MVVM stands for Model-View-ViewModel.It is an architectural pattern used in software development, which helps in organizing your code in a way that is easier to understand, test, and maintain. It the user interface from the business logic. The Model represents the data and business logic, the View represents the user interface, and the ViewModel acts as a mediator between the Model and the View. The ViewModel is responsible for handling the data and business logic and updating the View accordingly. This separation of concerns makes the code more modular, testable, and maintainable.
The Model represents the data and business logic of the application. It is responsible for managing the data and business logic and providing the data to the ViewModel. It is responsible for retrieving and storing data, as well as performing any necessary data processing. The Model is independent of the user interface and does not have any knowledge of the View or the ViewModel. This separation of concerns makes the code more modular and testable.
Example: If you have an app that displays a list of books, the Model would be responsible for fetching the book data from a database or an online source. It would also be responsible for updating the book data and providing the data to the ViewModel.
The following code snippet shows an example of a simple Model class in Kotlin:
data class Book(val title: String, val author: String, val year: Int)
What it is: The View is the user interface (UI) of the application. It displays the data to the user and interacts with the user. The View is responsible for displaying the data to the user and handling user input. It is also responsible for updating the ViewModel with user input and displaying the data provided by the ViewModel. The View is independent of the business logic and does not have any knowledge of the Model or the ViewModel. This separation of concerns makes the code more modular and testable.
Example: In the same book app, the View would be the actual screen that displays the list of books to the user.
The following code snippet shows an example of a simple View class in Kotlin:
class BookActivity : AppCompatActivity() {
private lateinit var viewModel: BookViewModel
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_book)
viewModel = BookViewModel()
val books = viewModel.getBooks()
// Now you can display the books in your UI
}
}
What it is: The ViewModel acts as a bridge between the Model and the View. It takes data from the Model, applies UI logic, and then formats it for display in the View. The ViewModel is responsible for handling the data and business logic and updating the View accordingly. It is also responsible for handling user input and updating the Model accordingly. The ViewModel is independent of the user interface and does not have any knowledge of the View or the Model. This separation of concerns makes the code more modular and testable. Example: For the book app, the ViewModel would take the raw book data, and format it nicely for display in the View. It would also handle any user input and update the Model accordingly.
The following code snippet shows an example of a simple ViewModel class in Kotlin:
In this example, the Book class is the Model, BookViewModel is the ViewModel, and BookActivity is the View. The ViewModel fetches the book data, and the View displays it.
A ViewModel Class in Android is a part of the MVVM architecture, as previously mentioned. It acts as a manager that handles the communication between the app’s data and the UI. The ViewModel class is designed to store and manage UI-related data in a lifecycle-conscious way. It allows data to survive configuration changes, such as screen rotations. The ViewModel is responsible for holding and processing all the data needed for the UI while respecting the lifecycle of the app’s activities or fragments. It also allows data to survive configuration changes, such as screen rotations.
To create a ViewModel class, you need to extend the ViewModel class provided by the Android Jetpack library. The ViewModel class is a part of the androidx.lifecycle.ViewModel package.
import androidx.lifecycle.ViewModel
class MyViewModel : ViewModel() {
var number: Int = 0
fun incrementNumber() {
number++
}
}
In this example, MyViewModel is a class that extends ViewModel(). It has a variable number and a function incrementNumber() to increase the number by one.
import androidx.activity.viewModels
import androidx.appcompat.app.AppCompatActivity
class MainActivity : AppCompatActivity() {
private val myViewModel: MyViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
// Use the ViewModel
myViewModel.incrementNumber()
}
}
In this example, we use the viewModels() function to create an instance of MyViewModel in the MainActivity. We then call the incrementNumber() function to increase the number by one.
Inheritance is a fundamental concept in object-oriented programming (OOP), and it is widely used in Android development.
Inheritance allows a class to use methods and fields of another class, promoting reusability and a hierarchical organization of code.
Base Class (Parent Class): The class whose properties and methods are inherited by another class. It is also known as the superclass or parent class.
Derived Class (Child Class): The class that inherits the properties and methods from another class. It is also known as the subclass or child class.
Open Keyword: In Kotlin, classes are final by default, which means they can’t be inherited. To allow a class to be inherited, it must be declared with the open keyword.
Super Keyword: The super keyword is used to refer to the parent class in the child class. It is used to call the parent class constructor and access the parent class properties and methods.
Imagine you are creating an application that has several types of vehicles, like cars and bicycles. You can create a general class named Vehicle and then create more specific classes like Car and Bicycle that inherit from Vehicle.
open class Vehicle {
fun start() {
println("The vehicle is starting.")
}
}
In this example, the Vehicle class is declared with the open keyword, allowing it to be inherited by other classes. It has a function start() that prints a message when the vehicle starts.
class Car : Vehicle() {
fun drive() {
println("The car is driving.")
}
}
In this example, the Car class inherits from the Vehicle class using the : symbol. It has a function drive() that prints a message when the car is driving.
val myCar = Car()
myCar.start() // Output: The vehicle is starting.
myCar.drive() // Output: The car is driving. fun main() {
val car = Car()
car.start()
car.drive()
}
In this example, we create an instance of the Car class and call the start() and drive() functions. The start() function is inherited from the Vehicle class, and the drive() function is specific to the Car class.
When you create an object of the Car class, you can access both the methods from the Car class and the inherited methods from the Vehicle class.
Interfaces in Kotlin are a way to define a contract for classes without implementing any behavior. Interfaces can contain abstract methods (methods without a body) and methods with a default implementation. Classes that implement an interface must provide implementations for all of its abstract methods.
Multiple Inheritance: Interfaces allow a class to inherit functionalities from multiple sources, as a class can implement multiple interfaces. Flexibility: Interfaces provide a way to define methods that must be implemented by a class, ensuring that certain functionalities are present.
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