Hi, how to use the "Options" from the API to change the temperature from fahrenheit to celsius?

You could either use the options in the forecast.io call like you mention e.g.

let forecastUrl = NSURL(string: "37.8267,-122.423?units=si", relativeToURL: baseUrl)

However be careful as that also changes lots of other units. The other way would be to create a new method in your Current.swift struct to change just the temperature to degrees using Maths similar to the dateStringFromUnixTime method for getting the current time.

For the simple route, you could just do a converstion method. I implemented the Swipe handler. Swipe left and it changes it to Celcius. Swipe right it switches it back.

func celsiusFromFahrenheit(fahrenheit:Int) -> Int {
    let c = ((Double(fahrenheit) - 32.0) * 5.0/9.0) // convert to Celsius
    return String.localizedStringWithFormat("%.0f",c).toInt()! // properly round. (This is SO much easier in C#)

}

I added a View level property isCelsiusSet to hold the Value.

In the closure I use a ternary operator to set the temp:

let temperature = inCelsius 
    ? self.celsiusFromFahrenheit(currentWeather.temperature) //true
    : currentWeather.temperature // false

(This could also be done in an if/else statement of course)

Obviously, you would need to use temperature when setting the label.

Finally I modified the getCurrentWeatherData signatuare to include a default parameter

func getCurrentWeatherData(inCelsius:Bool = false) -> Void{

However, that was before I checked to see if the API supported Metric... so I'll probably go back and try that out. It was a good exercise, anyhow.