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Абстрактная фабрика

Абстрактная фабрика на Rust

Абстрактная фабрика — это порождающий паттерн проектирования, который решает проблему создания целых семейств связанных продуктов, без указания конкретных классов продуктов.

Абстрактная фабрика задаёт интерфейс создания всех доступных типов продуктов, а каждая конкретная реализация фабрики порождает продукты одной из вариаций. Клиентский код вызывает методы фабрики для получения продуктов, вместо самостоятельного создания с помощью оператора new. При этом фабрика сама следит за тем, чтобы создать продукт нужной вариации.

GUI Elements Factory

This example illustrates how a GUI framework can organize its classes into independent libraries:

  1. The gui library defines interfaces for all the components.
    It has no external dependencies.
  2. The windows-gui library provides Windows implementation of the base GUI.
    Depends on gui.
  3. The macos-gui library provides Mac OS implementation of the base GUI.
    Depends on gui.

The app is a client application that can use several implementations of the GUI framework, depending on the current environment or configuration. However, most of the app code doesn't depend on specific types of GUI elements. All the client code works with GUI elements through abstract interfaces (traits) defined by the gui lib.

There are two approaches to implementing abstract factories in Rust:

  • using generics (static dispatch)
  • using dynamic allocation (dynamic dispatch)

When you're given a choice between static and dynamic dispatch, there is rarely a clear-cut correct answer. You'll want to use static dispatch in your libraries and dynamic dispatch in your binaries. In a library, you want to allow your users to decide what kind of dispatch is best for them since you don't know what their needs are. If you use dynamic dispatch, they're forced to do the same, whereas if you use static dispatch, they can choose whether to use dynamic dispatch or not.

gui: Abstract Factory and Abstract Products

gui/lib.rs

pub trait Button {
    fn press(&self);
}

pub trait Checkbox {
    fn switch(&self);
}

/// Abstract Factory defined using generics.
pub trait GuiFactory {
    type B: Button;
    type C: Checkbox;

    fn create_button(&self) -> Self::B;
    fn create_checkbox(&self) -> Self::C;
}

/// Abstract Factory defined using Box pointer.
pub trait GuiFactoryDynamic {
    fn create_button(&self) -> Box<dyn Button>;
    fn create_checkbox(&self) -> Box<dyn Checkbox>;
}

macos-gui: One family of products

macos-gui/lib.rs

pub mod button;
pub mod checkbox;
pub mod factory;

windows-gui: Another family of products

windows-gui/lib.rs

pub mod button;
pub mod checkbox;
pub mod factory;

Static dispatch

Here, the abstract factory is implemented via generics which lets the compiler create a code that does NOT require dynamic dispatch in runtime.

app: Client code with static dispatch

app/main.rs

mod render;

use render::render;

use macos_gui::factory::MacFactory;
use windows_gui::factory::WindowsFactory;

fn main() {
    let windows = true;

    if windows {
        render(WindowsFactory);
    } else {
        render(MacFactory);
    }
}

app/render.rs

//! The code demonstrates that it doesn't depend on a concrete
//! factory implementation.

use gui::GuiFactory;

// Renders GUI. Factory object must be passed as a parameter to such the
// generic function with factory invocation to utilize static dispatch.
pub fn render(factory: impl GuiFactory) {
    let button1 = factory.create_button();
    let button2 = factory.create_button();
    let checkbox1 = factory.create_checkbox();
    let checkbox2 = factory.create_checkbox();

    use gui::{Button, Checkbox};

    button1.press();
    button2.press();
    checkbox1.switch();
    checkbox2.switch();
}

Dynamic dispatch

If a concrete type of abstract factory is not known at the compilation time, then is should be implemented using Box pointers.

app-dyn: Client code with dynamic dispatch

app-dyn/main.rs

mod render;

use render::render;

use gui::GuiFactoryDynamic;
use macos_gui::factory::MacFactory;
use windows_gui::factory::WindowsFactory;

fn main() {
    let windows = false;

    // Allocate a factory object in runtime depending on unpredictable input.
    let factory: &dyn GuiFactoryDynamic = if windows {
        &WindowsFactory
    } else {
        &MacFactory
    };

    // Factory invocation can be inlined right here.
    let button = factory.create_button();
    button.press();

    // Factory object can be passed to a function as a parameter.
    render(factory);
}

app-dyn/render.rs

//! The code demonstrates that it doesn't depend on a concrete
//! factory implementation.

use gui::GuiFactoryDynamic;

/// Renders GUI.
pub fn render(factory: &dyn GuiFactoryDynamic) {
    let button1 = factory.create_button();
    let button2 = factory.create_button();
    let checkbox1 = factory.create_checkbox();
    let checkbox2 = factory.create_checkbox();

    button1.press();
    button2.press();
    checkbox1.switch();
    checkbox2.switch();
}

Output

Windows button has pressed
Windows button has pressed
Windows checkbox has switched
Windows checkbox has switched

Абстрактная фабрика на других языках программирования

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