Retrieve data from firebase database on button click

I'm writing an app that allows users to collaborate on inventory by uploading data to a database hosted by firebase. I can write data to it no problem, however it only wants to let me retrieve data when an event is triggered, such as the data being changed. All I want is to retrieve a string that I've saved to the database and set it as the text value of a TextView element on my page, and I want this to happen when I click a button. How do I do this?

Here's the code for the page where the user writes the data:

addButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
DatabaseReference mDatabase;
mDatabase = FirebaseDatabase.getInstance().getReference();

String dbLocationTag = location.getText().toString();
String itemInfo = "Item Description: " + itemDescription.getText().toString() + "\n" + "Job Number: " + jobNumber.getText().toString();

mDatabase.setValue(dbLocationTag, itemInfo);

Context context = getApplicationContext();
final Intent inventoryHomeScreen = new Intent();
inventoryHomeScreen.setClass(context, Inventory_Management.class);
startActivity(inventoryHomeScreen);
}
});

Here's where I would like to be able to retrieve the data:

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_inventory__view__item);

final TextView itemInfo = (TextView) findViewById(R.id.itemInfoTextView);

final Button viewButton = (Button) findViewById(R.id.viewButton);
assert viewButton != null;
viewButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {

//I would like to set the text of itemInfo to the data I just wrote

}
});
}

You need to use `addListenerForSingleValueEvent()` to retrieve data

DatabaseReference mDatabase;
mDatabase = FirebaseDatabase.getInstance().getReference();
mDatabase.child("<childName>").addListenerForSingleValueEvent(new ValueEventListener(){
@Override
public void onDataChange(DataSnapshot dataSnapshot) {

}

@Override
public void onCancelled(DatabaseError databaseError) {

}
});

Here in `child` you need to mention your child node name

If you want to get change of whole database, simply remove `child` so it look like `mDatabase.addListenerForSingleValueEvent()`

You can use `addValueEventListener()` to retrieve data when an event is triggered.

In your `onClick()` method you can write :

DatabaseReference mDatabase = FirebaseDatabase.getInstance().getReference();

// In the database whenever any data is changed then the below code snipped is going to be executed.
mDatabase.child("<dbLocationTag>").addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {

// the changed value is going to be stored into the dataSnapshot
// to retrieve value from dataSnapshot we write

String value = dataSnapshot.getValue(String.class);
mValueView.setText(value);

// NOTE: if you've multiple childs of same node then you can use Map to display all of them : EG:
/*Map<String, String> map = dataSnapshot.getValue(Map.class);
String val1 = map.get("val1");
String val2 = map.get("val2");
String val3 = map.get("val3");
Log.v("TAG", "Value 1 "+val1);
Log.v("TAG", "Value 2 "+val2);
Log.v("TAG", "Value 3 "+val3);
*/
}

@Override
public void onCancelled(FirebaseError firebaseError) {

mValueView.setText(firebaseError.toString());

}
});

addButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
DatabaseReference mRef = mDatabaseReference.getDatabase().getReference("users");
ValueEventListener valueEventListener = new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot ds : dataSnapshot.getChildren()) {
Log.d(TAG, "" + ds.getKey());
}
}

@Override
public void onCancelled(DatabaseError databaseError) {

}
};
}
});
mRef.addListenerForSingleValueEvent(valueEventListener);

The above method worked for me. Hope it helps for you, too.