Kotlin如何通过委托使用此实例化视图模型 [英] How does kotlin use this by delegate to instantiate the viewmodel

问题描述

我正在阅读google android体系结构示例，并遇到了这个问题.有人可以向我解释此委托的工作原理吗?

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() }

其中getViewModelFactory是返回ViewModelFactory的扩展方法，而TasksViewModel是ViewModel()的实例

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

我的阅读方式类似于:

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

有人能详细说明我的理解是否正确.

can someone elaborate on if my understanding is correct.

推荐答案

by viewModels(...)是 fragment-ktx 库，它是创建lazy委托以获得ViewModels的便捷捷径.

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)
}

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