Fetch data from Firebase by joining tables in iOS

Question

I am trying to fetch data from two different Firebase tables. Here is the structure of table:

Post {
    1{
       pImages{
           i1:true
           i2:true
       }
    }
    2{
       pImages{
           i3:true

       }
    }
}
Images{
       i1{
          iUrl : ....
          pId : 1
         }
       i2{
          iUrl :...
          pId : 1
         }
       i3{
         iUrl:....
          pId : 2
         }
 }

I need to retrieve images corresponding to post with id = 1. The following is my implementation to retrieve images:

 func retrieveImagesForPost(postId: String,completion: (result: AnyObject?, error: NSError?)->()){
        var imgArray:[Image]=[]
        let postsRef = self.ref.child("post")
        let imagesRef = self.ref.child("image")
        let postImagesRef = postsRef.child(postId).child("pImages");
        postImagesRef.observeEventType(FIRDataEventType.Value, withBlock: { (snapshot) in
            for item in snapshot.children{
                imagesRef.child(item.key).observeSingleEventOfType(.Value, withBlock: { (snap) in
                    let image = Image(snapshot: snap)
                    print(image)
                    imgArray.append(image)
                })
            }
            print(snapshot.key)
            print("called")
            completion(result:imgArray, error:nil)
        })
    }

But, the problem is I am not able to get all images in imgArray to be able to send to completion handler. Below is the output of calling retrieveImagesForPost with post id ==1.

pImages
called
<TestProject.Image: 0x7f9551e82000>
<TestProject.Image: 0x7f955466a150>

The images are retrieved after the completion handler is called. I tried the dispatch groups and the semaphores approach as described in the following post. But the results are still the same. How can I make completion handler to wait for all images to be fetched from Firebase?

Answer 1

Keep a counter that you increase as each image is loaded. Once the counter reaches the length of the snapshot.children list, you're done and call your completion handler.

let postImagesRef = postsRef.child(postId).child("pImages");
postImagesRef.observeEventType(FIRDataEventType.Value, withBlock: { (snapshot) in
    var counter = 0
    for item in snapshot.children{
        imagesRef.child(item.key).observeSingleEventOfType(.Value, withBlock: { (snap) in
            let image = Image(snapshot: snap)
            print(image)
            imgArray.append(image)
            counter = counter + 1
            if (counter == snapshot.childrenCount) {
                completion(result:imgArray, error:nil)
            }
        })
    }
})

You should probably add some error handling in the above, but in general this approach is tried and tested.