A simple Xcode extension that reverses code on selected lines

XcodeExtReverseLines.swift

import Foundation
import XcodeKit

class SourceEditorCommand: NSObject, XCSourceEditorCommand {

  // Reverses code on selected lines
  func perform(with invocation: XCSourceEditorCommandInvocation, completionHandler: @escaping (Error?) -> Void ) -> Void {

    let lines = invocation.buffer.lines as? [String] ?? []

    let selections = invocation.buffer.selections as? [XCSourceTextRange] ?? []
    for selection in selections {
      let indices: [Int] = Array(selection.start.line...selection.end.line)
      let reversedIndices: [Int] = indices.reversed()

      for i in 0..<indices.count {
        // Reverse the lines
        let from = reversedIndices[i]
        let to = indices[i]
        invocation.buffer.lines[to] = lines[from]
      }
    }

    // Signal to Xcode that the command has completed.
    completionHandler(nil)
  }

}