title: How cells are reused in UITableView?
body: <p>I think that I am missing something in a way that cells are reused in my UITableView.</p>
  
  <p>I have ViewController with TableView and custom cells with their own class and identifier. In cellForRowAt method I use</p>
  
  <pre><code>let cell: MyCell = myTableView.dequeueReusableCell(withIdentifier: "MyCell", for: indexPath) as! MyCell
      cell.objectForCell = myObject
  </code></pre>
  
  <p>And it works as it should, I got TableView with cells.
  I also have object that i pass to my cell. And inside cell I have this code</p>
  
  <pre><code>var objectForCell : ObjectClass? {
      didSet {
          if let object = objectForCell {
              someLabel.text = object.title
              ....
  </code></pre>
  
  <p>That also works fine. 
  But what if I want to add some subviews to cells? For example let it be each 3rd cell. When TableView is presented I see my subview in each 3rd cell as I expect but why I got this subview in every cell when I start to scroll in different directions? </p>
  
  <pre><code>var objectForCell : ObjectClass? {
      didSet {
          if let object = objectForCell {
              someLabel.text = object.title
              if object.id % 3 == 0 {
                 self.addSubview(......)
              }
  </code></pre>