您可以通过在屏幕的选项字典中为键NSWorkspaceDesktopImageScalingKey设置NSImageScaling.scaleProportionallyUpOrDown来获得“适合大小”行为。
Swift 3中的示例：

do {
    // here we use the first screen, adapt to your case
    guard let screens = NSScreen.screens(),
        let screen = screens.first else
    {
        // handle error if no screen is available
        return
    }
    let workspace = NSWorkspace.shared()
    // we get the screen's options dictionary in a variable
    guard var options = workspace.desktopImageOptions(for: screen) else {
        // handle error if no options dictionary is available for this screen
        return
    }
    // we add (or replace) our options in the dictionary
    // "size to fit" is NSImageScaling.scaleProportionallyUpOrDown
    options[NSWorkspaceDesktopImageScalingKey] = NSImageScaling.scaleProportionallyUpOrDown
    options[NSWorkspaceDesktopImageAllowClippingKey] = true
    // finally we write the image using the new options
    try workspace.setDesktopImageURL(destinationURL, for: screen, options: options)
} catch {
    print(error.localizedDescription)
}

出于可见性目的，.imageScaling需要一个NSNumber。
它看起来是这样的：

var options: [NSWorkspace.DesktopImageOptionKey: Any] = [:]
let scaleString: String = opts["scale"] as! String
if scaleString == "fill" {
       options[.imageScaling] = NSNumber(value: NSImageScaling.scaleAxesIndependently.rawValue)
    } else if scaleString == "fit" {
        options[.imageScaling] = NSNumber(value:NSImageScaling.scaleProportionallyUpOrDown.rawValue)
    } else if scaleString == "stretch" {
        options[.imageScaling] = NSNumber(value:NSImageScaling.scaleAxesIndependently.rawValue)
    } else if scaleString == "center" {
        options[.imageScaling] = NSNumber(value:NSImageScaling.scaleNone.rawValue)
    } else {
        options[.imageScaling] = NSNumber(value:NSImageScaling.scaleProportionallyUpOrDown.rawValue)
    }

try NSWorkspace.shared.setDesktopImageURL(destinationURL, for: NSScreen.main!, options: options)

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