關於by viewModels的實際實現方式

在創建viewModel時，通用的方法是

viewModel = ViewModelProvider(this,MainViewModel.MainViewModelFactory()).get(MainViewModel::class.java)
//viewModel = ViewModelProvider(this).get(MainViewModel::class.java)

對應MainViewModelFactory：

class MainViewModelFactory() : ViewModelProvider.Factory {
        override fun <T : ViewModel?> create(modelClass: Class<T>): T {
            return MainViewModel() as T
        }
    }

如果加了關於activity或者fragment的ktx拓展依賴，則可以簡化寫法

val viewModel by viewModels<MainViewModel>()

查看這個拓展的方法源碼可以看到

@MainThread
public inline fun <reified VM : ViewModel> ComponentActivity.viewModels(
    noinline factoryProducer: (() -> Factory)? = null
): Lazy<VM> {
    val factoryPromise = factoryProducer ?: {
        defaultViewModelProviderFactory
    }

    return ViewModelLazy(VM::class, { viewModelStore }, factoryPromise)
}

當factoryProducer為null時，調用了defaultViewModelProviderFactory的方法；

對應的代碼為ComponentActivity.java類中

  public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {
        if (getApplication() == null) {
            throw new IllegalStateException("Your activity is not yet attached to the "
                    + "Application instance. You can't request ViewModel before onCreate call.");
        }
        if (mDefaultFactory == null) {
            mDefaultFactory = new SavedStateViewModelFactory(
                    getApplication(),
                    this,
                    getIntent() != null ? getIntent().getExtras() : null);
        }
        return mDefaultFactory;
    }

這里創建了一個SavedStateViewModelFactory來簡化。