Wait for other processes - ios

Question

I have to wait for multiple background process before doing other, I am looking for and trying to do something with this, but I can't. Here is the code. I commented the parts to explain what I want to reach. I could do it with completion handler but this function is called for a few times, depending on number of photos.
func prepareArray(stext: NSMutableAttributedString, completionHandler: CompletionHandler) {
self.finalForUpload = [String]()
var attributedArray = [AnyObject]()
var lastRng = 0
let range:NSRange = NSMakeRange(0,stext.length)
stext.enumerateAttributesInRange(range, options: NSAttributedStringEnumerationOptions(rawValue: 0), usingBlock: {(dic: [String:AnyObject]?, range :NSRange!, stop: UnsafeMutablePointer<ObjCBool>) -> Void in
let attachement = dic as NSDictionary!
attachement.enumerateKeysAndObjectsUsingBlock({ (key:AnyObject?, obj:AnyObject?, stop:UnsafeMutablePointer<ObjCBool>) -> Void in
let stri = key as! NSString!
if stri == "NSAttachment"{
let att:NSTextAttachment = obj as! NSTextAttachment
if att.image != nil{
print("range location: \(range.location)")
if range.location != lastRng {
attributedArray.append(stext.attributedSubstringFromRange(NSMakeRange(lastRng, range.location-lastRng)))
lastRng = range.length
}
let im:UIImage = att.image!
self.httpManager.uploadPhoto(im, completionHandler: { (imgName) -> Void in
let imName = imgName.characters.dropFirst()
let imageName = String(imName.dropLast())
//WAIT FOR THIS
dispatch_async(dispatch_get_main_queue(), {
print("append")
//stext.attributedSubstringFromRange(range)
attributedArray.append(imageName)
lastRng = range.location
})
})
}
}
})
})
//BEFORE DOING THIS
dispatch_async(dispatch_get_main_queue(), {
if lastRng == 0 && attributedArray.count < 1{
attributedArray.append(stext)
} else {
attributedArray.append(stext.attributedSubstringFromRange(NSMakeRange(lastRng, stext.length-lastRng)))
}
for var i = 0; i<attributedArray.count; i++ {
print("\n \n \(i) : \(attributedArray[i])")
}
})
}

Answer 1

You can do this using dispatch_group methods.
However I would recommend using something like PromiseKit or similar which gives you a much nicer way of controlling dependencies of multiple async blocks.
Here is your code using dispatch_group methods:
func prepareArray(stext: NSMutableAttributedString, completionHandler: CompletionHandler) {
// create the dispatch group
dispatch_group_t aGroup = dispatch_group_create();
self.finalForUpload = [String]()
var attributedArray = [AnyObject]()
var lastRng = 0
let range:NSRange = NSMakeRange(0,stext.length)
stext.enumerateAttributesInRange(range, options: NSAttributedStringEnumerationOptions(rawValue: 0), usingBlock: {(dic: [String:AnyObject]?, range :NSRange!, stop: UnsafeMutablePointer<ObjCBool>) -> Void in
let attachement = dic as NSDictionary!
attachement.enumerateKeysAndObjectsUsingBlock({ (key:AnyObject?, obj:AnyObject?, stop:UnsafeMutablePointer<ObjCBool>) -> Void in
let stri = key as! NSString!
if stri == "NSAttachment"{
let att:NSTextAttachment = obj as! NSTextAttachment
if att.image != nil{
print("range location: \(range.location)")
if range.location != lastRng {
attributedArray.append(stext.attributedSubstringFromRange(NSMakeRange(lastRng, range.location-lastRng)))
lastRng = range.length
}
let im:UIImage = att.image!
// use dispatch_group to coordinate.
// must call before uploadPhoto as we don't know how long it will take
dispatch_group_enter(aGroup);
self.httpManager.uploadPhoto(im, completionHandler: { (imgName) -> Void in
let imName = imgName.characters.dropFirst()
let imageName = String(imName.dropLast())
dispatch_async(aGroup, dispatch_get_main_queue(), {
print("append")
//stext.attributedSubstringFromRange(range)
attributedArray.append(imageName)
lastRng = range.location
// indicate block has completed (reduced group count)
dispatch_group_leave(aGroup);
})
})
}
}
})
})
// wait for all items in group to finish before running this block
dispatch_group_notify(aGroup, dispatch_get_main_queue(), {
if lastRng == 0 && attributedArray.count < 1{
attributedArray.append(stext)
} else {
attributedArray.append(stext.attributedSubstringFromRange(NSMakeRange(lastRng, stext.length-lastRng)))
}
for var i = 0; i<attributedArray.count; i++ {
print("\n \n \(i) : \(attributedArray[i])")
}
})
}

Answer 2

I have to wait for multiple background process before doing other
You should really look at using NSOperationQueue for this requirement as it allows you to configure dependencies between all of the registered operations. In this way you could configure and add a number of downloads and then configure a post download action which is dependent on all of those downloads. The operation queue will deal with starting that operation when all others are complete.
The operations also give you a nice way to cancel the pending uploads / downloads if you don't need them any more.
You could alternatively look at using dispatch_groups to manage this. It's harder to ensure that what you've done is entirely correct.

Answer 3

The main queue is a serial queue, meaning that it executes on block at a time in first-in-first-out order. All of the dispatches in the enumerate should be done before the one at the bottom is run.
EDIT: just noticed that you async in a completion handler.
One simple thing to do is to just count down (because you know how many there are).
Just add var expectedCompletions = attachement.count before the enumerate and then count them down (expectedCompletions--) inside the block. When it gets to 0, run the code at the end.