Swift，parse.com：如何从查询传递数据

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
}

Answer 1

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.
    }
}

Answer 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.

Answer 3

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.