Fixing the keyboard: NotificationCenter
Fixing the keyboard: NotificationCenter 관련
Before we're done, there's a bug in our extension, and it's a bad one – or at least it's bad once you spot it. You see, when you tap to edit a text view, the iOS keyboard automatically appears so that user can start typing. But if you try typing lots, you'll notice that you can actually type underneath the keyboard because the text view hasn't adjusted its size because the keyboard appeared.
If you don't see a keyboard when you tap to edit, it probably means you have the Connect Hardware Keyboard setting turned on. Press Shift+Cmd+K to disable the hardware keyboard and use the on-screen one.
Having our view adjust to the presence of a keyboard is tricky, because there are a number of situations you need to cope with. For example, various keyboards are different heights, the user can rotate their device at will, they can connect a hardware keyboard when they need to, and there's even the QuickType bar that can be shown or hidden on demand.
In all the years I've done iOS development, I've seen at least a dozen ways of coping with keyboards, and few of them are easy. Even Apple's example solution requires fiddling around with constraints, which isn't ideal. I've tried to put together a solution that copes with all possibilities and also requires as little code as possible. If you manage to find something even simpler, do let me know!
We can ask to be told when the keyboard state changes by using a new class called NotificationCenter. Behind the scenes, iOS is constantly sending out notifications when things happen – keyboard changing, application moving to the background, as well as any custom events that applications post. We can add ourselves as an observer for certain notifications and a method we name will be called when the notification occurs, and will even be passed any useful information.
When working with the keyboard, the notifications we care about are keyboardWillHideNotification and keyboardWillChangeFrameNotification. The first will be sent when the keyboard has finished hiding, and the second will be shown when any keyboard state change happens – including showing and hiding, but also orientation, QuickType and more.
It might sound like we don't need keyboardWillHideNotification if we have keyboardWillChangeFrameNotification, but in my testing just using keyboardWillChangeFrameNotification isn't enough to catch a hardware keyboard being connected. Now, that's an extremely rare case, but we might as well be sure!
To register ourselves as an observer for a notification, we get a reference to the default notification center. We then use the addObserver() method, which takes four parameters: the object that should receive notifications (it's self), the method that should be called, the notification we want to receive, and the object we want to watch. We're going to pass nil to the last parameter, meaning "we don't care who sends the notification."
So, add this code to viewDidLoad():
let notificationCenter = NotificationCenter.default
notificationCenter.addObserver(self, selector: #selector(adjustForKeyboard), name: UIResponder.keyboardWillHideNotification, object: nil)
notificationCenter.addObserver(self, selector: #selector(adjustForKeyboard), name: UIResponder.keyboardWillChangeFrameNotification, object: nil)
The adjustForKeyboard() method is complicated, but that's because it has quite a bit of work to do. First, it will receive a parameter that is of type Notification. This will include the name of the notification as well as a Dictionary containing notification-specific information called userInfo.
When working with keyboards, the dictionary will contain a key called UIResponder.keyboardFrameEndUserInfoKey telling us the frame of the keyboard after it has finished animating. This will be of type NSValue, which in turn is of type CGRect. The CGRect struct holds both a CGPoint and a CGSize, so it can be used to describe a rectangle.
One of the quirks of Objective-C was that arrays and dictionaries couldn't contain structures like CGRect, so Apple had a special class called NSValue that acted as a wrapper around structures so they could be put into dictionaries and arrays. That's what's happening here: we're getting an NSValue object, but we know it contains a CGRect inside so we use its cgRectValue property to read that value.
Once we finally pull out the correct frame of the keyboard, we need to convert the rectangle to our view's co-ordinates. This is because rotation isn't factored into the frame, so if the user is in landscape we'll have the width and height flipped – using the convert() method will fix that.
The next thing we need to do in the adjustForKeyboard() method is to adjust the contentInset and scrollIndicatorInsets of our text view. These two essentially indent the edges of our text view so that it appears to occupy less space even though its constraints are still edge to edge in the view.
Finally, we're going to make the text view scroll so that the text entry cursor is visible. If the text view has shrunk this will now be off screen, so scrolling to find it again keeps the user experience intact.
It's not a lot of code, but it is complicated – par for the course on this project, it seems. Anyway, here's the method:
@objc func adjustForKeyboard(notification: Notification) {
guard let keyboardValue = notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue else { return }
let keyboardScreenEndFrame = keyboardValue.cgRectValue
let keyboardViewEndFrame = view.convert(keyboardScreenEndFrame, from: view.window)
if notification.name == UIResponder.keyboardWillHideNotification {
script.contentInset = .zero
} else {
script.contentInset = UIEdgeInsets(top: 0, left: 0, bottom: keyboardViewEndFrame.height - view.safeAreaInsets.bottom, right: 0)
}
script.scrollIndicatorInsets = script.contentInset
let selectedRange = script.selectedRange
script.scrollRangeToVisible(selectedRange)
}
As you can see, setting the inset of a text view is done using the UIEdgeInsets struct, which needs insets for all four edges. I'm using the text view's content inset for its scrollIndicatorInsets to save time.
Note there's a check in there for UIKeyboardWillHide, and that's the workaround for hardware keyboards being connected by explicitly setting the insets to be zero.