Crate chassis

Compile-time dependency injector.

Let the compiler generate your dependency injection code.

Goals

• Detect errors at compile time like missing dependencies or cyclic dependencies
• No need to annotate your classes (support for third-party classes)
• No required usage of std::sync::Arc
• Zero overhead: Fast as hand-written code
  • No use of runtime type information (Any)

Use

Add chassis to your crate dependencies

chassis = "^0.1.0"

Create a module for your dependency injection logic and annotate it with #[chassis::integration]. The code in this module will be inspected by this attribute.

#[chassis::integration]
mod integration {
   use super::*;

   // ...
}

Structs will be modules that can provide dependencies with functions and that itself can have dependencies. Note: Currently only associated functions are supported!

pub struct Module;
impl Module {
    pub fn provide_something(dep1: Dependency1, dep2: Dependency2) -> Dependency3 {
        Dependency3::new(dep1, dep2)
    }
    // ...
}

Traits will be components. For each trait a implemented component will be created. The generated implementation will have a Impl suffix, for example ComponentImpl. Also a ComponentImpl::new function is created.

pub trait Component {
    fn resolve_main_class(&self) -> MainClass;
}

Example

use std::rc::Rc;

// define your business logic

/// printer trait
pub trait Printer {
    fn print(&self, input: &str);
}

/// a printer implementation
pub struct StdoutPrinter;
impl Printer for StdoutPrinter {
    fn print(&self, input: &str) {
        println!("{}", input);
    }
}

/// greeter for messages
pub struct Greeter {
    message: String,
    printer: Rc<dyn Printer>,
}
impl Greeter {
    /// constructor with dependencies
    pub fn new(message: String, printer: Rc<dyn Printer>) -> Self {
        Self { message, printer }
    }

    /// your business logic
    pub fn say_hello(&self) {
        self.printer.print(&self.message);
    }
}

/// module that is parsed to create the dependency injection code
#[chassis::integration]
mod integration {
    use super::*;

    pub struct DemoModule;

    // use strong types when in need to distinguish
    pub struct Message(String);

    /// Define how to create your dependencies
    impl DemoModule {
        pub fn provide_printer() -> Rc<dyn Printer> {
            Rc::new(StdoutPrinter)
        }

        pub fn provide_message() -> Message {
            Message("Hello World".to_string())
        }

        pub fn provide_greeter(
            message: Message,
            printer: Rc<dyn Printer>
        ) -> Greeter {
            Greeter::new(message.0, printer)
        }
    }

    /// Define which dependencies you need.
    ///
    /// A struct `DemoComponentImpl` will be created for
    /// you which implements `DemoComponent`.
    pub trait DemoComponent {
        /// request the to create injection code for our main class `Greeter`
        fn resolve_greeter(&self) -> Greeter;
    }
}

fn main() {
    // import component trait
    use crate::integration::DemoComponent;

    // use generated component implementation
    let injector = integration::DemoComponentImpl::new();

    // Resolve main dependency
    // Note: it can not fail at runtime!
    let greeter = injector.resolve_greeter();

    // enjoy!
    greeter.say_hello();
}

The generated implementation will roughly look like this (Tip: use cargo-expand to inspect the code):

pub struct DemoComponentImpl{}

impl DemoComponentImpl {
    pub fn new() -> Self { Self {} }
}

impl DemoComponent for DemoComponentImpl {
    fn resolve_greeter(&self) -> Greeter {
        DemoModule::provide_greeter(
            DemoModule::provide_message(),
            DemoModule::provide_printer())
    }
}

Limitations

• Dependencies are looked up through the syntax token
  • Rc<Dep> and Rc< Dep > are the same
  • but Rc<Dep> and Rc<crate::Dep> never
  • Also types aliases with type result in different type keys
• Currently lifetimes in the types are not supported (also 'static)
• Currently generics are not handeled correctly
• Currently only the first error is show at compile time
• Currently modules can not have &self-methods, so inner data is useless
• Currently it is not possible to request a reference to a registered non-reference type
  • Like &MyType when MyType is provided by a module

Singletons

Normaly for every needed dependency the provider function on the module is called. This results in types created multiple times. This is maybe not intended. The solution is to use a singleton attribute. The provide method will than only called once at build time of the component (call to ComponentImpl::new). The requirement is that the type implements the Clone trait. It is recommendable to use a shared reference type like Rc or Arc for singletons so that really only one instance is created.

Example

impl Module {
    #[singleton]
    pub fn provide_printer() -> Rc<dyn Printer> {
        Rc::new(StdoutPrinter)
    }
}

Attribute Macros

integration

Attribute for modules