MVC Design Pattern in Cocoa

MVC Design Pattern in Cocoa

Posted on May
6, 2014 by Debasis
Das — No
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MVC Design Pattern in Cocoa

High level pattern that classifies objects based on the roles they play in an application

Its a compound design pattern and comprises of several elemental design pattern

e.g
of mvc pattern in Cocoa can be found in Cocoa Bindings, Cocoa Document Architecture, Scriptability etc

MVC Pattern is divided into objects

Model Objects

View Objects

Controller Objects

If developers are adhering to this Design pattern and are creating their custom classes, the custom classes must fall into one of the 3 categories (model, view or controller)

Model

Holds data and defines logic that manipulates that data

All persistent data must reside in models.

Model objects should not be concerned with interfaces and presentation issues.

Views

Knows how to display the data and allows the users to edit it

Views should not be responsible for storing the data that it is displaying

Caching Data in View Objects is an exception for performance reasons.

Views can be responsible for rendering a part of the model objects or the whole model object

A large number of view objects can be found in AppKit Framework

Controllers

Acts as an intermediary between the applications view and model objects

Acts as a conduit through which the view objects knows about the change in the model objects

Controllers are also responsible for managing the lifecycle of other objects

Combining Roles in MVC Design Pattern

View Controller

Fulfills the role of both the controller and the view

It owns the interface (views).

Main responsibility is to manage the interface and communicate with the models.

Model Controller

Owns the model.

Main responsibility is to manage the model and communicate its changes to view objects

Types of Controller Objects

Mediating Controller

Inherits from NSController class

Used in Cocoa Bindings

Some ready to use Mediating Controller classes are

NSObjectController

NSTreeController

NSArrayController

NSUserDefaultsController

Facilitates, Mediates the flow of data between view and model objects

Coordinating Controller

Provides services such as responding to delegation messages

Responding to action messages

Managing the lifecycle of objects (Windows, View Objects, Instance variables, Mediating Controllers etc)

Establishing connection between objects

Example of Co-ordinating controllers are

NSWindowController

NSDocumentController

The pattern as a (Composite, Strategy and Observer ) design
pattern

Composite

View Objects are a composite of nested views (view hierarchy)

Strategy

View objects confines itself to maintaining its visual aspect and delegates all the decision about the application specific meaning to the controller

Observer

Model objects keeps the view objects apprised/advised about the changes in its state.

 





Traditional MVC Design Pattern

 





Cocoa Version of MVC Design Pattern





MVC Coordinating Controller

Design Guidelines

For mediating controller, use an instance of NSObjectController, NSArrayController, NSTreeController rather than implementing all the functionalities in a NSObject subclass

Keep the roles separated for maximum reusability

In case you are planning to merge roles, then pick the predominant role for a class and use categories to extend the class to play other roles

Although it is possible to have views directly observe the properties of model objects, it is not a best practice. Avoid the temptation. A view should always go through a mediating controller to learn about the changes in a model object

A view class should not depend on a model class (If this is unavoidable, reduce the dependency)

A model class should not depend on anything other than model classes

A mediating controller should not depend on model classes, view classes or co-ordinating controllers.





Sample Implementation