I am following this tutorial by Raywenderlich on paging-library-for-android-with-kotlin on how to use android paging library. This is one of the easiest tutorials on the net and I have followed it thoroughly. However, I would like to make some changes so that I can intelligently switch between online data and offline data.
That is, I have old some posts in my database. Initially I have internet connection. So I load latest data from internet, then insert it into my database. Finally, I show this latest data in my recyclerView / PagedListAdapter. If for some reason, there is no internet connection after sometime, I should show the old posts from database.
How can I do this?
My attempts:
This is my code on github repository.
Here, I tried to create a factory pattern. It checks if initially I have internet, the factory returns pagedList from online dataSource. ELse, the factory returns pagedList from offline dataSource. But this doesnot intelligently switch between the 2 states.
I tried some random codes such as creating a boundary callback. But I am not sure how to make the necessary changes. I am not adding codes here (at least for now) to keep it short and precise.
Can anyone help me?
Edit:
To be specific, I am loading paged data primarily from network. If there is a network error, I don’t want to show the user an error. Instead I load paged data from cache / database and continuously show it to my user as long as possible. If the network is back,switch back to network paged data. (that’s what instagram / facebook does I think). What is the appropriate way to implement this? See my code / attemp in the answer.
Advertisement
Answer
Okay, so after trying out some codes for 2 days, this is what I came up with. However, I really don’t know if this is a good pratice or not. So I am open to any acceptable answers.
Explanation:
Since I have multiple data sources(network and database), I created ProfilePostDataSource: PageKeyedDataSource<Pair<Long, Long>, ProfilePost> here the key is a pair, the 1st one for network pagination, the 2nd one is for database pagination.
I used kotlin’s Coroutine to write some asynchronous codes in a simple if-else like manner. So we can write it in a psudo-code like this:
Database db;
Retrofit retrofit;
inside loadInitial/loadBefore / loadAfter:
currNetworkKey = params.key.first;
currDBKey = params.key.second;
ArrayList<Model> pagedList;
coroutine{
ArrayList<Model> onlineList = retrofit.getNetworkData(currNetworkKey); // <-- we primarily load data from network
if(onlineList != null) {
pagedList = onlineList;
db.insertAll(onlineList); // <-- update our cache
}else{
ArrayList<Model> offlineList = db.getOfflineData(currDBKey); // <-- incase the network fails, we load cache from database
if(offlineList !=null){
pagedList = offlineList;
}
}
if(pagedList != null or empty) {
nextNetworkKey = // update it accordingly
nextDBKey = // update it accordingly
Pair<int, int> nextKey = new Pair(nextNetworkKey, nextDBKey);
pagingLibraryCallBack.onResult(pagedList, nextKey); // <-- submit the data to paging library via callback. this updates your adapter, recyclerview etc...
}
}
So in apps like facebook, instagram etc, we see them primarily loading data from network. But if the network is down, they show you a cashed data. We can intelligently make this switch like this code.
Here is a relevant code snippet, the PageKeyedDataSource written in kotlin:
ProfilePostDataSource.kt
/** @brief: <Key, Value> = <Integer, ProfilePost>. The key = pageKey used in api. Value = single item data type in the recyclerView
*
* We have a situation. We need a 2nd id to fetch profilePosts from database.
* Change of plan: <Key, Value> = < Pair<Int, Int>, ProfilePost>. here the
*
* key.first = pageKey used in api. <-- Warning: Dont switch these 2!
* Key.second = db last items id
* used as out db page key
*
* Value = single item data type in the recyclerView
*
* */
class ProfilePostDataSource: PageKeyedDataSource<Pair<Long, Long>, ProfilePost> {
companion object{
val TAG: String = ProfilePostDataSource::class.java.simpleName;
val INVALID_KEY: Long = -1;
}
private val context: Context;
private val userId: Int;
private val liveLoaderState: MutableLiveData<NetworkState>;
private val profilePostLocalData: ProfilePostLocalDataProvider;
public constructor(context: Context, userId: Int, profilePostLocalData: ProfilePostLocalDataProvider, liveLoaderState: MutableLiveData<NetworkState>) {
this.context = context;
this.userId = userId;
this.profilePostLocalData = profilePostLocalData;
this.liveLoaderState = liveLoaderState;
}
override fun loadInitial(params: LoadInitialParams<Pair<Long, Long>>, pagingLibraryCallBack: LoadInitialCallback<Pair<Long, Long>, ProfilePost>) {
val initialNetworkKey: Long = 1L; // suffix = networkKey cz later we'll add dbKey
var nextNetworkKey = initialNetworkKey + 1;
val prevNetworkKey = null; // cz we wont be using it in this case
val initialDbKey: Long = Long.MAX_VALUE; // dont think I need it
var nextDBKey: Long = 0L;
GlobalScope.launch(Dispatchers.IO) {
val pagedProfilePosts: ArrayList<ProfilePost> = ArrayList(); // cz kotlin emptyList() sometimes gives a weird error. So use arraylist and be happy
val authorization : String = AuthManager.getInstance(context).authenticationToken;
try{
setLoading();
val res: Response<ProfileServerResponse> = getAPIService().getFeedProfile(
sessionToken = authorization, id = userId, withProfile = false, withPosts = true, page = initialNetworkKey.toInt()
);
if(res.isSuccessful && res.body()!=null) {
pagedProfilePosts.addAll(res.body()!!.posts);
}
}catch (x: Exception) {
Log.e(TAG, "Exception -> "+x.message);
}
if(pagedProfilePosts.isNotEmpty()) {
// this means network call is successfull
Log.e(TAG, "key -> "+initialNetworkKey+" size -> "+pagedProfilePosts.size+" "+pagedProfilePosts.toString());
nextDBKey = pagedProfilePosts.last().id;
val nextKey: Pair<Long, Long> = Pair(nextNetworkKey, nextDBKey);
pagingLibraryCallBack.onResult(pagedProfilePosts, prevNetworkKey, nextKey);
// <-- this is paging library's callback to a pipeline that updates data which inturn updates the recyclerView. There is a line: adapter.submitPost(list) in FeedProfileFragment. this callback is related to that line...
