This code demonstrates how display refresh rates can be retrieved in Cocoa framework with swift

GetDisplayRefreshRates.swift

// Prints refresh rates of displays.
func debugPrintDisplayRefreshRates() {
    var displayCount: UInt32 = 0
    var result = CGGetActiveDisplayList(0, nil, &displayCount)

    let allocated = Int(displayCount)
    let activeDisplays = UnsafeMutablePointer<CGDirectDisplayID>.allocate(capacity: allocated)

    defer { activeDisplays.deallocate() }

    result = CGGetActiveDisplayList(displayCount, activeDisplays, &displayCount)
    if result != .success {
        print("error: \(result)")
        return
    }

    for i in 0 ..< displayCount {
        let displayId = activeDisplays[Int(i)]

        if CGDisplayIsBuiltin(displayId) == 1 {
            continue
        }

        // NOTE: For some reason or another, zero is printed for intrinsic display of my Macbook Pro :(
        if let mode = CGDisplayCopyDisplayMode(displayId) {
            debugPrint("[\(i)] - \(activeDisplays[Int(i)]) \(mode.refreshRate)Hz")
        }
    }
}