ios之为什么添加可选时此代码有效

在我制作的程序中，我试图访问网络内容以显示天气。我正在为可选的概念而苦苦挣扎，我想知道为什么这行代码在“？”时起作用。添加，但并非没有。

   @IBAction func pressedSearch(sender: AnyObject) { 
 
        var urlString = "http://www.weather-forecast.com/locations/" + cityField.text.stringByReplacingOccurrencesOfString(" ", withString: "") + "/forecasts/latest" 
 
        var url = NSURL(string: urlString) 
 
        println(urlString) 
 
        let task = NSURLSession.sharedSession().dataTaskWithURL(url!) { (data, response, error) in 
 
            var urlContent = NSString(data: data, encoding: NSUTF8StringEncoding) 
 
            var contentArray = urlContent?.componentsSeparatedByString("<span class=\"phrase\">") as Array<String> 
 
            println(contentArray[1]) 
 
        } 
        task.resume() 
    } 

这是“？”所在的代码行添加以使代码工作:

var contentArray = urlContent?.componentsSeparatedByString("<span class=\"phrase\">") as Array<String> 

“？”是什么意思？做修复错误，为什么我们必须添加“？”和 ”!”在整个 swift 中对某些变量而不是其他变量？

请您参考如下方法:

这里有一些代码可以帮助您更好地理解 ? 和 !(包括您告诉机器的内容):

var foo = resultOfAFunctionThatReturnsAQuestionMarkValue() 
foo.accessFunction() // "<silence>..." 
                     // this could break if foo is nil 
 
var bar = resultOfAFunctionThatReturnsAQuestionMarkValue() 
bar?.accessFunction() // "Hey, bro. This might be nil. 
                      //       If it is, just ignore the line." 
                      // accessFunction() might not be called, 
                      // but it won't explode. 
 
var asdf = resultOfAFunctionThatReturnsAQuestionMarkValue() 
asdf!.accessFunction() // "Yo, dude. I know for sure this is not nil. 
                       //      Don't bother checking first. It's cool." 
                       // this could break if asdf if nil 
                       // You just lied to the computer. 
                       //            (They will remember.)