Multi tool use

No adapter attached; skipping layout when clicking back button and starting the app again

Android Studio 3.2 Canary 18
kotlin_version = 1.2.50


I have a simple app that uses a recyclerview and adapter. When the app starts is load all the data.
However, when I click the back button and start the app again. It won't display the data (blank).
If I clear the app from memory and start the app. The data will load as normal.

I am loading the data from sqlite and the data is loaded each time. as it populates the insectDataModelList.

insectDataModelList

After going into the RecyclerView.java source code the reason is the mAdapter is null. However, I have
checked that the adapter is correct when I set it to the recyclerview.

RecyclerView.java

mAdapter

void dispatchLayout() {
if (mAdapter == null) {
Log.e(TAG, "No adapter attached; skipping layout");
// leave the state in START
return;
}
...
}


My MainActivity.java is Java

public class MainActivity extends AppCompatActivity {
private RecyclerView rvInsects;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
rvInsects = (RecyclerView)findViewById(R.id.recycler_view);

DatabaseManager databaseManager = DatabaseManager.getInstance(this);
databaseManager.queryAllInsects("friendlyName");
}

private void setupAdapter(List<InsectDataModel> insectDataModelList) {
final LayoutManager layoutManager = new LinearLayoutManager(
this, LinearLayoutManager.VERTICAL, false);
rvInsects.setLayoutManager(layoutManager);
rvInsects.setHasFixedSize(true);
final InsectAdapter insectAdapter = new InsectAdapter(insectDataModelList);
rvInsects.setAdapter(insectAdapter);
insectAdapter.notifyDataSetChanged();
}

/* Callback from database */
public void loadAllInsects(final Cursor cursor) {
InsectInteractorMapper insectInteractorMapper = new InsectInteractorMapperImp();
final List<InsectDataModel> insectDataModelList = insectInteractorMapper.map(cursor);
/* data loaded with 24 items */
setupAdapter(insectDataModelList);
}
}


InsectAdapter.kt is Kotlin.

class InsectAdapter(private val insectList: MutableList<InsectDataModel>)
: RecyclerView.Adapter<InsectAdapter.CustomInsectHolder>() {

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): CustomInsectHolder {
val view = LayoutInflater.from(parent.context)
.inflate(R.layout.insect_row_item, parent, false)
return CustomInsectHolder(view)
}

override fun onBindViewHolder(holder: CustomInsectHolder, position: Int) {
holder.tvFriendlyName.text = insectList[position].friendlyName
holder.tvScientificName.text = insectList[position].scientificName
}

override fun getItemCount(): Int {
return insectList.size
}

class CustomInsectHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
val ivDangerLevel: DangerLevelView = itemView.findViewById(R.id.ivDangerLevel)
val tvFriendlyName: TextView = itemView.findViewById(R.id.tvFriendlyName)
val tvScientificName: TextView = itemView.findViewById(R.id.tvScientificName)
}
}


The database I use rxjava2 to do the query

public class DatabaseManager {
private static DatabaseManager sInstance;
private MainActivity mainActivity;
private BugsDbHelper mBugsDbHelper;

public static synchronized DatabaseManager getInstance(MainActivity context) {
if (sInstance == null) {
sInstance = new DatabaseManager(context);
}

return sInstance;
}

private DatabaseManager(MainActivity context) {
mBugsDbHelper = new BugsDbHelper(context);

mainActivity = context;
}

@SuppressLint("CheckResult")
public void queryAllInsects(String sortOrder) {
final InsectStorageInteractorImp insectStorageInteractorImp
= new InsectStorageInteractorImp(new InsectStorageImp(mBugsDbHelper.getReadableDatabase()));

insectStorageInteractorImp.getAllSortedInsects(sortOrder)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new SingleObserver<Cursor>() {
Disposable disposable;

@Override
public void onSubscribe(Disposable d) {
disposable = d;
}

@Override
public void onSuccess(Cursor cursor) {
mainActivity.loadAllInsects(cursor);
disposable.dispose();
}

@Override
public void onError(Throwable e) {
disposable.dispose();
}
});
}
}


Everything works as expected when the apps installs for the first time. And if you clear it out of memory.
However, its only when you click the back button, and then try and start the app it will not load any data
because of the mAdapter being null in the RecyclerView class.

When I click the back button and then start the app again. All I get is a blank screen i.e.

Updated DatabaseManager class that removes the singleton and used a weakreference to ensure that the MainActivity instance is garbage collected.

public class DatabaseManager {
private WeakReference<MainActivity> mainActivity;
private BugsDbHelper mBugsDbHelper;

public DatabaseManager(MainActivity context) {
mBugsDbHelper = new BugsDbHelper(context);
mainActivity = new WeakReference<>(context);
}

@SuppressLint("CheckResult")
public void queryAllInsects(String sortOrder) {
final InsectStorageInteractorImp insectStorageInteractorImp
= new InsectStorageInteractorImp(new InsectStorageImp(mBugsDbHelper.getReadableDatabase()));

insectStorageInteractorImp.getAllSortedInsects(sortOrder)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new SingleObserver<Cursor>() {
Disposable disposable;

@Override
public void onSubscribe(Disposable d) {
disposable = d;
}

@Override
public void onSuccess(Cursor cursor) {
mainActivity.loadAllInsects(cursor);
disposable.dispose();
}

@Override
public void onError(Throwable e) {
disposable.dispose();
}
});
}
}


Many thanks for any suggestions,

5 Answers
5

When you click the back button and relaunch the app, a new instance of MainActivity is started.

