android - 是否可以在没有工厂的情况下在公共(public)方法中将 id 参数传递给 AndroidViewModel？

Question

我正在查看 Android 架构组件示例中的 BasicSample 应用。在 ProductViewModel.java 文件中，一些注释如下:

It's not actually necessary in this case, as the product ID can be passed in a public method.

根据我对评论的理解，我想知道是否可以在不使用工厂的情况下将 productId 传递给 ProductViewModel，以及如何做到这一点。

我已经实现了转换，并且我知道我可以使用 switchMap 传递 productId。但我一直在寻找一种使用单个 ID 初始化模型的方法。

public class ProductViewModel extends AndroidViewModel {

    private final LiveData<ProductEntity> mObservableProduct;

    public ObservableField<ProductEntity> product = new ObservableField<>();

    private final int mProductId;

    private final LiveData<List<CommentEntity>> mObservableComments;

    public ProductViewModel(@NonNull Application application, DataRepository repository,
            final int productId) {
        super(application);
        mProductId = productId;

        mObservableComments = repository.loadComments(mProductId);
        mObservableProduct = repository.loadProduct(mProductId);
    }

....

   /**
     * A creator is used to inject the product ID into the ViewModel
     * <p>
     * This creator is to showcase how to inject dependencies into ViewModels. It's not
     * actually necessary in this case, as the product ID can be passed in a public method.
     */
    public static class Factory extends ViewModelProvider.NewInstanceFactory {

        @NonNull
        private final Application mApplication;

        private final int mProductId;

        private final DataRepository mRepository;

        public Factory(@NonNull Application application, int productId) {
            mApplication = application;
            mProductId = productId;
            mRepository = ((BasicApp) application).getRepository();
        }

        @Override
        public <T extends ViewModel> T create(Class<T> modelClass) {
            //noinspection unchecked
            return (T) new ProductViewModel(mApplication, mRepository, mProductId);
        }
    }
}

Answer 1

引用示例，当评论说:

the product ID can be passed in a public method

这是指您可以创建公共(public) setter 方法这一事实。

由于 productId 用于从您的数据库中获取 LiveData，因此您应该使用 switchMap，如您所提到的。这是因为 switchMap 允许您查找和更新 LiveData 指向的内容，而无需重新设置观察者。如果您没有使用 switchMap，您需要告诉您的 Activity 观察新查找的 LiveData，并可能停止观察旧的 LiveData 对象。 More description of this is included in the docs .

请注意 - 工厂在这里也很有用，因为您要通过构造函数传入或注入(inject) DataRepository 依赖项。这是将存储库放入类中的更好方法，因为在测试时很容易模拟存储库。

考虑到这一点，如果您想在工厂中执行此操作，您的代码可能类似于:

public class ProductViewModel extends AndroidViewModel {

    private final LiveData<ProductEntity> mProduct;
    private final LiveData<List<CommentEntity>> mComments;
    private final MutableLiveData<Integer> mProductId = new MutableLiveData<>();


    public ProductViewModel(@NonNull Application application) {
        super(application);

        // Have some way to get your repository, this is not great for testing...
        Repository repository = ((BasicApp) application).getRepository();

            mProduct = Transformations.switchMap(mProductId, id -> {
            return repository.loadProduct(id);
        }

            mComments = Transformations.switchMap(mComments, id -> {
            return repository.loadComments(id);
        }
    }

    public void setProductId(int productId) {
       mProductId.setValue(productId); // This will trigger both of those switchMap statements
    }

}