Is livedata builder ok for one-shot operations?

Question

For example, let's say that we have a product catalog view with an option to add product to a cart. Each time when user clicks add to cart, a viewModel method addToCart is called, that could look like this:

//inside viewModel
fun addToCart(item:Item): LiveData<Result> = liveData {
    val result = repository.addToCart(item) // loadUser is a suspend function.
    emit(result)
}


//inside view
addButton.onClickListener = {
     viewModel.addToCart(selectedItem).observe (viewLifecycleOwner, Observer () {
          result -> //show result
    }
}

What happens after adding for example, 5 items -> will there be 5 livedata objects in memory observed by the view?

If yes, when will they be cleanup? And if yes, should we avoid livedata builder for one-shot operations that can be called multiple times?

Answer 1

您的实施似乎是错误的！LiveData您不断地为每个addToCard函数调用返回一个新对象。关于你的第一个问题，这是一个Yes.

如果你想通过 liveData 正确地做到这一点。

// In ViewModel

private val _result = MutableLiveData<Result>()
val result: LiveData<Result>
   get() = _result;

fun addToCart(item: Item) {
   viewModelScope.launch {
      // Call suspend functions
      result.value = ...
   }
}

// Activity/Fragment

viewModel.result.observe(lifecycleOwner) { result ->
   // Process the result  
   ...
}

viewModel.addToCart(selectedItem)

您所要做的就是从活动中调用它并处理结果。您也可以用于StateFlow此目的。它还有一个扩展asLiveData，可以转换 Flow -> LiveData。