Introduction to Model View View Model (MVVM)

Description of Model is as follows:

• MODEL: ( Reusable Code – DATA ) Business Objects that encapsulate data and behavior of application domain, Simply hold the data. 
• VIEW: ( Platform Specific Code – USER INTERFACE ) What the user sees, The Formatted data. 
• VIEWMODEL: ( Reusable Code – LOGIC ) Link between Model and View OR It Retrieves data from Model and exposes it to the View. This is the model specifically designed for the View. 

Note: Link between Model and View Model is Manipulating Data and between ViewModel and View is 2-way Data Binding. 

BASIC INTRODUCTION: [ Way to Structure Code]

FEATURES:  

• Life Cycle state of Application will be maintained.
• The application will be in the same position as where the user left it.
• UI Components are kept away from Business Logic.
• Business Logic is kept away from Database operations.
• Easy to understand and read.

BASIC EXAMPLE: We want to display Name in Purple Color (not written in the proper format, proper length) or Display Purple Color if Age of a person is > 18 years, Display Pink Color if Age of a person is < 18 years, Then the Logic of Purple and Pink Color would be present in ViewModel. 

SUMMARY: From Server, Get Data(available in Model Objects), View Model reads Model Objects and then facilitates the easy presentation of data on the view.

The primary differences between MVVM AND MVC are as follows: 

ADVANTAGES:  

• Maintainability – Can remain agile and keep releasing successive versions quickly.
• Extensibility – Have the ability to replace or add new pieces of code.
• Testability – Easier to write unit tests against a core logic.
• Transparent Communication – The view model provides a transparent interface to the view controller, which it uses to populate the view layer and interact with the model layer, which results in a transparent communication between the layers of your application.

DISADVANTAGES: 

• Some people think that for simple UIs, MVVM can be overkill.
• In bigger cases, it can be hard to design the ViewModel.
• Debugging would be a bit difficult when we have complex data bindings.