adding catextlayers to calayer in a background task - ios

Question

I have a situation, which I do not understand.
I like to create CATextLayers in a background task and show them in my view, because it takes some time.
This works perfect without a background task. I can see the text immediately in my view "worldmapview"
#IBOutlet var worldmapview: Worldmapview! // This view is just an empty view.
override func viewDidLoad(){
view.addSubview(worldmapview);
}
func addlayerandrefresh_direct(){
var calayer=CALayer();
var newtextlayer:CATextLayer=create_text_layer(0,y1: 0,width:1000,heigth:1000,text:"My Text ....",fontsize:5);
self.worldmapview.layer.addSublayer(newtextlayer)
//calayer.addSublayer(newtextlayer);
calayer.addSublayer(newtextlayer)
self.worldmapview.layer.addSublayer(calayer);
self.worldmapview.setNeedsDisplay();
}
When doing this in a backgroundtask, the text does not appear in my view. Sometimes, not always, it appears after some seconds (10 for example).
func addlayerandrefresh_background(){
let qualityOfServiceClass = QOS_CLASS_BACKGROUND
let backgroundQueue = dispatch_get_global_queue(qualityOfServiceClass, 0)
dispatch_async(backgroundQueue, {
var calayer=CALayer();
var newtextlayer:CATextLayer=create_text_layer(0,y1: 0,width:1000,heigth:1000,text:"My Text ....",fontsize:5);
dispatch_async(dispatch_get_main_queue(),{
self.worldmapview.layer.addSublayer(newtextlayer)
//calayer.addSublayer(newtextlayer);
calayer.addSublayer(newtextlayer)
self.worldmapview.layer.addSublayer(calayer);
self.worldmapview.setNeedsDisplay();
})
})
}
func create_text_layer(x1:CGFloat,y1:CGFloat,width:CGFloat,heigth:CGFloat,text:String,fontsize:CGFloat) -> CATextLayer {
let textLayer = CATextLayer()
textLayer.frame = CGRectMake(x1, y1, width, heigth);
textLayer.string = text
let fontName: CFStringRef = "ArialMT"
textLayer.font = CTFontCreateWithName(fontName, fontsize, nil)
textLayer.fontSize=fontsize;
textLayer.foregroundColor = UIColor.darkGrayColor().CGColor
textLayer.wrapped = true
textLayer.alignmentMode = kCAAlignmentLeft
textLayer.contentsScale = UIScreen.mainScreen().scale
return textLayer;
}
Does someone see, what is wrong ?
What is very confusing : The same doing with CAShapeLayer works in the background.

Answer 1

Looks like setNeedDisplay not cause sublayers redraw and we need call it on all layers we need, in this case it's newly added layer
func addlayerandrefresh_background(){
let calayer = CALayer()
calayer.frame = self.worldmapview.bounds
dispatch_async(backgroundQueue, {
for var i = 0; i < 100 ; i+=10 {
let newtextlayer:CATextLayer=self.create_text_layer(0, y1: CGFloat(i) ,width:200,heigth:200,text:"My Text ....",fontsize:5)
calayer.addSublayer(newtextlayer)
}
dispatch_async(dispatch_get_main_queue(),{
self.worldmapview.layer.addSublayer(calayer)
for l in calayer.sublayers! {
l.setNeedsDisplay()
}
})
})
}

Answer 2

All the changes on UI are performed on main thread. You cannot update User Interface on background thread. According to Apple's documentation
Work involving views, Core Animation, and many other UIKit classes
usually must occur on the app’s main thread. There are some exceptions
to this rule—for example, image-based manipulations can often occur on
background threads—but when in doubt, assume that work needs to happen
on the main thread.