How to check does method systemFontOfSize(fontSize: weight:) exist in iOS

My application deployment target is iOS 7.0 I want to use method systemFontOfSize(fontSize: weight:) on devices with iOS 8+. iOS 7 doesn't support this method (with weight: parameter) and my application crashes. Specially for iOS 7 I want to set Helvetica Light font instead of SystemFont Light.

What is the best way to check it? Do I need to check iOS version or I can check the method? how?

I use swift and tried

if let font = UIFont.systemFontOfSize(12, weight: UIFontWeightLight)

respondsToSelector method. It didn't work for me.

Solution

respondsToSelector works as expected:

    let font: UIFont
    if UIFont.respondsToSelector("systemFontOfSize:weight:") {
        println("YES")
        font = .systemFontOfSize(12, weight: UIFontWeightLight)
    }
    else {
        println("NO")
        font = UIFont(name: "HelveticaNeue-Light", size: 12)!
    }

And I would recommend HelveticaNeue instead of Helvetica, since the system font is Neue one.