Bridge Design Pattern in Go (Golang)

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Note: Interested in understanding how all other design patterns can be implemented in GO. Please see this full reference – All Design Patterns in Go (Golang)

Introduction:

Bridge design pattern is a structural design pattern that allows the separation of abstraction from its implementation. Sounds confusing? Don’t worry, it will be more clear as we go along.

This pattern suggests dividing a large class into two separate hierarchy

  • Abstraction – It is an interface and children of the Abstraction are referred to as Refined Abstraction. The abstraction contains a reference to the implementation.
  • Implementation – It is also an interface and children’s of the Implementation are referred to as Concrete Implementation

Abstraction hierarchy is being referred to by clients without worrying about the implementation. Let’s take an example. Assume you have two types of computer mac and windows. Also, let’s say two types of printer epson and hp . Both computers and printers needs to work with each other in any combination.  The client will only access the computer without worrying about how print is happening. Instead of creating four structs for the 2*2 combination, we create two hierarchies

  • Abstraction Hierarchy
  • Implementation Hierarchy

See the below figure. These two hierarchies communicate with each other via a bridge where Abstraction (computer here) contains a reference to the Implementation(printer here). Both the abstraction and implementation can continue to develop independently without affecting each other.  Notice how win and mac embed the reference to printer. We can change the Abstraction’sImplementation (i.e., computer’s printer) at run time as abstraction refers to implementation via the interface. On calling mac.print() or windows.print() it dispatches the request to printer.printFile(). This acts as a bridge and provides a loose coupling between the two.

UML Diagram:

Mapping 

The below table represents the mapping from the UML diagram actors to actual implementation actors in “Practical Example” below

Abstractioncomputer.go
Refined Abstraction 1win.go
Refined Abstraction 2mac.go
Implementationprinter.go
Concrete Implementation 1epson.go
Concrete Implementation 2hp.go
Clientmain.go

Practical Example

computer.go

package main

type computer interface {
    print()
    setPrinter(printer)
}

mac.go

package main

import "fmt"

type mac struct {
    printer printer
}

func (m *mac) print() {
    fmt.Println("Print request for mac")
    m.printer.printFile()
}

func (m *mac) setPrinter(p printer) {
    m.printer = p
}

windows.go

package main

import "fmt"

type windows struct {
    printer printer
}

func (w *windows) print() {
    fmt.Println("Print request for windows")
    w.printer.printFile()
}

func (w *windows) setPrinter(p printer) {
    w.printer = p
}

printer.go

package main

type printer interface {
    printFile()
}

epson.go

package main

import "fmt"

type epson struct {
}

func (p *epson) printFile() {
    fmt.Println("Printing by a EPSON Printer")
}

hp.go

package main

import "fmt"

type hp struct {
}

func (p *hp) printFile() {
    fmt.Println("Printing by a HP Printer")
}

main.go

package main

import "fmt"

func main() {
    hpPrinter := &hp{}
    epsonPrinter := &epson{}
    macComputer := &mac{}
    macComputer.setPrinter(hpPrinter)
    macComputer.print()
    fmt.Println()
    macComputer.setPrinter(epsonPrinter)
    macComputer.print()
    fmt.Println()
    winComputer := &windows{}
    winComputer.setPrinter(hpPrinter)
    winComputer.print()
    fmt.Println()
    winComputer.setPrinter(epsonPrinter)
    winComputer.print()
    fmt.Println()
}

Output:

Print request for mac
Printing by a HP Printer

Print request for mac
Printing by a EPSON Printer

Print request for windows
Printing by a HP Printer

Print request for windows

Full Working Code:

package main

import "fmt"

type computer interface {
    print()
    setPrinter(printer)
}

type mac struct {
    printer printer
}

func (m *mac) print() {
    fmt.Println("Print request for mac")
    m.printer.printFile()
}

func (m *mac) setPrinter(p printer) {
    m.printer = p
}

type windows struct {
    printer printer
}

func (w *windows) print() {
    fmt.Println("Print request for windows")
    w.printer.printFile()
}

func (w *windows) setPrinter(p printer) {
    w.printer = p
}

type printer interface {
    printFile()
}

type epson struct {
}

func (p *epson) printFile() {
    fmt.Println("Printing by a EPSON Printer")
}

type hp struct {
}

func (p *hp) printFile() {
    fmt.Println("Printing by a HP Printer")
}

func main() {
    hpPrinter := &hp{}
    epsonPrinter := &epson{}
    macComputer := &mac{}
    macComputer.setPrinter(hpPrinter)
    macComputer.print()
    fmt.Println()
    macComputer.setPrinter(epsonPrinter)
    macComputer.print()
    fmt.Println()
    winComputer := &windows{}
    winComputer.setPrinter(hpPrinter)
    winComputer.print()
    fmt.Println()
    winComputer.setPrinter(epsonPrinter)
    winComputer.print()
    fmt.Println()
}

Output:

Print request for mac
Printing by a HP Printer

Print request for mac
Printing by a EPSON Printer

Print request for windows
Printing by a HP Printer

Print request for windows
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