CGGetActiveDisplayList.swift

import CoreGraphics

func printDisplays() {
    var displayCount: UInt32 = 0;
    var result = CGGetActiveDisplayList(0, nil, &displayCount)
    if (result != 0) {
        println("error: \(result)")
        return
    }
    let allocated = Int(displayCount)
    var activeDisplays = UnsafeMutablePointer<CGDirectDisplayID>.alloc(allocated)
    result = CGGetActiveDisplayList(displayCount, activeDisplays, &displayCount)
    if (result != 0) {
        println("error: \(result)")
        return
    }
    println("\(displayCount) displays:")
    for i in 0..<displayCount {
        println("[\(i)] - \(activeDisplays[Int(i)])")
    }
    activeDisplays.dealloc(allocated)
}

printDisplays()

-  var activeDisplays
+ let activeDisplays

adjusted for swift 5.2:

import AVFoundation

    func printDisplays() {
        var displayCount: UInt32 = 0;
        var result = CGGetActiveDisplayList(0, nil, &displayCount)
            
        if result != .success {
            print("error: \(result)")
            return
        }

        let allocated = Int(displayCount)
        let activeDisplays = UnsafeMutablePointer<CGDirectDisplayID>.allocate(capacity: allocated)
        result = CGGetActiveDisplayList(displayCount, activeDisplays, &displayCount)
        if result != .success {
            print("error: \(result)")
            return
        }

        print("\(displayCount) displays:")
        for i in 0..<displayCount {
            print("[\(i)] - \(activeDisplays[Int(i)])")
        }
        activeDisplays.deallocate()
    }