Demonstrates the implementation of the Mediator Design Pattern in PowerShell

ChatMediator.ps1

<#
    Define an object that encapsulates how a set of objects interact.
    Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
#>

class ChatMediator {
    $users

    ChatMediator() {
        $this.users = New-Object System.Collections.ArrayList
    }

    AddUser($user) {
        $this.users.add($user)
    }

    RemoveUser($user) {
        $this.users.remove($user)
    }

    SendMessage($msg, $user) {
        foreach ($targetUser in $this.users) {
            # message should not be received by the user sending it
            if ($targetUser -ne $user) {
                $targetUser.receive($msg)
            }
        }
    }
}

class User {

    $mediator
    $name

    User($mediator, $name) {
        $this.mediator = $mediator
        $this.name = $name
    }

    Send($msg) {
        "$($this.name): Sending Message: $msg " | Out-Host
        $this.mediator.SendMessage($msg, $this)
    }

    Receive($msg) {
        "$($this.name): Received Message: $msg " | Out-Host
    }
}

$script:mediator = $null

function New-ChatUser {
    param($name)

    # Singleton - Lazy Evaluation
    if ($null -eq $script:mediator) {
        $script:mediator = [ChatMediator]::new()
    }

    $chatUser = [User]::new($mediator, $name)
    $mediator.AddUser($chatUser)

    $chatUser
}

$John = New-ChatUser John
$Lisa = New-ChatUser Lisa
$Maria = New-ChatUser Maria
$David = New-ChatUser David

''
$John.Send("Hello everyone")

$script:mediator.RemoveUser($John)
"`nUser John has left the chat room`n"

$David.Send("Hi")
''

John: Sending Message: Hello everyone
Lisa: Received Message: Hello everyone
Maria: Received Message: Hello everyone
David: Received Message: Hello everyone

User John has left the chat room

David: Sending Message: Hi
Lisa: Received Message: Hi
Maria: Received Message: Hi