How to Parse a Single Dictionary from Firebase Snapshot? - ios

Question

My code is:
_ = DataService.ds.REF_POSTS.child(key!).observe(.value, with: { (snapshot) in
})
the key is equal to a post key(-Kef2J6vJBCjHGYjSlkI). I want to retrieve only one element at a time.
There is only 1 dictionary under that child:
I need to create a dictionary out of the contents of this. However, when I implement regular algorithm:
_ = DataService.ds.REF_POSTS.child(key!).observe(.value, with: { (snapshot) in
if let snapshot = snapshot.children.allObjects as? [FIRDataSnapshot] {
for snap in snapshot {
print("Zhenya: here is the snap: \(snap)")
if let postDict = snap.value as? Dictionary<String, AnyObject> {
let key = snap.key
let post = Post(postKey: key, postData: postDict)
self.posts.append(post)
} else {
print("Zhenya: failed to convert")
}
}
}
})
The 'snap' prints elements of the dictionary itself, not the whole dictionary. So it always fails the conversion on the following line.
Bonus for ninjas:
If you can suggest a better architecture, I would appreciate that.
It seems that retrieving posts from FB one by one will slow down performance considerably.
The reason it is 'one by one' is this one post is the result of user's GeoFire specification. Geofire returns keys to posts that are nearby.
I want to filter Firebase's main branch of posts based on those returned keys, but there is no way to do that - only load one by one (.orderByChild - doesn't returns posts, but instead, orders all the posts. If you you know how it can help me, please advice)

Answer 1

This is the code provided in your question:
_ = DataService.ds.REF_POSTS.child(key!).observe(.value, with: { (snapshot) in
if let snapshot = snapshot.children.allObjects as? [FIRDataSnapshot] {
for snap in snapshot {
print("Zhenya: here is the snap: \(snap)")
if let postDict = snap.value as? Dictionary<String, AnyObject> {
let key = snap.key
let post = Post(postKey: key, postData: postDict)
self.posts.append(post)
} else {
print("Zhenya: failed to convert")
}
}
}
})
Let's take a look at what's happening here.
The first line is observing posts/$post-id in Firebase using the .value method. The .value method returns the reference you provide in a FIRDataSnapshot, where snapshot.key is the name of the child and snapshot.value is a dictionary containing its children.
So, snapshot in DataService.ds.REF_POSTS.child(key!).observe(.value, with: { (snapshot) in ... }) should look like this:
$post-id
imageUrl: http://...
likes: 0
userKey: ...
But on the very next line you redefine snapshot using a local definition. That is why you are getting the children of the post, because you explicitly define it that way:
if let snapshot = snapshot.children.allObjects as? [FIRDataSnapshot] { ... }
If you remove this, you should see the behavior you want.
So what should you really be doing? Well I don't like using .value because it has a few drawbacks:
It downloads the entire node requested. If you mess up your reference for a large node (i.e, ref/users), it's easy for a .value operation to take several seconds before you see any data.
It is only called once. This is fine if your database is static, but whose database is static?
This is why I prefer using .childAdded for things like posts, messages, etc. It returns a single node at a time, which means you can populate a list progressively as new data becomes available. Using a few clever UI functions (such as tableView.insertRows(...) you can fairly smoothly display large lists of data without waiting for results. I believe it also supports paging, where you get them in batches of 10, for example. Here is a quick primer on how you can begin using .childAdded to make the most of large lists of data.