Loose Coupling
This guide demonstrates how to reuse a function or type from one pallet in another pallet using a technique called loose coupling.
Loose coupling enables you to reuse part of the logic defined in an external pallet inside your current pallet. This guide illustrates loose coupling by using the trait bounds in a pallet's configuration trait to reuse a type that's declared in an external pallet. In this example, the current pallet makes use of a method for the Currency
trait from the frame_support
pallet.
Steps preview
Configure your workspace manifest to include the external pallet.
Import the trait you want to use.
Add a type to the configuration trait.
Provide the implementation in the runtime.
Configure your workspace manifest
For the pallet in your working directory to reuse code from another pallet, the external pallet must be included in the list of package dependencies that are imported for your pallet. Therefore, the manifest in the Cargo.toml
file for your project must specify the pallet you want to import. In this example, you are reusing Currency
trait information from the frame-support
pallet, so you must ensure that frame-support
is included in the dependencies
and features
sections of the manifest.
To configure your workspace manifest:
Open a terminal shell on your computer and navigate to the root directory for your project.
Open the manifest
Cargo.toml
file in a text editor.Add the you are loosely coupling with to the dependencies.
For example:
Note that you should use the same branch and version information for all of the pallets to ensure that the imported pallets are compatible with each other. Using pallets from different branches can result in compiler errors. This example illustrates adding the
frame-support
pallet to theCargo.toml
file if the other pallets usebranch = "polkadot-v1.0.0"
.Because the build process compiles both a standard binary and the WebAssembly target, you must also include
frame-support/std
in the features for your pallet.Add
frame-support/std
to thestd
features for your pallet.For example:
Save your changes and close the
Cargo.toml
file.
Import the trait you want to use
In this example, you want to use the Currency
trait from the frame_support
pallet so that your current pallet can access its methods.
To import a trait from another pallet:
Open a terminal shell on your computer and navigate to the root directory for your project.
Open the
src/lib.rs
file for your current pallet in a text editor.Import the
Currency
trait by adding the following line:Save your change.
Add a type to the configuration trait
The next step is to create a type that is bound by the type you want to expose in your pallet.
To update the configuration trait for your pallet:
Open a terminal shell on your computer and navigate to the root directory for your project.
Open the
src/lib.rs
file for your current pallet in a text editor.Create a type to use in your pallet that is bound by the type you want to access in the external pallet:
For example:
Use a method that the trait of your loosely-coupled pallet provides with the type you've created to access the method.
For example:
In this example,
total_issuance
is a method that theCurrency
trait exposes from theframe_support
pallet.Save your changes and close the
src/lib.rs
file for your project.
Provide the implementation in the runtime
After you have completed the updates in your project, you are ready to implement the runtime configuration to include the LocalCurrency
that is based on the the Currency
trait.
To update the runtime configuration for your pallet:
Open a terminal shell on your computer and navigate to the root directory for the node template.
Open the
runtime/src/lib.rs
file in a text editor.Add the runtime configuration for your pallet to specify the
LocalCurrency
type to use the implementation defined for theBalances
pallet.Check the
Balances
definition insideconstruct_runtime!
macro.In this example, your pallet can inherit the implementation of the
Currency
trait from thepallet_balances
pallet and access methods from the loosely-coupledframe-support
pallet.By default, the
construct_runtime!
macro includes all pallet attributes for all pallets listed in the macro definition.
Examples
EnsureOrigin
trait in the Democracy pallet
Resources
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