How to drawInRect asynchronously - ios

Question

I'm trying to load a huge image (talking about 131.072x131.072 pixels) tiled up nicely into 512x512 tiles of 256x256 pixels from a bunch of URLs.
Once my function returns the Image I want to draw it in a Rect on the proper position.
Since this process takes a while, I want to run the whole thing asynchronously.
Below is what I've tried so far:
override func drawRect(rect: CGRect) {
let firstColumn = Int(CGRectGetMinX(rect) / sideLength)
let lastColumn = Int(CGRectGetMaxX(rect) / sideLength)
let firstRow = Int(CGRectGetMinY(rect) / sideLength)
let lastRow = Int(CGRectGetMaxY(rect) / sideLength)
let qos = Int(QOS_CLASS_USER_INITIATED.rawValue)
dispatch_async(dispatch_get_global_queue(qos, 0)) { () -> Void in
for row in firstRow...lastRow {
for column in firstColumn...lastColumn {
let url = NSURL(string: "https://someURL/\(row)/\(column).jpg")
let tile = UIImage(data: NSData(contentsOfURL: url!)!)!
let x = self.sideLength * CGFloat(column)
let y = self.sideLength * CGFloat(row)
let point = CGPoint(x: x, y: y)
let size = CGSize(width: self.sideLength, height: self.sideLength)
var tileRect = CGRect(origin: point, size: size)
tileRect = CGRectIntersection(self.bounds, tileRect)
dispatch_async(dispatch_get_main_queue()) {
tile.drawInRect(tileRect)
}
}
}
}
}
And I'm getting this error:
<Error>: CGContextRestoreGState: invalid context 0x0. Backtrace:
<-[UIImage drawInRect:]+66>
<_TFFFC6H1Z1DB15MyClass8drawRectFS0_FVSC6CGRectT_U_FT_T_U_FT_T_+122>
<_TTRXFo__dT__XFdCb__dT__+39>
<_dispatch_call_block_and_release+12>
<_dispatch_client_callout+8>
<_dispatch_main_queue_callback_4CF+1738>
<__CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__+9>
<__CFRunLoopRun+2073>
<CFRunLoopRunSpecific+488>
<GSEventRunModal+161>
<UIApplicationMain+171>
<main+109>
Can anybody give me a hint on how to retrieve and draw the image asynchronously?

Answer 1

Don't use drawRect. There's no reason to use drawRect in this scenario. Simply use views, or layers, or SpriteKit, or OpenGL ES (there are probably more possible choices). In the first two cases, you'll probably have to add/remove bits and pieces based on the part of the view which is visible on screen, but using standard views/layers will get you much better performance. Apple strongly recommends against using drawRect.
If you do use drawRect, certainly don't load data while in there. Apple clearly states that you should be drawing, not doing anything else while in there. And you certainly don't want to start asynchronous tasks while in there, this will just lead to a catastrophe. Load the data beforehand, store it somewhere, and just do the drawing while in drawRect. If you load data as the user moves around, do the loading as the user moves, not when you draw. You'll probably need to invalidate rects when the image has actually been loaded so that drawRect is then called. But again, don`t use drawRect. Just add/remove views/layers.
Also I recommend not using NSData(contentsOfURL:). Use an NSURLSession dataTask* with the appropriate completion handler. This way, all of your loads will happen simultaneously (up to the set limits), not one after the other.