Does anyone know how to use `CGDisplayMode` from m...

# kotlin-native

[Dmitry Kandalov]

Does anyone know how to use CGDisplayMode from macos CoreGraphics in Kotlin (1.6.10)? In particular, I can’t make the following code work:

fun main() {
    val displayModes = CGDisplayCopyAllDisplayModes(1, null)!!.toNsArray()
    val displayMode = displayModes.objectAtIndex(0)
    println(displayMode) // prints <CGDisplayMode 0x600003648840> [{ BitsPerPixel = 32; ... }]
    println(displayMode as? CGDisplayMode) // compilation error: java.lang.IllegalStateException: class CGDisplayMode... long stacktrace
    println(displayMode as? CGDisplayModeRef) // prints null
    println((displayMode as? NSObject)?.className) // prints __NSCFType
}

private fun CFArrayRef.toNsArray() = CFBridgingRelease(this) as NSArray

[Sam]

Your toNSArray function looks problematic. Does it work if you just cast the CFArrayRef to an NSArray without releasing it? You’ll have to release it at some point. It just looks like you’re casting a null to an NSArray. Maybe,

private fun CFArrayRef.toNSArray(): NSArrray {
   val arr = this as NSArray
   CFBridgingRelease(this)
   return arr
}

It does feel like an awkward side affect to be releasing the reference in that function. You would get a crash if you called toNSArray twice on the same reference.

[Dmitry Kandalov]

This CGDisplayCopyAllDisplayModes(1, null) as NSArray fails at runtime with kotlin.TypeCastException.

[Dmitry Kandalov]

Using CFArrayRef directly works:

fun main() {
    val displayModes = CGDisplayCopyAllDisplayModes(1, null) as CFArrayRef
    val displayMode = CFArrayGetValueAtIndex(displayModes, 0)!!
    println(displayMode as? CGDisplayModeRef) // prints CPointer(raw=0x...)
    println(CGDisplayModeGetWidth(displayMode as? CGDisplayModeRef)) // prints screen width
    println(CGDisplayModeGetHeight(displayMode as? CGDisplayModeRef)) // prints screen height
}

But it’s not obvious to me how to convert it to NSArray (which as I understand should be convertable to List in Kotlin).

[Dmitry Kandalov]

@Sam just checked with the function you suggested and it fails at runtime 😞

private fun CFArrayRef.toNsArray(): NSArray {
    val result = this as NSArray // fails at runtime with kotlin.TypeCastException
    CFBridgingRelease(this)
    return result
}

I think overall it feels like the interop with macos API is a bit difficult and confusing.

[Sam]

CFArrayRef is a type alias for a CPointer pointing to a CFArray. Using the pointed property, you should be able to get to the actual array.

public typealias CFArrayRef = kotlinx.cinterop.CPointer<cnames.structs.__CFArray>

Interacting with the lower level Core Foundation API is not fun in either Swift or Kotlin. It usually takes a while to work out the exact magic needed and unfortunately there are more people doing it in Swift than in Kotlin so it isn’t easy to google for.

[russhwolf]

You might be able to do val arr = CFBridgingRelease(this) as NSArray.

[russhwolf]

Where this is the CFArrayRef in Sam's function

[russhwolf]

I have some CFBridging Kotlin code in Multiplatform Settings for Keychain stuff if you want a reference https://github.com/russhwolf/multiplatform-settings/blob/master/multiplatform-settings/src/appleMain/kotlin/com/russhwolf/settings/KeychainSettings.kt#L209

[russhwolf]

It's a bit of a mess but works fine as far as I know

[russhwolf]

But I mostly use NSData and NSString and arrays might be more complicated

[Dmitry Kandalov]

@russhwolf hm, I’m not sure how

val arr = CFBridgingRelease(this) as NSArray

is different from

private fun CFArrayRef.toNsArray() = CFBridgingRelease(this) as NSArray

[russhwolf]

oh sorry you were already doing that

[russhwolf]

I don't have any more helpful advice then, sorry