A generic implementation of the `UITextDocumentProxy` protocol that should work for anything that conforms to `UIResponder` and `UITextInput`. Useful to put text fields inside custom keyboards and then reuse your keyboard's regular handling logic with this text field. See https://github.com/danielsaidi/KeyboardKit/issues/45 for more info.

TextDocumentProxy.swift

//
//  documentProxy.swift
//  KeyboardKitDemoKeyboard
//
//  Created by Jeffrey Wear on 4/28/20.
//

import UIKit

class TextDocumentProxy<TextDocument: UIResponder & UITextInput>: NSObject, UITextDocumentProxy {
    init(document: TextDocument) {
        self.document = document
        super.init()
    }
    
    private unowned let document: TextDocument
    
    // MARK: - UITextDocumentProxy
    
    var documentInputMode: UITextInputMode? {
        document.textInputMode
    }
    
    var documentContextAfterInput: String? {
        guard let selectedTextRange = document.selectedTextRange else {
            return nil
        }
        guard let rangeAfterInput = document.textRange(from: selectedTextRange.end, to: document.endOfDocument) else {
            return nil
        }
        return document.text(in: rangeAfterInput)
    }
    
    var documentContextBeforeInput: String? {
        guard let selectedTextRange = document.selectedTextRange else {
            return nil
        }
        guard let rangeBeforeInput = document.textRange(from: document.beginningOfDocument, to: selectedTextRange.start) else {
            return nil
        }
        return document.text(in: rangeBeforeInput)
    }
    
    // https://stackoverflow.com/a/41023439/495611 suggests adjusting the text
    // position (i.e. moving the cursor) by adjusting the selected text range.
    func adjustTextPosition(byCharacterOffset offset: Int) {
        guard let selectedTextRange = document.selectedTextRange else { return }
        
        // Not sure what's supposed to happen if the range is non-empty. Let's
        // abort if it is.
        guard selectedTextRange.isEmpty else { return }
        
        // Now that it's empty, the start and end should be the same. Move that position.
        // The guard is a bounds check.
        guard let newPosition = document.position(from: selectedTextRange.start, offset: offset) else { return }
        
        document.selectedTextRange = document.textRange(from: newPosition, to: newPosition)
    }
    
    var selectedText: String? {
        guard let selectedTextRange = document.selectedTextRange else {
            return nil
        }
        return document.text(in: selectedTextRange)
    }
    
    let documentIdentifier: UUID = UUID()
    
    func setMarkedText(_ markedText: String, selectedRange: NSRange) {
        document.setMarkedText(markedText, selectedRange: selectedRange)
    }
    
    func unmarkText() {
        document.unmarkText()
    }
    
    // MARK: - UIKeyInput
    
    func insertText(_ text: String) {
        document.insertText(text)
    }
    
    func deleteBackward() {
        document.deleteBackward()
    }
    
    var hasText: Bool {
        document.hasText
    }
}

Thanks @danielsaidi for Reply
No, I didn't use KeyboardKit Pro.
I just tested your demo code.
In my custom keyboard, I used textDocumentProxy.documentContextBeforeInput, but it can't read more than 300 characters — unlike your fullDocumentContext, which reads all the text.
How can I handle large text like that? without using KeyboardKit Pro?

I see, yes the native text document proxy APIs have always been very lacking.

The full document context implementation is a Pro feature with a lot of time and effort put into it. You need a Silver license to use it. Without KeyboardKit Pro, you have to implement the logic yourself.

Best,
Daniel

@danielsaidi I respect your effort and time.
However, I am a student and don't have the budget to purchase a Silver license.
I only need this one feature to implement in my code, as it's currently blocking my app's progress.
If you could offer some support, I would be very grateful.