Get HashMap<Model1, List<Model2>> in an MVVM+Repostiory+LiveData setting? - kotlin

Question

So I was working on this silly little app for practicing MVVM and Repository Pattern. I have two model classes at the moment. They are Category and SubCategory for which I have defined the following data classes:
#Entity(tableName = "categories")
data class Category(
#PrimaryKey(autoGenerate = true)
#ColumnInfo(name = "id")
val id: Int,
#ColumnInfo(name = "name")
val name: String
) {
}
And
/**
* One to many Relationship from Category to SubCategory
*/
#Entity(
tableName = "sub_categories", foreignKeys = arrayOf(
ForeignKey(
entity = Category::class,
parentColumns = arrayOf("id"),
childColumns = arrayOf("category_id")
)
)
)
data class SubCategory(
#PrimaryKey(autoGenerate = true)
#ColumnInfo(name = "id")
val id: Int,
#ColumnInfo(name = "name")
val name: String,
#ColumnInfo(name = "category_id")
val categoryId: Int
) {
}
As you can see, I have modeled the resources such that we will need categoryId to be passed to get SubCategories related to a Category.
Now I am pretty new with this MVVM and LiveData and Repository Pattern.
My Problem is that I am using an ExpandableListView to populate SubCategories under Categories and the Adapter for it requires a HashMap<Category, List<SubCategory> for it to display the expandable listview.
So my question is how do I get a HashMap<Category, List<SubCategory> from my database using an approach of db->dao->repository->viewmodel and wherever the adpater goes.
I suppose creating a separate repository like CategorySubCategoryRespository whereby I can do something like following is not going to help?:
class CategorySubCategoryRepository(
private val categoryDao: CategoryDao,
private val subCategoryDao: SubCategoryDao
) {
val allCategoriesSubCategories: LiveData<HashMap<Category, List<SubCategory>>>
get() {
val hashMap: HashMap<Category, List<SubCategory>> = hashMapOf()
for (category in categoryDao.getList()) {
hashMap[category] = subCategoryDao.getSubCategoriesListForCategory(category.id)
}
return hashMap
}
}
}
PS: I think I want to use LiveData wherever possible

Answer 1

So what I ended up doing was, in my CategorySubcategoryRepository I constructed the Hashmap from the CategoryDao and SubcategoryDao as follows:
class CategorySubCategoryRepository(
private val categoryDao: CategoryDao,
private val subCategoryDao: SubCategoryDao
) {
fun getHashMap(): LiveData<HashMap<Category, List<SubCategory>>> {
val data = MutableLiveData<HashMap<Category, List<SubCategory>>>()
val hashMap: HashMap<Category, List<SubCategory>> = hashMapOf()
Executors.newSingleThreadExecutor().execute {
for (category in categoryDao.getList()) {
hashMap[category] = subCategoryDao.getSubCategoriesListForCategory(category.id)
}
}
data.value = hashMap
return data
}
}
Then I used this in my viewmodel's init{} block like:
hashMap = categorySubCategoryRepository.getHashMap()
Then I observed it in my Fragment's onCreateView as:
myViewModel.hashMap.observe(this, Observer {
adapter.setCategoryList(it.keys.toList())
adapter.setCategorySubCategoriesMap(it)
elv_categories.setAdapter(adapter)
adapter.notifyDataSetChanged()
})
Do Comment if this is not the right thing to do. I am doing this only to increase my skills and would love to hear if there's a better way to go about things or if my approach is completely absurd.
Edit:
As per #SanlokLee's comment. The getHashMap function has been changed to:
fun getHashMap(): LiveData<HashMap<Category, List<SubCategory>>> {
val data = MutableLiveData<HashMap<Category, List<SubCategory>>>()
Executors.newSingleThreadExecutor().execute {
val hashMap: HashMap<Category, List<SubCategory>> = hashMapOf()
for (category in categoryDao.getList()) {
hashMap[category] = subCategoryDao.getSubCategoriesListForCategory(category.id)
}
data.postValue(hashMap)
}
return data
}