Adding values to Array in Swift Class but Array claims it is empty when tested - arrays

Question

I am trying to make an array of integers with data I have stored on an online database (Parse). I know for a fact that I am receiving the data and that my variable is storing that data. I also know for a fact that the data value is being added as when I print out the size of the array under my append line, the size constantly increases until it reaches the size of my database (90). However, when I print the size of the array at the end of the method or in my constructor, I get a value of 0. I have a feeling that this problem is happening because of the "self" keyword but I am not sure how to get around it. My end goal is store all the values from the database into an array that I create and can globally access.
Here is the code:
import Foundation
class DataLoader
{
var allData: [Int] = []
init()
{
allData = []
generateAllData()
println(allData.count)
}
private func generateAllData()
{
var tempVal: Int = 0
var tempArray: [Int] = []
Parse.setApplicationId("CENSORED FOR PRIVACY REASONS", clientKey: "CENSORED FOR PRIVACY REASONS")
var query = PFQuery(className: "Snapshot")
query.selectKeys(["objectID"])
query.findObjectsInBackgroundWithBlock
{
(objects: [AnyObject]!, error: NSError!) -> Void in
if error == nil
{
for obj in objects
{
var temp: String = obj.objectId
var newQuery = PFQuery(className: "Data")
newQuery.getObjectInBackgroundWithId(temp)
{
(dataValue: PFObject!, error: NSError!) -> Void in
if error == nil && dataValue != nil
{
tempVal = dataValue["Year"].integerValue
}
else
{
println(error)
}
}
self.allData.append(tempVal)
println(self.allData.count)
}
}
}
println(allData.count)
}
}

Answer 1

The problem is that the println's at the end of the method and the end of the init are happening while the background operation is still getting the data.
When you call "query.findObjectsInBackgroundWithBlock", it will go off into the background and get the data. But that does not block the next line from being called and it does not stop the function returning.
I suggest you add a callback or delegate method so that the rest of your code knows when the data has completely downloaded.
And you use of "self" is correct. Inside a block, you have to provide that context.
Update:
Adding more info about blocks and callbacks.
When you are using a block, the code after the block continues on without waiting for the block to complete. So a good way to know when the block has finished it to send a callback function as one of the parameters to the function containing the block. Once the block has completed, it can use the callback function to report back.
As a shorter example than your code, have a look at this animation function:
doSomeAnimation() // call the function below
func doSomeAnimation() {
println("Starting doSomeAnimation")
UIView.animateWithDuration(2.0,
animations: { () -> Void in
// animate something here
self.alphaButton.alpha = 1.0
}) { (animationDone) -> Void in
// animation over
println("Animation complete")
}
println("Ending doSomeAnimation")
}
You will see it contains 3 println() statements and the order in which they appear may surprise you:
Starting doSomeAnimation
Ending doSomeAnimation
Animation complete
So the doSomeAnimation() function is over long before the animation has completed. In this trivial example, that doesn't matter, but what if you need to know before the rest of your code could proceed?
One solution is to send a callback function to the function with the block. Here is the previous example with a callback in place:
doSomeAnimation() { (complete : Bool) in
println("Animation callback")
}
func doSomeAnimation(callback : ( Bool ) -> Void) {
println("Starting doSomeAnimation")
UIView.animateWithDuration(2.0,
animations: { () -> Void in
// animate something here
self.alphaButton.alpha = 1.0
}) { (animationDone) -> Void in
// animation over
println("Animation complete")
callback(animationDone)
}
println("Ending doSomeAnimation")
}
So the doSomeAnimation() function is called, but it is provided with a function as its parameter. It performs the animation and when the animation has finished, it uses this supplied callback function to report back to the caller.
And now the println() statements show as follows:
Starting doSomeAnimation
Ending doSomeAnimation
Animation complete
Animation callback
Hopefully this makes it clearer, both what the issue is an a possible solution.