var v : UIView?
the variable v may be nil or may be a valid UIView Object
var uv = v!
although v is optional, but when the above line is executed, developer promises the v points to a valid uiView object, otherwise, the app will crash
var m = v?.backgroundColor
if v is nil, the express return nil. Otherwise, returning the UIView's background color
if let t = v {
var h = t.backgroundColor
}
else{
}
combine if check with optional variable. t is not optional
var k : UIView!
var bc = k.backgroundColor
k, as an
implicitly unwrapped optional type, is similar to option type, so we do no need to set an initial value when declaring and defining the variable, also we do no need to explicitly unwrap the variable with ! when assigning it to a non optional variable.
However, when defining implicitly unwrapped option type, developer promised that whenever the variable is used, it will always have a valid value assign to it. so the variable will be set before its value is used. This type is mostly used for IBOutlet.
let a : UIView?= v as? UIView
case v to UIView and assign to a. The result may be nil or an option variable points to a valid object
let m: UIView = v as UIView
Developer guarantees the v points to a valid object
option chain
When accessing an optional variable's property or method, the variable must be unwrapped with either ? or !, so that if the variable is nil, the express can return nil gracefully. If the variable definitely has a value, then ! can also be used to unwrapped the variable.
var mainStoryBoard = window?.rootViewController?.storyboard
