CGDisplayStream only capturing a single frame

Question

I'm finally starting to play with Swift (and macOS development) for the first time. I'm trying to write a simple screen capture app to get started. I've already taken a look at and succeeded in using the AVFoundation APIs for doing this ( AVCaptureSession , AVCaptureScreenInput , etc). But now I'd like to attempt to go a little lower-level and play with the closer-to-the-metal CGDisplayStream API.

Unfortunately I've only been able to get it to capture a single frame. I suspect I may be missing something regarding potential interaction between the main Runloop and the DispatchQueue I'm passing in? Not really clear on if those things interact in the first place.

Here's a small reproduction of my issue:

import Foundation
import AVFoundation
import CoreGraphics

let mainDisplay    = CGMainDisplayID()
let displayBounds  = CGDisplayBounds(mainDisplay)
let recordingQueue = DispatchQueue.global(qos: .background)

let displayStreamProps : [CFString : Any] = [
  CGDisplayStream.preserveAspectRatio: kCFBooleanTrue,
  CGDisplayStream.showCursor:          kCFBooleanTrue,
  CGDisplayStream.minimumFrameTime:    60,
]

let displayStream = CGDisplayStream(
  dispatchQueueDisplay: mainDisplay,
  outputWidth:          Int(displayBounds.width),
  outputHeight:         Int(displayBounds.height),
  pixelFormat:          Int32(kCVPixelFormatType_32BGRA),
  properties:           displayStreamProps as CFDictionary,
  queue:                recordingQueue,

  handler: { status, displayTime, frameSurface, updateRef in
    print("is only called once")
  }
)

func quit(_ : Int32) {
  displayStream?.stop()
}

signal(SIGINT, quit)

displayStream?.start()

RunLoop.current.run()

Any help would be massively appreciated!!

Answer 1

Removing this line seems to fix the issue:

CGDisplayStream.minimumFrameTime:    60,

The docs don't mention what the unit for this "time" field is, but it appears to be in seconds. So you could change it to 1.0/60.0 for 60fps capture.