Using protocols for dependency injection.

LoadContentVC3.swift

/// Protocol definition for the FileManager
protocol AppFileManager {
  func contents(atPath: String)
}

/// Protocol conformance of the system FileManager
extension FileManager: AppFileManager {}

/// ViewController
class MyViewController: UIViewController {
   var myPath: String
   var content: [MyModel]
  
   // ...
   
  func loadContent(
    fileManager: AppFileManager, 
    path: String, 
    jsonDecoder: JSONDecoder = JSONDecoder()
  ) throws {    
      let data = fileManager.contents(atPath: path),
      self.content = try jsonDecoder.decode([MyModel].self, from data)
    }      
  }
}