我们可以使用以下命令初始化ViewModel类
private val viewModel: CharactersViewModel by viewModels()
或
viewModel = ViewModelProvider(this).get(CharactersViewModel::class.java)
Herer CharactersViewModel is our ViewModel class. My question is when to use which?
Do both contain the same purpose? I have read the android official documentation of ViewModel.The documentation says by viewModels() Kotlin property delegate. But unf或tunately failed to understand it. Can anyone help me understand this?
Both of them do the same thing, but there is a discriminative advantage for the first one. Kotlin property delegation uses the idea of Lazy Initialization. On Wikipedia you can find a brief definition for it:
In computer programming, lazy initialization is the tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed. It is a kind of lazy evaluation that refers specifically to the instantiation of objects or other resources.
因此，当您使用您提到的第一种方法时，您可以利用惰性属性.这意味着仅在首次访问时才创建ViewModel实例.
Given below code as an example:
class YourFragment : Fragment() {
private val viewModel: CharactersViewModel by viewModels()
// other codes ...
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
// doing some view initialization ...
viewModel.someLiveData.observe(viewLifecycleOwner) {
// ...
}
}
}
如果viewModel.someLiveData.observe(viewLifecycleOwner)是第一次接触viewModel字段，则它的实例化将在那里发生.(创建CharactersViewModel实例)
因此，使用视图模型等对象的延迟初始化可以减少片段的启动影响，从而更快地加载和显示其内容，而不是直接初始化它们.