Factory Method: decouple traits and implementations

The aim is to demonstrate a value of the Factory Method
by decoupling interfaces and implementations.
Although, the full decoupling is going to be demonstrated in
the Abstract Factory example.

Author: fadeevab

creational/factory-method/README.md

@@ -6,10 +6,9 @@ of objects that will be created._
 
 ## Maze Game
 
-Implementing the Factory Method pattern using **static dispatch** (generics).
-
 This example reproduces one from the GoF Design Patterns book:
-https://en.wikipedia.org/wiki/Factory_method_pattern
+https://en.wikipedia.org/wiki/Factory_method_pattern, implementing
+the Factory Method pattern using generics (_static dispatch_).
 
 ### How to Run
 
@@ -32,10 +31,24 @@ Ordinary Room: #1
 
 ## Render Dialog
 
-Implementing the Factory Method via **dynamic dispatch**.
+This example shows a GUI framework can organize its types into
+independent modules:
+
+1. The `gui` module defines interfaces for all the components.
+   It has no external dependencies.
+2. The `html_gui` module provides HTML implementation of the base GUI.
+   Depends on `gui`.
+3. The `windows_gui` module provides Windows 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
+defined by the `gui` module.
 
-See [Factory Method Java Example](https://refactoring.guru/design-patterns/factory-method/java/example)
-as a reference.
+The [Abstract Factory example](../abstract-factory/) demonstrates even greater
+separation of Factory interface and its implementations.
 
 ### How to Run
 
@@ -46,7 +59,12 @@ cargo run --bin factory-method-render-dialog
 ### Output
 
 ```
+-- No OS detected, creating the HTML GUI --
 <button>Test Button</button>
 Click! Button says - 'Hello World!'
 Dialog - Refresh
 ```
+
+### Reference
+
+This example reproduces [Factory Method Java Example](https://refactoring.guru/design-patterns/factory-method/java/example).

creational/factory-method/render-dialog/button/mod.rs

@@ -1,12 +1,13 @@
-pub mod html;
-pub mod windows;
-
-use crate::button::Button;
+pub trait Button {
+    fn render(&self);
+    fn on_click(&self);
+}
 
 /// Dialog has a factory method `create_button`.
 ///
 /// It creates different buttons depending on a factory implementation.
 pub trait Dialog {
+    /// The factory method. It must be overridden with a concrete implementation.
     fn create_button(&self) -> Box<dyn Button>;
 
     fn render(&self) {

creational/factory-method/render-dialog/dialog/html.rs

@@ -1,4 +1,4 @@
-use super::Button;
+use crate::gui::{Button, Dialog};
 
 pub struct HtmlButton;
 
@@ -12,3 +12,12 @@ impl Button for HtmlButton {
         println!("Click! Button says - 'Hello World!'");
     }
 }
+
+pub struct HtmlDialog;
+
+impl Dialog for HtmlDialog {
+    /// Creates an HTML button.
+    fn create_button(&self) -> Box<dyn Button> {
+        Box::new(HtmlButton)
+    }
+}

creational/factory-method/render-dialog/dialog/windows.rs

@@ -0,0 +1,14 @@
+use crate::gui::Dialog;
+use crate::html_gui::HtmlDialog;
+use crate::windows_gui::WindowsDialog;
+
+pub fn initialize() -> &'static dyn Dialog {
+    // The dialog type is selected depending on the environment settings or configuration.
+    if cfg!(windows) {
+        println!("-- Windows detected, creating Windows GUI --");
+        return &WindowsDialog;
+    } else {
+        println!("-- No OS detected, creating the HTML GUI --");
+        return &HtmlDialog;
+    };
+}

creational/factory-method/render-dialog/dialog/mod.rs renamed to creational/factory-method/render-dialog/gui.rs

@@ -1,19 +1,15 @@
-mod button;
-mod dialog;
+mod gui;
+mod html_gui;
+mod init;
+mod windows_gui;
 
-use dialog::{html::HtmlDialog, windows::WindowsDialog, Dialog};
+use init::initialize;
 
 fn main() {
-    // The dialog factory is constructed depending on unpredictable input
-    // (not really unpredictable in this case, but let's imagine it).
-    // We need to put a new object into a `Box` pointer, then it's going to be
-    // used uniformly throughout a code.
-    let dialog: Box<dyn Dialog> = if cfg!(windows) {
-        Box::new(WindowsDialog)
-    } else {
-        Box::new(HtmlDialog)
-    };
-
+    // The rest of the code doesn't depend on specific dialog types, because
+    // it works with all dialog objects via the abstract `Dialog` trait
+    // which is defined in the `gui` module.
+    let dialog = initialize();
     dialog.render();
     dialog.refresh();
 }

creational/factory-method/render-dialog/button/html.rs renamed to creational/factory-method/render-dialog/html_gui.rs

@@ -1,4 +1,4 @@
-use super::Button;
+use crate::gui::{Button, Dialog};
 
 pub struct WindowsButton;
 
@@ -12,3 +12,12 @@ impl Button for WindowsButton {
         println!("Click! Hello, Windows!");
     }
 }
+
+pub struct WindowsDialog;
+
+impl Dialog for WindowsDialog {
+    /// Creates a Windows button.
+    fn create_button(&self) -> Box<dyn Button> {
+        Box::new(WindowsButton)
+    }
+}