How to swap from MTKView to UIView display seamlessly - swift

Question

I have an MTKView whose contents I draw into a UIView. I want to swap display from MTKView to UIView without perceptible changes. How to achieve?
Currently, I have
let strokeCIImage = CIImage(mtlTexture: metalTextureComposite...) // get MTLTexture
let imageCropCG = cicontext.createCGImage(strokeCIImage...) // convert to CGImage
let layerStroke = CALayer() // create layer
layerStroke.contents = imageCropCG // populate with CGImage
strokeUIView.layer.addSublayer(layerStroke) // add to view
strokeUIView.layerWillDraw(layerStroke) //heads up to strokeUIView
and a delegate method within layerWillDraw() that clears the MTKView.
strokeViewMetal.metalClearDisplay()
The result is that I'll see a frame drop every so often in which nothing is displayed.
In the hopes of cleanly separating the two tasks, I also tried the following:
let dispatchWorkItem = DispatchWorkItem{
print("lyr add start")
self.pageCanvasImage.layer.addSublayer(sublayer)
print("lyr add end")
}
let dg = DispatchGroup()
DispatchQueue.main.async(group: dg, execute: dispatchWorkItem)
//print message when all blocks in the group finish
dg.notify(queue: DispatchQueue.main) {
print("dispatch mtl clear")
self.strokeCanvasMetal.setNeedsDisplay() // clear MTKView
}
The idea being add the new CALayer to UIImageView, and THEN clear the MTKView.
Over many screen draws, I think this result in fewer frame drops during the View swap, but I'd like a foolproof solution with NO drops. Basically what I'm after is to only clear strokeViewMetal once strokeUIView is ready to display. Any pointers would be appreciated

Answer 1

Synchronicity issues between MTKView and UIView are resolved for 99% of my tests when I set MTKView's presentsWithTransaction property to true. According to Apple's documentation:
Setting this value to true changes this default behavior so that your
MTKView displays its drawable content synchronously, using whichever
Core Animation transaction is current at the time the drawable’s
present() method is called.
Once that is done, the draw loop has to be modified from:
commandEncoder.endEncoding()
commandBuffer.present(drawable)
commandBuffer.commit()
to:
commandEncoder.endEncoding()
commandBuffer.commit()
commandBuffer.waitUntilScheduled() // synchronously wait until the drawable is ready
drawable.present() // call the drawable’s present() method directly
This is done to prevent Core activities to end before we're ready to present MTKView's drawable.
With all of this set up, I can simply:
let strokeCIImage = CIImage(mtlTexture: metalTextureComposite...) // get MTLTexture
let imageCropCG = cicontext.createCGImage(strokeCIImage...) // convert to CGImage
let layerStroke = CALayer() // create layer
layerStroke.contents = imageCropCG // populate with CGImage
// the last two events will happen synchronously
strokeUIView.layer.addSublayer(layerStroke) // add to view
strokeViewMetal.metalClearDisplay() // empty out MTKView
With all of this said, I do see overlapping of the views every now and then, but at a much, much lower frequency