Crate shaku

Shaku is a compile time dependency injection library. It can be used directly or through integration with application frameworks such as Rocket (see shaku_rocket).

Getting started

Note: This getting started guide focuses on components, which live for the lifetime of the application (or, technically, the container). After reading this getting started guide, check out Provider to learn how to create services with shorter lifetimes.

Structure your application

Start with your application's structs and traits. Use Arc<dyn T> for dependencies.

use std::sync::Arc;

trait IOutput {
    fn write(&self, content: String);
}

trait IDateWriter {
    fn write_date(&self);
}

struct ConsoleOutput;

impl IOutput for ConsoleOutput {
    fn write(&self, content: String) {
        println!("{}", content);
    }
}

struct TodayWriter {
    output: Arc<dyn IOutput>,
    today: String,
    year: usize,
}

impl IDateWriter for TodayWriter {
    fn write_date(&self) {
        self.output.write(format!("Today is {}, {}", self.today, self.year));
    }
}

Inherit "Interface" for the interface traits

Interface traits require certain bounds, such as 'static and optionally Send + Sync if using the thread_safe feature. The Interface trait acts as a trait alias for these bounds, and is automatically implemented on types which implement the bounds.

In our example, the two interface traits would become:

use shaku::Interface;

trait IOutput: Interface {
    fn write(&self, content: String);
}

trait IDateWriter: Interface {
    fn write_date(&self);
}

Implement Component

A component is a struct that implements an Interface trait. In our example, we have 2 components:

• TodayWriter of type IDateWriter
• ConsoleOutput of type IOutput

These components must implement Component, which can either be done manually or through a derive macro (using the derive feature):

use shaku::Component;

#[derive(Component)]
#[shaku(interface = IOutput)]
struct ConsoleOutput;

Express dependencies

Components can depend on other components. In our example, TodayWriter requires an IOutput component.

To express this dependency, first make sure the property is declared as a trait object wrapped in an Arc. Then (when using the derive macro) use the #[shaku(inject)] attribute on the property to tell shaku to inject the dependency.

In our example:

use shaku::Component;

#[derive(Component)]
#[shaku(interface = IDateWriter)]
struct TodayWriter {
    #[shaku(inject)]
    output: Arc<dyn IOutput>,
    today: String,
    year: usize,
}

If you don't use the derive macro, add HasComponent bounds to your module generic and inject the dependencies manually with ContainerBuildContext::resolve.

Create a Module

Modules link together components and providers, and are core to providing shaku's compile time guarentees. A Module can be created manually or via the module macro (the derive feature is not necessary):

use shaku::module;

module! {
    MyModule {
        components = [ConsoleOutput, TodayWriter],
        providers = []
    }
}

This module implements HasComponent<dyn IOutput> and HasComponent<dyn IDateWriter> using the provided component implementations.

Build a Container

At application startup, create a Container using a ContainerBuilder. You can use this container to resolve the module's services.

use shaku::{Container, ContainerBuilder};

let container: Container<MyModule> = ContainerBuilder::new().build();
// Alternatively, let container = Container::<MyModule>::default();

Passing parameters

In many cases you need to pass parameters to a component. This can be done during container creation. Each component has an associated parameters type, and the derive generates a *Parameters struct for you (named after the component struct). Use this struct to pass in the parameters.

Note that if you don't pass in parameters, the parameters' default values will be used. You can override the default value by annotating the property with #[shaku(default = ...)].

let container: Container<MyModule> = ContainerBuilder::new()
    .with_component_parameters::<TodayWriter>(TodayWriterParameters {
        today: "Jan 26".to_string(),
        year: 2020
    })
    .build();

Resolve components

Once you created the Container, you can resolve the components.

let writer: &dyn IDateWriter = container.resolve_ref();
writer.write_date(); // Prints "Today is Jan 26, 2020"

Overriding components

Although shaku is a compile time DI library, you can override the implementation of a service during the container build. This can be useful during testing, for example using an in-memory database while doing integration tests. For components, simply pass in a struct instance which implements the interface you want to override to with_component_override:

#[derive(Component)]
#[shaku(interface = IOutput)]
struct FakeOutput;

impl IOutput for FakeOutput {
    fn write(&self, _content: String) {
        // We don't want to actually log stuff during tests
    }
}

let container: Container<MyModule> = ContainerBuilder::new()
    .with_component_override::<dyn IOutput>(Box::new(FakeOutput))
    .with_component_parameters::<TodayWriter>(TodayWriterParameters {
        today: "Jan 26".to_string(),
        year: 2020
    })
    .build();

let writer: &dyn IDateWriter = container.resolve_ref();
writer.write_date(); // Nothing will be printed

The full example

use shaku::{module, Component, Container, ContainerBuilder, Interface};
use std::sync::Arc;

trait IOutput: Interface {
    fn write(&self, content: String);
}

trait IDateWriter: Interface {
    fn write_date(&self);
}

#[derive(Component)]
#[shaku(interface = IOutput)]
struct ConsoleOutput;

impl IOutput for ConsoleOutput {
    fn write(&self, content: String) {
        println!("{}", content);
    }
}

#[derive(Component)]
#[shaku(interface = IDateWriter)]
struct TodayWriter {
    #[shaku(inject)]
    output: Arc<dyn IOutput>,
    today: String,
    year: usize,
}

impl IDateWriter for TodayWriter {
    fn write_date(&self) {
        self.output.write(format!("Today is {}, {}", self.today, self.year));
    }
}

module! {
    MyModule {
        components = [ConsoleOutput, TodayWriter],
        providers = []
    }
}

let container: Container<MyModule> = ContainerBuilder::new()
    .with_component_parameters::<TodayWriter>(TodayWriterParameters {
        today: "Jan 26".to_string(),
        year: 2020
    })
    .build();

let writer: &dyn IDateWriter = container.resolve_ref();
writer.write_date();

Crate features

By default shaku is thread-safe and exposes derive macros, but these can be disabled by opting out of the following features:

• thread_safe: Requires components to be Send + Sync and provided services to be Send
• derive: Uses the shaku_derive crate to provide proc-macro derives of Component and Provider.

Macros

module

Create a Module which is associated with some components and providers.

Structs

Container	Resolves services associated with a Module. A Container is built by a ContainerBuilder, or through the shortcut Container::default
ContainerBuildContext	Builds a Container. This struct is used during Component::build.
ContainerBuilder	Builds a Container. Component parameters can be set, and both components and providers implementations can be overridden.

Traits

Component	Components provide a service by implementing an interface. They may use other components as dependencies.
HasComponent	Indicates that a module contains a component which implements the interface.
HasProvider	Indicates that a module contains a provider which implements the interface.
Interface	Interfaces must be 'static in order to be stored in the container (hence the Any requirement).
Module	A module represents a group of services. By implementing traits such as HasComponent on a module, service dependencies are checked at compile time. At runtime, modules hold the components they are associated with.
ProvidedInterface	Provided interfaces must be 'static in order for the provider to be stored in the container (hence the Any requirement).
Provider	Like Components, providers provide a service by implementing an interface.

Type Definitions

ProviderFn

The type signature of Provider::provide. This is used when overriding a provider via ContainerBuilder::with_provider_override

Derive Macros

Component
Provider