How does kotlin use this by delegate to instantiate the viewmodel

问题

I was reading through the google android architecture example and came across this.Can someone explain to me how this delegate works?

private val viewModel by viewModels<TasksViewModel> { getViewModelFactory() }

where getViewModelFactory is an extension method that returns a ViewModelFactory and TasksViewModel is an instance of ViewModel()

The way that I read this is similar to:

private val viewModel: TasksViewModel by Fragment.ViewModel(ViewModelFactory)

can someone elaborate on if my understanding is correct.

回答1:

by viewModels(...) is part of fragment-ktx library, it's a convienience short hand for creating a lazy delegate obtaining ViewModels.

// creates lazy delegate for obtaining zero-argument MyViewModel
private val viewModel : MyViewModel by viewModels()
// it's functionally equal to:
private val viewModel by lazy {
    ViewModelProvider(this).get(MyViewModel::class.java)
}

// with factory:
private val viewModel : MyViewModel by viewModels { getViewModelFactory() }
// is equal to:
private val viewModel by lazy {
    ViewModelProvider(this, getViewModelFactory()).get(MyViewModel::class.java)
}

来源：https://stackoverflow.com/questions/58106707/how-does-kotlin-use-this-by-delegate-to-instantiate-the-viewmodel