Counter App Jetpack Compose

3 min readMay 23, 2024

Overview

we can increment or decrement the counter and the counter will survive or remain the same on configuration change.

We are following MVVM pattern here.
We have view model for business logic and interacting with model and views
We have repository as a model, to store and update data.
UI layer is basic and already covered earlier, so will cover only business logic here

Our model is defined inside the file CounterModel.kt
Inside our model we have data class named CounterModel
CounterModel has an attribute named count
We have an repository named CounterRepository
Repository is used to fetch data from local or remote source
Here repository CounterRepository is fetching data from local source by creating an object of CounterModel named _counter by initializing its count value to 0.
We have defined a method named getCounter inside the CounterRepository to fetch _counter object
we have defined method incrementCounter and decrementCounter inside CounterRepository to increment or decrement count value of the _counter object

we have used CounterViewModel as the view model for our view
we have created an object of CounterRepository named _repository
we have created a state variable named _count of type _counter object’s count value, which holds the data, and act as single source of truth along with CounterRepository
_count will fetch the data from CounterRepository and acts as a data source for CounterViewModel
we have declared _count as private and exposing its value using public state variable count
we have defined method increment and decrement , which is calling repository increment and decrement method to update count value of _counter object.
then we are assigning updated count value using repository object to _count variable.

we have defined our view using TheCounterApp composable
TheCounterApp contains viewModelCounter object of type CounterViewModel
Notice here we have used inbuilt viewModel() class to create viewModelCounter object of type CounterViewModel
we are updating Text composable of TheCounterApp using count state variable of the viewmodel, whenever the count state variable value or data changes Text composable is recomposed automatically to reflect current value.
Whenever we click on Button increment or decrement, viewmodel increment or decrement method is called which update the data in the repository.
The updated repository data is again fetched by the viewmodel increment and decrement methods to update its local data which in turn updates the UI automatically.

Why do we declare Private variables in Java?

It comes under the concept of data hiding
we are not protecting the variable’s value from other devs, it comes under another topic of data integrity
When the variable is private its value is protected when the code runs.
It keeps the variable hidden from the other classes which use the class
Other classes should only access behavior by calling methods on the class and not changing variable value directly.
If any class can change the variable value then it is difficult to determine the validity of the value. eg — you have a variable to count manufactured products of the factory. If you make the variable public any class can make its value negative and it can crash the other parts of the code. On the other hand if the variable is private, you can ensure its value is never negative.
Sometimes even the methods that modifies these variables locally are made private for extra security.

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