MainActivity

At the same time, your DatabaseManager is a singleton. Its reference is stored as a static variable. It survives the activity recreation. It will live until the process is killed.

DatabaseManager

So, when you run queryAllInsects for the second time, the callback is sent to the old instance of MainActivity, which is not visible anymore.

queryAllInsects

MainActivity

You should not keep a reference to MainActivity in DatabaseManager. It's a memory leak, because it cannot be garbage collected.

MainActivity

DatabaseManager

DatabaseManager.getInstance(this)

DatabaseManager

queryAllInsects

The issue is most likely that you are loading the data in your onCreate() and not in onResume(). When you press back to "close the app" you are not necessarily clearing the UI stack from memory. That's why when you go back into the app, it doesn't invoke onCreate() again, and doesn't load your data again.

Keep everything the same, just move your data loading from onCreate() to onResume(). That way, whenever the screen is shown to the user, the data will load.

Few observations:

Put this 2 lines in onResume() and remove from onCreate() and try it.

DatabaseManager databaseManager = DatabaseManager.getInstance(this);
databaseManager.queryAllInsects("friendlyName");


I suggest the following changes:

MainActivity, the less code you write in the activity the better, move all the data retrieval part to the DatabaseManager. Also setup the RecyclerView once and only update the dataset when appropriate:

DatabaseManager

RecyclerView

public class MainActivity extends AppCompatActivity {

private List<InsectDataModel> insectDataModelList = new ArrayList<>();

private Disposable disposable;
private RecyclerView rvInsects;
private InsectAdapter insectAdapter;


@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
setupAdapter();

//Request Data, take advantage of RxJava to load data asynchronously
DatabaseManager.getInstance(this)
.queryAllInsects("friendlyName")
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new SingleObserver<List<InsectDataModel>>() {
@Override
public void onSubscribe(Disposable d) {
disposable = d;
}

@Override
public void onSuccess(List<InsectDataModel> response) {
insectDataModelList.clear();
insectDataModelList.addAll(response);
insectAdapter.notifyDatasetChanged();
}

@Override
public void onError(Throwable e) {
Toast.makeText(this, e.getMessage(), Toast.LENGTH_SHORT).show();
}
});;
}

private void setupAdapter() {
//Setup RecyclerView Only need to be called once
rvInsects = (RecyclerView)findViewById(R.id.recycler_view);
LayoutManager layoutManager = new LinearLayoutManager(this); // LinearLayoutManager is Vertical by default
rvInsects.setLayoutManager(layoutManager); // You don't event have to define it as RecyclerView use LinearLayoutManager.Vertical by default
rvInsects.setHasFixedSize(true);
insectAdapter = new InsectAdapter(insectDataModelList);
rvInsects.setAdapter(insectAdapter);
}


@Override
protected void onDestroy() {
//Dispose observer if activity is destroyed to prevent memory leak
if(disposable != null && !disposable.isDisposed())
disposable.dispose();

super.onDestroy();
}
}


And in DatabaseManager, instead of observing the data source(Cursor) and notify the requester(Activity) via callback, we get the data stream and pass it the caller to observe:

public class DatabaseManager {
private static DatabaseManager sInstance;
private BugsDbHelper mBugsDbHelper;

public static synchronized DatabaseManager getInstance() {
if (sInstance == null) {
sInstance = new DatabaseManager();
}

return sInstance;
}

private DatabaseManager() {
// Move the actualy database initiation to application class or singleton
mBugsDbHelper = BugsDbHelper.getInstance(); // or ApplicationController.getDbHelper();
}

@SuppressLint("CheckResult")
public SingleObserver<List<InsectDataModel>> queryAllInsects(String sortOrder) {
final InsectStorageInteractorImp insectStorageInteractorImp
= new InsectStorageInteractorImp(new InsectStorageImp(mBugsDbHelper.getReadableDatabase()));

insectStorageInteractorImp.getAllSortedInsects(sortOrder)
.map(new Function<Cursor, List<Object>>() {
@Override
public List<Object> apply(Cursor cursor) throws Exception {
InsectInteractorMapper insectInteractorMapper = new InsectInteractorMapperImp();
return insectInteractorMapper.map(cursor);
}
});
}
}


Now the solution here is to rely on the RxJava to change the callback pattern to the observer pattern. So instead of passing the activity (callback) and waiting to be called, we get the data steram (observable) and observe it for the response. This eliminate the leak problem all together and enhance the readability and maintainability.

RxJava

Also don't forget to move the Database initialization to the Application class or a Singleton instance to prevent multiple instantiation. The easier solution would be like:

Database

public class ApplicationController extends Application {

private BugsDbHelper mBugsDbHelper;

@Override
public void onCreate() {
super.onCreate();
mBugsDbHelper = new BugsDbHelper(this);
}

public BugsDbHelper getDbHelper(){
return mBugsDbHelper ;
}
}


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