is publicly readable anyway, so it is a logical feature for a blockchain programming language to have.
A script can get the AuthAccount for an account address using the built-in getAuthAccount function:
_10fun getAuthAccount(_ address: Address): AuthAccount
See the language reference for more information about accounts.
This tutorial expanded on the idea of resources in Cadence by expanding access scope to a resource using capabilities and covering more account storage API use-cases.
You deployed a smart contract with a resource, then created a capability to grant access to that resource.
With the capability, you used the borrow method to create a reference and used the reference to call the
Finally, you used a script to borrow the same capability and create a reference so that the script can
call the resource's
hello() function. This is important because script's cannot access account storage
without using capabilities.
Now that you have completed the tutorial, you have the basic knowledge to write a simple Cadence program that can:
- Implement a resource in a smart contract
- Create capabilities to grant access to resources in an account
- Interact with resources using both signed transactions and scripts
Feel free to modify the smart contract to create different resources, experiment with the available account storage API, and write new transactions and scripts that execute different functions from your smart contract. Have a look at the capability-based access control page to find out more about what you can do with capabilities.
You're on the right track to building more complex applications with Cadence, now is a great time to check out the Cadence Best Practices document and Anti-patterns document as your applications become more complex.