处理复杂的Firebase数据库查询的空返回

Question

I have a search feature built into my app that filters users from my firebase database.

I am able to do this correctly to an extent. The issue is whenever I search for a user that doesn't exist the app crashes with the error: Index out of range

These are UICollectionViewCells so I handle the cells returned in numberOfItemsInSection and I thought I had coded for the condition nil or zero but apparently not.

Here is where the search is performed

func searchUsers(searchText: String) {

    let ref = Database.database().reference()
    ref.child("users").queryOrdered(byChild: "name").queryStarting(atValue: searchText).queryEnding(atValue: "\(searchText)\u{f8ff}")
        .observe(.childAdded, with: {(snapshot) in

            if let dictionary = snapshot.value as? [String: AnyObject] {
                let user = User()
                user.setValuesForKeys(dictionary)

                self.users.append(user)
                print(snapshot)

                DispatchQueue.main.async(execute: {
                    self.collectionView?.reloadData()
                })
            }

                print(self.users.count)
        }, withCancel: nil)

}

Any suggestions?

Answer 1

you should not return Int() from the numberOfItemsInSection return 0 instead

override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
   switch section {
   case 0:
      return users.count
   case 1:
     // handle based on section 1 datasource count
}

Answer 2

After carefully reading the documentation I was able to deduce the source of the error. Here is a line from the FireBase documentation:

"If no data exists at the specified location, the FDataSnapshot will return an NSNull."

So to circumnavigate the error normally you would be able to add something along the lines of:

if snapshot.value == NSNULL {

    print("no users returned") 
} else {
//do something with data

However, with the FireBase query I was performing, the observation was that of .childAdded events.

The FireBase documentaion calls these queries, complex queriers. In my case I was using .queryStarting and .queryEnding which only return .childAdded events so checking the value wouldn't work inside the query parameters and the application would crash.

The solution is to check if the query returns null BEFORE you preform the search.

This is especially important if you are using the data to return something like collectionView or tableView Cells.

So to wrap up, all I had to do, was simply add a reference observation on the query return value before I used it to create my UICollectionViewCells. That ended up looking like this.

ref.observe(.value, with: { snap in
    if snap.value is NSNull {
        print("no users returned")
        self.users.removeAll()

        DispatchQueue.main.async(execute: {
        self.collectionView?.reloadData()
        })

    }
})

Hope this helps someone.