FirebaseLoaders

FirebaseItemLoader.java

package nl.remcomokveld.firebaseloaders;

import android.content.Context;
import android.support.annotation.NonNull;
import android.support.v4.content.Loader;

import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.ValueEventListener;

public class FirebaseItemLoader<Type> extends Loader<Type> implements ValueEventListener {

    private final DatabaseReference databaseReference;
    private final SnapshotAdapter<Type> snapshotAdapter;

    public FirebaseItemLoader(@NonNull Context context,
                              @NonNull DatabaseReference databaseReference,
                              @NonNull SnapshotAdapter<Type> snapshotAdapter) {
        super(context);
        this.databaseReference = databaseReference;
        this.snapshotAdapter = snapshotAdapter;
    }

    @Override
    protected void onStartLoading() {
        super.onStartLoading();
        databaseReference.addValueEventListener(this);
    }

    @Override
    protected void onStopLoading() {
        super.onStopLoading();
        databaseReference.removeEventListener(this);
    }

    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        deliverResult(snapshotAdapter.adapt(dataSnapshot));
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
}

FirebaseListLoader.java

package nl.remcomokveld.firebaseloaders;

import android.content.Context;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.content.Loader;

import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class FirebaseListLoader<Type> extends Loader<List<Type>> implements ChildEventListener {

    private final SnapshotAdapter<Type> snapshotAdapter;
    private final DatabaseReference mDatabaseReference;

    /** Both a List and a HashMap are used for storing references to the result because
     *  the list allows keeping the data sorted and the Map allows O(n) access times when updating
     *  or deleting data
     */
    List<Item> mItems = new ArrayList<>();
    Map<String, Item> mItemMap = new HashMap<>();

    public FirebaseListLoader(@NonNull Context context,
                              @NonNull DatabaseReference databaseReference,
                              @NonNull SnapshotAdapter<Type> snapshotAdapter) {
        super(context);
        this.snapshotAdapter = snapshotAdapter;
        this.mDatabaseReference = databaseReference;
    }

    @Override
    protected void onStartLoading() {
        super.onStartLoading();
        mDatabaseReference.addChildEventListener(this);
    }

    @Override
    protected void onStopLoading() {
        super.onStopLoading();
        mDatabaseReference.removeEventListener(this);
    }

    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String s) {
        if (mItemMap.containsKey(dataSnapshot.getKey())) return;
        Type appWrapper = snapshotAdapter.adapt(dataSnapshot);
        Item newItem = new Item(dataSnapshot.getKey(), appWrapper, s);
        mItems.add(newItem);
        mItemMap.put(newItem.key, newItem);
        updateResult();
    }

    private void updateResult() {
        Collections.sort(mItems);
        ArrayList<Type> types = new ArrayList<>(mItems.size());
        for (Item item : mItems) {
            types.add(item.value);
        }
        deliverResult(types);
    }

    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String s) {
        mItemMap.get(dataSnapshot.getKey()).value = snapshotAdapter.adapt(dataSnapshot);
        updateResult();
    }

    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {
        Item item = mItemMap.get(dataSnapshot.getKey());
        mItems.remove(item);
        updateResult();
    }

    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String s) {
        Item item = mItemMap.get(dataSnapshot.getKey());
        item.previousKey = s;
        updateResult();
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }


    private class Item implements Comparable<Item> {

        @NonNull private String key;
        @Nullable private String previousKey;
        @NonNull private Type value;

        public Item(@NonNull String key, @NonNull Type value, @Nullable String previousKey) {
            this.key = key;
            this.previousKey = previousKey;
            this.value = value;
        }

        @Override
        public int compareTo(@NonNull Item item) {
            if (previousKey == null || key.equals(item.previousKey)) return -1;
            if (item.key.equals(previousKey)) return 1;
            return 0;
        }

    }

}

SampleSnapshotAdapter.java

private static class AppSnapshotAdapter implements SnapshotAdapter<SampleValueObject> {

  @Override
  public SampleValueObject adapt(DataSnapshot dataSnapshot) {
    return new SampleValueObject(dataSnapshot);
  }
}

SampleValueObject.java

private static class SampleValueObject {
  private final DataSnapshot dataSnapshot;

  public SampleValueObject(DataSnapshot dataSnapshot) {
    this.dataSnapshot = dataSnapshot;
  }

  @Override
  public String getId() {
    return dataSnapshot.getKey();
  }

  @Override
  public String getName() {
    return dataSnapshot.child("name").getValue(String.class);
  }
}

SnapshotAdapter.java

package nl.remcomokveld.firebaseloaders;

import com.google.firebase.database.DataSnapshot;

public interface SnapshotAdapter<Type> {
    Type adapt(DataSnapshot dataSnapshot);
}