How to create custom text input using UIKeyInput

Swift version: 5.6

Paul Hudson @twostraws April 23rd 2020

Although we can use pressesBegan() and pressesEnded() to read keypresses, they won’t show the on-screen keyboard and so won’t let you provide custom text input for users without a hardware keyboard. If you need that keyboard to be shown, you should create a class that adopts the UIKeyInput protocol instead, which has just three requirements:

What to do when text is inserted.
What to do when text is deleted.
Whether your custom text input currently has text or not.

The only other thing you need to know is that your custom input control will show the keyboard when it becomes first responder. So, you should override the canBecomeFirstResponder property of your subclass, setting it to true rather than the default of false.

To demonstrate this, we could create a simple UIView subclass that draws text to the screen as it’s typed, like this:

class TextRenderingView: UIView, UIKeyInput {
    // the string we'll be drawing
    var input = ""

    override var canBecomeFirstResponder: Bool {
        true
    }

    var hasText: Bool {
        input.isEmpty == false
    }

    func insertText(_ text: String) {
        input += text
        setNeedsDisplay()
    }

    func deleteBackward() {
        _ = input.popLast()
        setNeedsDisplay()
    }

    override func draw(_ rect: CGRect) {
        let attrs: [NSAttributedString.Key: Any] = [.font: UIFont.systemFont(ofSize: 32)]
        let attributedString = NSAttributedString(string: input, attributes: attrs)
        attributedString.draw(in: rect)
    }
}

If you want to handle more complex user input, such as selecting ranges of text or drawing the caret, you should use the more advanced UITextInput protocol instead.