ControlledTextView.swift

//
//  ControlledTextView.swift
//
//  Created by devin_abbott on 4/21/18.
//  Copyright © 2018 devin_abbott. All rights reserved.
//

import AppKit

fileprivate extension String {
  func clamp(index targetIndex: Int) -> Int {
    return min(max(targetIndex, 0), count)
  }

  func slice(start: Int, end: Int? = nil) -> String {
    let startIndex = self.index(self.startIndex, offsetBy: clamp(index: start))
    let endIndex = self.index(self.startIndex, offsetBy: clamp(index: end ?? count))

    return String(self[startIndex..<endIndex])
  }
}

class ControlledTextView: NSTextView,
  NSTextFieldDelegate,
  NSControlTextEditingDelegate
{

  // MARK: - Lifecycle

  override init(frame frameRect: NSRect) {
    super.init(frame: frameRect)
    self.delegate = self
  }

  override init(frame frameRect: NSRect, textContainer container: NSTextContainer?) {
    super.init(frame: frameRect, textContainer: container)
    self.delegate = self
  }

  required init?(coder: NSCoder) {
    super.init(coder: coder)
  }

  // MARK: - Public

  var onChangeText: ((String) -> Void)?

  override var string: String {
    get {
      return super.string
    }
    set {
      let oldValue = string
      let oldRange = selectedRange()

      var index = newValue.count - oldValue.count + oldRange.upperBound

      // Handle forward-deletion, e.g. Ctrl+K.
      // The index may be negative at this point. If we detect that the string hasn't changed
      // before the cursor, then we shouldn't move the cursor toward the start of the string.
      if newValue.slice(start: 0, end: oldRange.lowerBound) ==
        oldValue.slice(start: 0, end: oldRange.lowerBound) {
        index = max(index, oldRange.lowerBound)
      }

      let updatedRange = NSRange(location: index, length: 0)

      // Perform UI updates

      super.string = newValue

      setSelectedRange(updatedRange)
    }
  }
}

extension ControlledTextView: NSTextViewDelegate {
  func textView(
    _ textView: NSTextView,
    shouldChangeTextIn affectedCharRange: NSRange,
    replacementString: String?) -> Bool {

    if let replacementString = replacementString,
      let range = Range(affectedCharRange, in: string) {
      let updated = string.replacingCharacters(in: range, with: replacementString)

      onChangeText?(updated)
    }

    return false
  }
}