How do I call a function only if another function is complete? (Swift and retrieving from Parse)

Question

I am trying to only call a function only if I have retrieved a certain PFObjectfrom my Parse backend in a separate function. At the moment I am calling this second function after a set delay of 3.0 and it is working, but only if the first query function is called within 3.0, otherwise I have to pullToRefresh the tableView after the first function is eventually finished for it to populate with the Parse data. (I am using a PFQueryTableViewController by the way)

Here is my code at the moment (I am aware queryForTable is an override so is being called regardless, so is there a way to change this to a normal func?) :

override func viewDidLoad() {

    // ideally want something like "if getX'IsComplete' then queryForTable() and self.loadObjects()"

    retrieveFirstX()

    let delay = 3.0 * Double(NSEC_PER_SEC) // retrieveFirstX must load within 3.0 for table to populate without needing to pullToRefresh
    let time = dispatch_time(DISPATCH_TIME_NOW, Int64(delay))
    dispatch_after(time, dispatch_get_main_queue(), {

        self.queryForTable()
        self.loadObjects()
    })
}

var X = PFObject(className: "X")

func retrieveFirstX() -> Bool {

    let xQuery = PFQuery(className: "X")
    xQuery.orderByDescending("createdAt")
    xQuery.getFirstObjectInBackgroundWithBlock {
        (object: PFObject?, error: NSError?) -> Void in
        if error == nil {

            self.X = object!
        }
    }
    return true
}

 override func queryForTable() -> PFQuery {

    let xRelation = X.relationForKey("xPosts") as PFRelation!
    let relationQuery = xRelation.query()

    let xPostsQuery = PFQuery(className: "Posts")
    xPostsQuery.includeKey("postUser")
    xPostsQuery.whereKey("objectId", matchesKey: "objectId", inQuery: relationQuery)
    xPostsQuery.cachePolicy = .NetworkElseCache
    xPostsQuery.orderByDescending("createdAt")
    return xPostsQuery
}

Do I need to use completion handlers, and if so how do I do that as I have never used them before?

Thanks in advance!

score 3 · Accepted Answer · answered Dec 13 '15 at 16:05

3

A completion handler sounds right, something like:

override func viewDidLoad() {
    let completionHandler = {
        self.queryForTable()
        self.loadObjects()
    }
    retrieveFirstX(completionHandler)
}

func retrieveFirstX(completion: ()->()) -> Bool {
    let xQuery = PFQuery(className: "X")
    xQuery.orderByDescending("createdAt")
    xQuery.getFirstObjectInBackgroundWithBlock {
        (object: PFObject?, error: NSError?) -> Void in
        if error == nil {
            self.X = object!
            completion()
        }
    }
    return true
}

answered Dec 13 '15 at 16:05

Phillip Mills

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That worked! Thanks! -- by the way, the queryForTable() is now being called twice, which I am assuming is down to it being called in viewDidLoad() and with it being an override func. Do you know if it is possible to make queryForTable() not an override func so that it is only called once upon opening the app? – Chris Wright Dec 13 '15 at 16:30
I don't know but you might want to put a breakpoint in that function and look at the stack trace each time to see where it is being called from. – Phillip Mills Dec 13 '15 at 16:34

How do I call a function only if another function is complete? (Swift and retrieving from Parse)

1 Answers1