profilePostLocalData.insertProfilePosts(pagedProfilePosts, userId); // insert the latest data in db
}else{
// fetch data from cache
val cachedList: List<ProfilePost> = profilePostLocalData.getProfilePosts(userId);
pagedProfilePosts.addAll(cachedList);
if(pagedProfilePosts.size>0) {
nextDBKey = cachedList.last().id;
}else{
nextDBKey = INVALID_KEY;
}
nextNetworkKey = INVALID_KEY; // <-- probably there is a network error / sth like that. So no need to execute further network call. thus pass invalid key
val nextKey: Pair<Long, Long> = Pair(nextNetworkKey, nextDBKey);
pagingLibraryCallBack.onResult(pagedProfilePosts, prevNetworkKey, nextKey);
}
setLoaded();
}
}
override fun loadBefore(params: LoadParams<Pair<Long, Long>>, pagingLibraryCallBack: LoadCallback<Pair<Long, Long>, ProfilePost>) {} // we dont need it in feedProflie
override fun loadAfter(params: LoadParams<Pair<Long, Long>>, pagingLibraryCallBack: LoadCallback<Pair<Long, Long>, ProfilePost>) {
val currentNetworkKey: Long = params.key.first;
var nextNetworkKey = currentNetworkKey; // assuming invalid key
if(nextNetworkKey!= INVALID_KEY) {
nextNetworkKey = currentNetworkKey + 1;
}
val currentDBKey: Long = params.key.second;
var nextDBKey: Long = 0;
if(currentDBKey!= INVALID_KEY || currentNetworkKey!= INVALID_KEY) {
GlobalScope.launch(Dispatchers.IO) {
val pagedProfilePosts: ArrayList<ProfilePost> = ArrayList(); // cz kotlin emptyList() sometimes gives a weird error. So use arraylist and be happy
val authorization : String = AuthManager.getInstance(context).authenticationToken;
try{
setLoading();
if(currentNetworkKey!= INVALID_KEY) {
val res: Response<ProfileServerResponse> = getAPIService().getFeedProfile(
sessionToken = authorization, id = userId, withProfile = false, withPosts = true, page = currentNetworkKey.toInt()
);
if(res.isSuccessful && res.body()!=null) {
pagedProfilePosts.addAll(res.body()!!.posts);
}
}
}catch (x: Exception) {
Log.e(TAG, "Exception -> "+x.message);
}
if(pagedProfilePosts.isNotEmpty()) {
// this means network call is successfull
Log.e(TAG, "key -> "+currentNetworkKey+" size -> "+pagedProfilePosts.size+" "+pagedProfilePosts.toString());
nextDBKey = pagedProfilePosts.last().id;
val nextKey: Pair<Long, Long> = Pair(nextNetworkKey, nextDBKey);
pagingLibraryCallBack.onResult(pagedProfilePosts, nextKey);
setLoaded();
// <-- this is paging library's callback to a pipeline that updates data which inturn updates the recyclerView. There is a line: adapter.submitPost(list) in FeedProfileFragment. this callback is related to that line...
profilePostLocalData.insertProfilePosts(pagedProfilePosts, userId); // insert the latest data in db
}else{
// fetch data from cache
// val cachedList: List<ProfilePost> = profilePostLocalData.getProfilePosts(userId);
val cachedList: List<ProfilePost> = profilePostLocalData.getPagedProfilePosts(userId, nextDBKey, 20);
pagedProfilePosts.addAll(cachedList);
if(pagedProfilePosts.size>0) {
nextDBKey = cachedList.last().id;
}else{
nextDBKey = INVALID_KEY;
}
nextNetworkKey = INVALID_KEY; // <-- probably there is a network error / sth like that. So no need to execute further network call. thus pass invalid key
val nextKey: Pair<Long, Long> = Pair(nextNetworkKey, nextDBKey);
pagingLibraryCallBack.onResult(pagedProfilePosts, nextKey);
setLoaded();
}
}
}
}
private suspend fun setLoading() {
withContext(Dispatchers.Main) {
liveLoaderState.value = NetworkState.LOADING;
}
}
private suspend fun setLoaded() {
withContext(Dispatchers.Main) {
liveLoaderState.value = NetworkState.LOADED;
}
}
}
Thank you for reading this far. If you have a better solution, feel free to let me know. I’m open to any working solutions.