Clearing Firebase observations from a UITableViewCell - ios

Question

In all iOS classes that use Firebase you will have code like this,
private func clearObservations() {
// your method for clearing observations, probably something like
blah blah. removeAllObservers()
}
In view controllers, it's essential that you call this in viewWillDisappear (or viewDidDisappear)
override func viewWillDisappear(_ animated: Bool) {
super.viewWillDisappear(animated)
clearObservations()
}
That's fine.
Assume that you have created an observation in a UITableViewCell.
What is the best place in a cell to "clear observations" ?
Note that prepareForReuse is useless, try it.
The only approach we've found is
override func willMove(toSuperview newSuperview: UIView?) {
if newSuperview == nil {
clearObservations()
}
super.willMove(toSuperview: newSuperview)
}
Seems flakey/bizarre though.
What's the deal on this?
Update
Note while "XY Answers" are interesting and informative, if anyone knows the answer to the question that would be great also!

Answer 1

Preface
This was an attempt to answer the question but the question was misunderstood. I'll leave it here as it does have some relevance regarding observers, handles and tableView cell interaction.
While you can go through those gyrations, it's not really needed in most use cases.
For example, if you add and observer to a node, there wouldn't necessarily be a someRef? variable hanging around. So here we are watching the Posts node for new posts
let postsRef = self.ref.child("Posts")
postsRef.observe(.childAdded, with: { snapshot in
print(snapshot) //add the post to the dataSource and reloadTableview/cell
})
Here's another example of watching for any posts that are changed by uid_2
let postsRef = self.ref.child("Posts")
let queryRef = postsRef.queryOrdered(byChild: "poster_id").queryEqual(toValue: "uid_2")
queryRef.observe(.childChanged) { (snapshot) in
print(snapshot) //change the post in the dataSource and reloadTableview/cell
}
No class vars are needed for this functionality and nothing needs be nil'd. The point here being that you do not have to have class vars to get observing functionality and you do not need to keep a handle for every observer (keep reading)
In view controllers, it's essential that you call this
(someRef?.removeAllObservers()) in viewWillDisappear (or Did)..
will use Firebase in the cells of tables.
To clarify; I wouldn't want to put Firebase observers in the cells of tables. The observers should be in whichever viewController controls the tableView that has cells. Cells should pull data from the dataSource array (which is backed by Firebase)
There are some circumstances where you may want to remove all observers, again no need to have a class var or a need to nil a var.
let postsRef = self.ref.child("Posts")
postsRef.removeAllObservers()
There are times when a specific observer needs to be removed (in the case where a node has observers on it's child nodes for example), and in those cases, we store a handle to that observer as say, a class var (keeping them in an array is a tidy way to do it)
class ViewController: UIViewController {
var myPostHandle : DatabaseHandle?
func addObserver() {
let postsRef = self.ref.child("Posts")
self.myPostHandle = postsRef.observe(.childAdded, with: { snapshot in
print(snapshot)
})
func stopObserving() {
if self.myPostHandle != nil {
let postsRef = self.ref.child("Posts")
postsRef.removeObserver(withHandle: self.myPostHandle) //remove only the .childAdded observer
}
}
}
Again though, once the observer is removed, the handle would go out of scope once the class closes.
Tableviews that contain cells are backed by a dataSource and that dataSource get's it's data from firebase. When something is added, changed or removed from Firebase, your app is notified and the array is updated and then the cell refreshed. No need for an observer in the cell itself.
There's no need to add dozens of observers (in the cells) - add one central observer and let it keep the array current. Refresh tableView only when something changes.
EDIT
To Address a comment regarding the use of removeAllObservers: code is worth 1000 words:
Create a new Firebase project with two button actions. Here's the code for button0 which adds an observer to a node:
func button0() {
let testRef = self.ref.child("test_node")
testRef.observe( .value) { snapshot in
print(snapshot)
}
}
when this button0 is clicked, from there on, any adds, changes, or deletes to the test node will print it's contents to the log.
func button1() {
let testRef = self.ref.child("test_node")
testRef.removeAllObservers()
}
This will remove all observers for the node specified. Once clicked, no events will print to the console.
Try it!

Answer 2

It is not right to clear observations in cell and therefore there is not a best place to do it in cell, because, firstly, this approach contradicts MVC pattern. Views only responsible for displaying content and they should only contain code that describes how they must be draw. And in the view controller you give the content for showing by views. Usually content has provided by your model. So controller connects views and model. In your case, when you place clearObservations() in cell class, you also have someRef as a class property, so you have a model in your view class and this is incorrect.
Secondly, if you try to clear observations in table cell you definitely make logic of showing some content in table in wrong way. Cell only show data that has to be generated by some object that conforms to UITableViewDataSource protocol and implements protocol methods. For instance, in cellForRow method you generate cell and setup it with some content from array. This array is generated from model (Firebase service). Your view controller may be this data source object. You have to include array property to controller class and someRef, than you fill array and reload table data. If controller's view disappeared you clear observations, but you do it only inside view controller (in viewWillDisappear()).
Overall, all manipulations with someRef you should do in view controller and therefore "clear observations" also inside controller.