ViewModelFactory in Kotlin

During the development of Android applications, the ViewModel is used to store and manage data that is needed for the user interface. However, creating a ViewModel instance can be challenging when the constructor requires parameters. This is where the ViewModelFactory comes into play. In this article, we will discuss how to use ViewModelFactory in Kotlin.

What is ViewModelFactory?

ViewModelFactory is a class provided by the Android Architecture Components library that provides a way to create ViewModel instances along with any necessary dependencies. It separates the creation of ViewModel from the activity or fragment responsible for using it.

How to use ViewModelFactory?

To use ViewModelFactory in Kotlin, we need to create a ViewModelFactory class that extends the ViewModelProvider.Factory interface. This interface has one method, create, which is responsible for creating a new ViewModel instance.

Here is a sample code snippet of a ViewModelFactory class :

class MyViewModelFactory(private val myRepository: MyRepository) : ViewModelProvider.Factory {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(MyViewModel::class.java)) {
            return MyViewModel(myRepository) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

In the above example, we are creating a ViewModelFactory called MyViewModelFactory that takes a dependency on a MyRepository instance. The create method checks if the requested model class is MyViewModel and returns a new instance of the class with the dependency injected into it.

Now, we can use this factory in our activity or fragment to create an instance of MyViewModel as shown below:

val myRepository = MyRepository()
val myViewModelFactory = MyViewModelFactory(myRepository)
val myViewModel = ViewModelProvider(this, myViewModelFactory).get(MyViewModel::class.java)

In the above code, we first create an instance of the MyRepository class, then we create an instance of the MyViewModelFactory with this repository instance. Lastly, we use the ViewModelProvider to get an instance of the MyViewModel by passing in the activity or fragment context and the MyViewModelFactory instance.

Conclusion

In this article, we have discussed what ViewModelFactory is, and how it can be used in Kotlin to create ViewModel instances with dependencies. ViewModelFactory is a useful tool that can simplify the creation of ViewModel instances, especially when the constructor requires dependencies.