Discussion on "Recommended Ways To Create ViewModel or AndroidViewModel"

Vincent Tsen · 2021-09-18T00:08:55.706Z

There are few ways to create ViewModel and AndroidViewModel. This article shows you the Kotlin examples of creating them. This is an example of ViewModel or AndroidViewModel class that you may have. class MyViewModel: ViewModel() { } class MyAndroidV...

I tried to implement your solution but it does not seem to work. Do you have any idea what Im doing wrong?

What I did: Added implementation "androidx.fragment:fragment-ktx:1.3.6"

My Android View Model constructor looks like this:

class MainViewModel(private val app: Application, webApiRepository: IWebApiRepository = WebApiRepository.getInstance(app)) : ViewModelProvider.AndroidViewModelFactory(app)

I also added the create function:

    override fun <T : ViewModel> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(
                MainViewModel::class.java)) {

            return MainViewModel(app, webApiRepository) as T
        }

        throw IllegalArgumentException("Unknown ViewModel class")
    }

But when I try to use by activityViewModels or viewModels it does not work:

(Remove the two empty spaces) https:// cdn.hashnode .com/res/hashnode/image/upload/v1657015361719/7gYkfO5k8.png

I need to use activityViewModel in order to get a shared ViewModel across fragments. Thanks

Did you name your factory class wrongly? It should be something like MainViewModelFactory instead of MainViewModel. Maybe it conflicts with your MainViewModel class, thus it is not working. Just guessing. :)

Vincent Tsen You are the man! Im an idiot, didnt understand I had to create new seperate class haha

Thank you for the article, it was very helpful for me!

If just want to mention I had to remove "?" on line "override fun <T : ViewModel?> create(modelClass: Class<T>): T {" in order to remove a "'create' overrides nothing" error

class MyAndroidViewModelFactory(
    private val app: Application,
    private val repository: Repository)
    : ViewModelProvider.AndroidViewModelFactory(app) {

    override fun <T : ViewModel> create(modelClass: Class<T>): T {

        if (modelClass.isAssignableFrom(
                MyAndroidViewModel::class.java)) {

            return MyAndroidViewModel(app, repository) as T
        }

        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

You're welcome, I'm glad it helps. It looks like the API has changed, I will fix that. Thanks for pointing that out.

Useful info, thanks. But I'm wondering if there's a way to use the view models to share data between an activity and the fragments it contains, or is it only possible between fragments?

I can't find a way to get an activity to be able to set up a view model that the fragments also use if I need a custom factory. They all seem to need the factory to be provided, which then means that the factory has to be provided via some other means. If there are multiple additional arguments then that is slightly better, but otherwise the shared data might just as well be passed more direclty. Even if the factory is passed it's not clear that the same view model would be used rather than the factory being used to construct separate instances.

Craig You're welcome! Have you tried by activityViewModels? If both fragments use this, only one ViewModel is created. It works with custom ViewModel factory too.

It supposes to work. You save the data in View Model before you navigate to the next fragment, and you retrieve the data from the ViewModel in another fragment. I have done this in one of my apps.

Another way is using Dependency Injection, and I haven't tried this out yet. This seems like a better approach than the custom ViewModel factory.

In fact, if we want to do this properly, I think each fragment should have their own independent ViewModel. There shouldn't share the same ViewModel. That data is shared by using Safe Args which allows you to save the data into the bundle and retrieve it from the target fragment during the navigation. I have tried this too, but by acitivtyViewModels is a lot more convenient.

I'm not sure if I answer your questions, but I hope it helps, and thanks for the comment.

By the way, I just noticed I mentioned to use by Lazy to replace the both by ViewModels and by ActivityViewModels which is NOT correct.

It can be used to replace by ViewModels only, and NOT to be used to replace by AcitivityViewModels.

I will fix it.

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