Desing Patterns in Golang: Bridge

Introduction

The Bridge Pattern is a creational design pattern used to decouple an abstraction from its implementation. The bridge uses encapsulation, aggregation, and can use inheritance to separate responsibilities into different classes.

Purpose

Design Pattern Diagram

The objects participating in this pattern are presented on the following diagram:

Bridge Class Diagram

Implementation

Consider building an UI package that supports drawing different shapes on the screen by supporting OpenGL and Direct2D rendering technologies. In our particular example should be able to draw a circle by supporting both rendering systems.

In order to do that we should separate the Circle struct from its drawing implementation:

// Circle represents a circle shape
type Circle struct {
	// DrawingContext for this circle
	DrawingContext Drawer
	// Center of the circle
	Center Point
	// Radius of the circle
	Radius float64
}

// Draw draws a circle
func (circle *Circle) Draw() error {
	rect := Rect{
		Location: Point{
			X: circle.Center.X - circle.Radius,
			Y: circle.Center.Y - circle.Radius,
		},
		Size: Size{
			Width:  2 * circle.Radius,
			Height: 2 * circle.Radius,
		},
	}

	return circle.DrawingContext.DrawEllipseInRect(rect)
}

The Drawer defines the contract between the Circle abstraction and its implementation:

// Drawer draws on the underlying graphics device
type Drawer interface {
	// DrawEllipseInRect draws an ellipse in rectanlge
	DrawEllipseInRect(Rect) error
}

For every of the supported rendering technologies we should implement a struct that obeys the Drawer interface:

This is a sample implementation of OpenGL drawer:

// OpenGL drawer
type OpenGL struct{}

// DrawEllipseInRect draws an ellipse in rectangle
func (gl *OpenGL) DrawEllipseInRect(r Rect) error {
	fmt.Printf("OpenGL is drawing ellipse in rect %v", r)
	return nil
}

This is a sample implementation of Direct2D drawer:

// Direct2D drawer
type Direct2D struct{}

// DrawEllipseInRect draws an ellipse in rectangle
func (d2d *Direct2D) DrawEllipseInRect(r Rect) error {
	fmt.Printf("Direct2D is drawing ellipse in rect %v", r)
	return nil
}

Then we can easily render a circle by using the desired drawing system. We should just change the implementation by setting DrawingContext field:

openGL := &uikit.OpenGL{}
direct2D := &uikit.Direct2D{}

circle := &uikit.Circle{
	Center: uikit.Point{X: 100, Y: 100},
	Radius: 50,
}

circle.DrawingContext = openGL
circle.Draw()

circle.DrawingContext = direct2D
circle.Draw()

Verdict

The Bridge Pattern is designed up-front to let the abstraction and the implementation vary independently. That allows us to independently change the implementation from its abstraction.