How to use GCD to draw image?

Question

I drawn an image previously (on main thread, synchronously), it works well. Below is what I drawn:

But now, I'd like to utilize GCD to redraw it for improving performance. But I got an image like below after I turn the code to use GCD:

Why the gray circle goes sideway?

This is the latest code (asynchronously):

let queue3 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
print("frame: \(self.frame)")
self.attachSunMoon()
self.attachClock(rect)

dispatch_async(queue3, {
   self.buildCircle(circleRadius: self.radius)
})

Before, the code is (synchronously, this works well, but not what I want, I prefer to utilize GCD):

print("frame: \(self.frame)")
self.attachSunMoon()
self.attachClock(rect)
self.buildCircle(circleRadius: self.radius)

At last, the buildCircle function:

    /// Draw the big circle
private func buildCircle(circleRadius radius: CGFloat) {

    UIGraphicsPushContext(self.ccontext!)
    defer { UIGraphicsPopContext() }


    print("frame: \(self.frame)")
    // Drawing code
    let center = CGPoint(x: self.frame.width / 2, y: self.frame.height / 2)
    self.centerPoint = center
    //let radius = max(rect.width, rect.height) - 40
    let startAngle: CGFloat = 0
    let endAngle: CGFloat = 2*π

    let path = UIBezierPath(arcCenter: center, radius: radius, startAngle: startAngle, endAngle: endAngle, clockwise: true)
    path.lineWidth = self.arcWidth
    clockBorderColor.setStroke()
    path.stroke()
}

Appreciate your help! :)

score 0 · Answer 1 · answered Jul 27 '16 at 05:04

0

You have to do all UI operations on the main thread.

answered Jul 27 '16 at 05:04

Gargoyle

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score 0 · Accepted Answer · edited May 23 '17 at 11:51

0

You can move your drawing code to a background thread. You just need to be sure to update the actual UI back on the main thread. There seems to be a few good resources on StackOverflow that cover this. Check out: Having UIView drawRect occur in a background thread

edited May 23 '17 at 11:51

Community

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answered Jul 27 '16 at 05:11

D.C.

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Yes, you are right, I've already fixed that problem via drawing content in a background thread, and when the drawing operation complete, I show the image on the UI in the main thread. – Rex Tsao Jul 27 '16 at 06:24

How to use GCD to draw image?

2 Answers2