Swift, parse.com: how to pass data from query

Question

I have such query to parse.com. Why the numObjects variable has different values inside the findObjectsInBackgroundWithBlock and on the function exits

func searchUserInParse () -> Int {

    var numObjects : Int = 0 // the num return objects from query

    var query = PFQuery(className:"Bets")
    query.whereKey("user", equalTo: "Bob")
    query.findObjectsInBackgroundWithBlock {
        (objects: AnyObject[]!, error: NSError!) -> Void in
        if !error {
            numObjects = objects.count
            println(numObjects) // at this point the value = 1
        } else {
            // Log details of the failure
            NSLog("Error: %@ %@", error, error.userInfo)
        }
    }
    println(numObjects) // at this point the value = 0

    return numObjects
}

This is because the block runs on a background thread asynchronously (take a look at the method name). Because it's asynchronous, it returns immediately and moves on to the code after it. This means that the second println(numObjects) is called before the above code block. — Milo
– Milo, Commented Jun 29, 2014 at 9:50

Milo · Accepted Answer · 2014-06-29 20:20:47Z

4

Instead of using findObjectsInBackgroundWithBlock which runs asynchronously, try using findObjects which runs synchronously:

//Set up query...
var objects = query.findObjects()
numObjects = objects.count
println(numObjects)

Then when running your function, do it like so:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0) {
    //Search users
    searchUserInParse()

    dispatch_async(dispatch_get_main_queue()) {
        //Show number of objects etc.
    }
}

edited Jun 29, 2014 at 20:20

answered Jun 29, 2014 at 10:01

Milo

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1 Comment

hodov Over a year ago

Thanks, it works. But I get Warning: A long-running Parse operation is being executed on the main thread. Break on warnParseOperationOnMainThread() to debug.

Timothy Walters · Accepted Answer · 2014-06-29 19:20:58Z

2

That's the nature of async code, your function will run the outer code to completion, then some time later (depending on connection speed and query complexity) the completion block will run.

Your calling code should do something like the following:

create the query
start the query with a completion block
show a loading animation
return (query results still unknown)

Then you should think about the inside of the block:

check for errors
update values that the UI is bound to
tell the UI to refresh

You can't have a function that will return the count, but you could write a function that takes a completion block as a parameter, and executes it in the query completion block. That's a bit more advanced though.

answered Jun 29, 2014 at 19:20

Timothy Walters

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Comments

Yatheesha · Accepted Answer · 2014-06-29 09:52:32Z

1

query.findObjectsInBackgroundWithBlock is will be executed asynchronously , the completion block is called after fetching objects. therefore the code after the block called first hence numObjects value is 0.

answered Jun 29, 2014 at 9:52

Yatheesha

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1 Comment

Unbreakable Over a year ago

then what is the solution to it. I mean what if we need to do the processing asynchrously.

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