unexpectedly found nil while unwrapping an Optional value keyboardWillShow

问题

I have this code below which runs when the keyboardWillShowNotification is called:

func keyboardWillShow(_ notification: Notification) {
    //ERROR IN THE LINE BELOW            
    keyboard = (notification.userInfo?[UIKeyboardFrameEndUserInfoKey] as AnyObject).cgRectValue
    animaton = (notification.userInfo?[UIKeyboardAnimationDurationUserInfoKey] as AnyObject).doubleValue

    UIView.animate(withDuration: 0.4, animations: { () -> Void in
       self.scrollView.frame.size.height = self.scrollViewHeight - self.keyboard.height
    }) 
}

I am getting an error on the second line saying: unexpectedly found nil while unwrapping an Optional value. Basically whenever I click on one of the textFields this notification for the keyboard will be called and the code in keyboardWillShow will run. I know I put if...let statements but I want to know why I am getting nil for this.

I am not sure how I am getting this error or how to debug it either. Is it because I am running it from the simulator?

Here is what printing the notification.userInfo gives:

Optional([AnyHashable("UIKeyboardFrameEndUserInfoKey"): NSRect: {{0, 315}, {320, 253}}, AnyHashable("UIKeyboardIsLocalUserInfoKey"): 1, AnyHashable("UIKeyboardBoundsUserInfoKey"): NSRect: {{0, 0}, {320, 253}}, AnyHashable("UIKeyboardAnimationCurveUserInfoKey"): 7, AnyHashable("UIKeyboardCenterBeginUserInfoKey"): NSPoint: {160, 694.5}, AnyHashable("UIKeyboardCenterEndUserInfoKey"): NSPoint: {160, 441.5}, AnyHashable("UIKeyboardFrameBeginUserInfoKey"): NSRect: {{0, 568}, {320, 253}}, AnyHashable("UIKeyboardAnimationDurationUserInfoKey"): 0.25])

回答1:

From the docs:

let UIKeyboardFrameEndUserInfoKey: String 

Description

The key for an NSValue object containing a CGRect that identifies the end frame of the keyboard in screen coordinates

Your second key:

let UIKeyboardAnimationDurationUserInfoKey: String

Description The key for an NSNumber object containing a double that identifies the duration of the animation in seconds.

So you need to cast the first one to NSValue and the second one to NSNumber:

func keyboardWillShow(_ notification: Notification) {
    print("keyboardWillShow")
    guard let userInfo = notification.userInfo else { return }
    keyboard = (userInfo[UIKeyboardFrameEndUserInfoKey] as! NSValue).cgRectValue
    animaton = (userInfo[UIKeyboardAnimationDurationUserInfoKey] as! NSNumber).doubleValue
    // your code
}

回答2:

(How to fix your issue is clearly written in Leo Dabus's answer, so I will try to explain the error I was getting before I added the ! .)

In Swift 3, as AnyObject has become one of the most risky operation. It's related to the worst new feature called as id-as-Any.

In this line of your code:

    keyboard = (notification.userInfo?[UIKeyboardFrameEndUserInfoKey] as AnyObject).cgRectValue

The type of expression notification.userInfo?[UIKeyboardFrameEndUserInfoKey] is Any?. As you see, an Optional type Any? should not be safely converted to non-Optional AnyObject. But Swift 3 converts it with creating non-Optional _SwiftValue.

You can check this behaviour with inserting this code:

print(type(of: notification.userInfo?[UIKeyboardFrameEndUserInfoKey] as AnyObject))

So, you are trying to apply non-optional-chaining .cgRectValue to _SwiftValue, which may be confusing the Swift 3's feature: "implicit type conversion _SwiftValue back to the Swift value".

Getting too long...

---

Do NOT use as AnyObject casting in Swift 3

来源：https://stackoverflow.com/questions/39519822/unexpectedly-found-nil-while-unwrapping-an-optional-value-keyboardwillshow