ViewModel的创建方式

在Jetpack组件中，我们一般会用到ViewModel，那么我们来说下它的几种创建模式和应用场景

1.第一种方式

 val viewModel by lazy {
      ViewModelProvider(this,ViewModelProvider.NewInstanceFactory()).get(modelClass)
 }

一般在Activity中使用，它只会被创建一次

2.第二种方式

 val viewModel by lazy {
      ViewModelProvider(requireActivity(),ViewModelProvider.NewInstanceFactory()).get(modelClass)
 }

一般被用于Fragment，它也只有一个实例，被用于Fragment的数据共享

3.第三种方式

 val viewModel by lazy {
      ViewModelProvider.NewInstanceFactory().create(modelClass)
 }

一般被用于Fragment，但是它会随着Fragment的重新创建,也会随着Fragment销毁而销毁，一般配合着ViewPager 里面的 Fragment使用。

4.第四种方式

private val vm:PlayerViewModel by viewModels()

private val vm:PlayerViewModel by activityViewModels()

第一种，viewModelStore是fragment本身；
第二种，viewModelStore是Activity，这个其实是获取Activity的ViewModel；

@MainThread
inline fun  Fragment.viewModels(
    noinline ownerProducer: () -> ViewModelStoreOwner = { this },
    noinline factoryProducer: (() -> Factory)? = null
) = createViewModelLazy(VM::class, { ownerProducer().viewModelStore }, factoryProducer)

@MainThread
fun  Fragment.createViewModelLazy(
    viewModelClass: KClass,
    storeProducer: () -> ViewModelStore,
    factoryProducer: (() -> Factory)? = null
): Lazy {
    val factoryPromise = factoryProducer ?: {
        defaultViewModelProviderFactory
    }
    return ViewModelLazy(viewModelClass, storeProducer, factoryPromise)
}

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@MainThread
inline fun  Fragment.activityViewModels(
    noinline factoryProducer: (() -> Factory)? = null
) = createViewModelLazy(VM::class, { requireActivity().viewModelStore },
    factoryProducer ?: { requireActivity().defaultViewModelProviderFactory })