Array retuning a blank array outside of PFQuery wi

问题:

Array returning a blank array when outside of the PFQuery. For some reason, the items are not being passed to the array when compiled.

class DriverViewController: UIViewController {
var placesArr : Array<Place> = []


    override func viewDidLoad() {
    super.viewDidLoad()
    self.window = UIWindow(frame: UIScreen.mainScreen().bounds)


    var query = PFQuery(className:"places")
    query.whereKey("username", equalTo:"[e-mail]")
    query.findObjectsInBackgroundWithBlock {
        (objects: [AnyObject]?, error: NSError?) -> Void in

        if error == nil {
            println("Successfully retrieved \(objects!.count) scores.")

            if let objects = objects as? [PFObject] {
                for object in objects {
             let x = Place(aIdent: (object["Ident"] as! Int), aName: (object["name"] as! String), aAddress: (object["originAddress"] as! String), aCity: (object["originCity"] as! String), aCategoryName: (object["catName"] as! String), aLat: (object["aLat"] as! String), aLng: (object["aLng"] as! String))

                    self.placesArr.append(x)
                    println(placesArr) //****It works here and prints an array****
                }
            }
        } else {
            // Log details of the failure
            println("Error: \(error!) \(error!.userInfo!)")
        }
    }
 println(placesArr) //****But here it returns a blank array and this is where I need it to return an array****

回答1:

This a very common misunderstanding relating to threading, the issue is what order events run:

// Runs 1st
query.findObjectsInBackgroundWithBlock {
    (objects: [AnyObject]?, error: NSError?) -> Void in
    // Runs 3rd
}
// Runs 2nd
println(placesArr)

The execution of the program doesn't halt when you call findObjectsInBackground, it finds objects: inBackground which means the heavy lifting of a network request is dispatched to a different queue so that the user can still interact with the screen. A simple way to do this would be to do:

var placesArray: [Place] = [] {
    didSet {
        // Do any execution that needs to wait for places array here.
    }
}

You can also trigger subsequent actions within the parse response block, I just personally find executing behavior dependent on a property update being executed in didSet to be a nice way to control flow.

回答2:

Logan's answer is right in the money.

See my answer in this thread: Storing values in completionHandlers - Swift

I wrote up a detailed description of how async completion handlers work, and in the comments there is a link to a working example project the illustrates it.

回答3:

query.findObjectsInBackgroundWithBlock is a block operation that performs in the background - it's asynchronous.

The line println(placesArr) actually executes before the block is finished - that's why you see nil there.