How to use livedata with Transformations.switchMap correctly to get initial data?

问题

right now I am starting to use LiveData for the first time. First I put all of my code in the viewModel including the code to start a search in the server. I used LiveData like this:

Fragment onViewCreated()

        viewModel.changeNotifierContacts.observe(this, androidx.lifecycle.Observer { value -> value?.let {
        recyclerViewAdapter.setData(value)
    } })

This was working as expected. Now I add a repository layer following MVVM pattern. (For this I moved my contact search functionality to repository class) First I implemented the connection between ViewModel and repository like this:

ViewModel code:

fun getContacts(): MutableLiveData<ContactGroup> {
   return contactSearchRepository.changeNotifierContacts;
}

fun search(newSearchInput: String) {
   contactSearchRepository.searchInRepository(newSearchInput)
}

Now I read this article that told us to not use LiveData like this: https://developer.android.com/topic/libraries/architecture/livedata#merge_livedata

Example from this page:

class MyViewModel(private val repository: PostalCodeRepository) : ViewModel() {

private fun getPostalCode(address: String): LiveData<String> {
    // DON'T DO THIS
    return repository.getPostCode(address)
}

}

Instead we should use something like this:

var changeNotifierContacts : LiveData<ContactGroup> = Transformations.switchMap(searchInput) {
    address -> contactSearchRepository.getPostCode(address) }

Questions:

Did I understand this article correctly or can I use my first implementation?
In my constructor of the viewModel I am creating an instance of my repository object that is starting to observe server data and it is getting initial data. (For example I am getting a list of all my friends). I am getting this initial data if I am using my first implementation. If I am using Transformations.switchMap implementation I am not getting this initial data. I first have to start a search here to get updated data then. This is not what I want, I also need to display "my friends" list without doing a search.
Is there another approach I can use here? Maybe LiveData is not the best solution to connect ViewModel with Repository?

Thanks for responses and suggestions!

回答1:

Did I understand this article correctly or can I use my first implementation?

I think you did, but I believe you have expanded the concept too much.

If you are expecting the user to enter a search to receive an answer, you should do like they said:

class MyViewModel(private val repository: PostalCodeRepository) : ViewModel() {
    private val addressInput = MutableLiveData<String>()
    val postalCode: LiveData<String> = Transformations.switchMap(addressInput) {
            address -> repository.getPostCode(address) }


    fun setInput(address: String) {
        addressInput.value = address
    }
}

However, if you are loading a default list, you should do it like you did in your first example:

val getContact = contactSearchRepository.changeNotifierContacts

In this case you will have to observe getContact and postalCode.

In my constructor of the viewModel I am creating an instance of my repository object that is starting to observe server data and it is getting initial data. (For example I am getting a list of all my friends). I am getting this initial data if I am using my first implementation. If I am using Transformations.switchMap implementation I am not getting this initial data. I first have to start a search here to get updated data then. This is not what I want, I also need to display "my friends" list without doing a search.

You can start your fragment/activity with a default search, like this:

MyViewModel.setInput("Friends")

This way you do not need to observe two objects as postalCode will provide all answers.

Is there another approach I can use here? Maybe LiveData is not the best solution to connect ViewModel with Repository?

I think live data is your answer. After done with the learning curve it becomes easier to deal with!

I hope it helps!

来源：https://stackoverflow.com/questions/61748362/how-to-use-livedata-with-transformations-switchmap-correctly-to-get-initial-data

标签

android

kotlin

mvvm

repository

android-livedata