Simple way to define ViewModel Factories

I started working on my final project for Udacity’s Kotlin Android Developer Nanodegree, where I started noticing that my ViewModel factories are becoming large and complex. So I wanted to keep them simple.

What did I already have?

To have an idea of how complex they have become, see the following ViewModelProvider.Factory that I have created for VoterInfoViewModel that I have in my project.

class VoterInfoViewModelFactory(
        private val context: Context,
        private val election: Election
) : ViewModelProvider.Factory {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        val database = ElectionDatabase.getInstance(context)
        val repository = ElectionRepository(database.electionDao)

        val followString = context.getString(R.string.follow_election)
        val unFollowString = context.getString(R.string.unfollow_election)

        return modelClass.getConstructor(
                ElectionRepository::class.java,
                Election::class.java,
                String::class.java,
                String::class.java
        ).newInstance(repository, election, followString, unFollowString)
    }
}

The problems I have with it are the following:

It is too long for a simple ViewModel creation.
Getting the constructor and creating a new instance isn’t intuitive.
I don’t want to write a class every time I make such simple factories in the project.

How did I abstract it out?

The first thing I noticed is that the structure is almost the same for all the factories I currently have in the project, and am sure that only part that changes is the body of the factory. So I created a class which delegates the body to a provider.

class SimpleViewModelFactory(
        private val `class`: Class<*>,
        private val provider: () -> ViewModel
) : ViewModelProvider.Factory {

    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (!modelClass.isAssignableFrom(`class`)) {
            throw IllegalArgumentException("ViewModel type is non assignable")
        }

        @Suppress("UNCHECKED_CAST")
        return provider() as T
    }
}

So what is changed here? I delegated the body of the function to a provider which is a lambda. Also, the check is now done before hand, and now a proper exception is thrown in case of a misfire, which is pretty nice.

Also, I didn’t want to provide the class to the constructor always, so I made one inline function using reified generics to help me out.

inline fun <reified T : ViewModel> viewModelFactory(noinline provider: () -> T) =
        SimpleViewModelFactory(T::class.java, provider)

The use of reified generics is only allowed for inline functions, but care has to be taken so that provider doesn’t get inlined.

What is the end result?

So after adding this utility, I was able to refactor the factory to an even simpler and nice blob of code. See it for yourself.

fun voterInfoViewModelFactory(context: Context, election: Election) = viewModelFactory {
    val database = ElectionDatabase.getInstance(context)
    val repository = ElectionRepository(database.electionDao)

    val followString = context.getString(R.string.follow_election)
    val unFollowString = context.getString(R.string.unfollow_election)

    VoterInfoViewModel(repository, election, followString, unFollowString)
}

Now it is a lot more cleaner and easy enough to follow! Do let me know what you think about this in the comments. Happy hacking!

Simple way to define ViewModel Factories

Written by Sri Harsha Chilakapati on Feb 16, 2021

What did I already have?

How did I abstract it out?

What is the end result?