title: self-sizing uitableviewcell not recalculate content size when subview instrinsic content size has changed
body: <p>everyone, i have issue about uitableviewcell</p>
  
  <p>custom uitableview cell which has subviews, one of which is custom image view named CustomIntrinsicImageView, and code as below:</p>
  
  <pre><code>class CustomIntrinsicImageView: UIImageView {
      var _preferredMaxLayoutWidth: CGFloat!
      var preferredMaxLayoutWidth: CGFloat! {
          get {
               return _preferredMaxLayoutWidth
          }
  
          set {
               _preferredMaxLayoutWidth = newValue
              invalidateIntrinsicContentSize()
          }
      }
      override func intrinsicContentSize() -&gt; CGSize {
          if let size = image?.size {
              let width: CGFloat
              let height: CGFloat
  
              if let maxLayoutWidth = preferredMaxLayoutWidth where size.width &gt; maxLayoutWidth {
                  width = maxLayoutWidth
                  height = maxLayoutWidth * size.height/size.width
             } else {
                  width = size.width
                  height = size.height
             }
  
             return CGSizeMake(width, height)
         }
  
          return CGSizeMake(UIViewNoIntrinsicMetric, UIViewNoIntrinsicMetric)
      }
  }
  </code></pre>
  
  <p>the subviews have layout constraints to cell, and the image view update preferredMaxLayoutWidth in cell's layoutSubview body, code like this</p>
  
  <pre><code>override func layoutSubviews() {
      super.layoutSubviews()
  
      image.preferredMaxLayoutWidth = CGRectGetWidth(contentView.bounds) - CGFloat(leftMarginImage + rightMarginImage)
  
  }
  </code></pre>
  
  <p>the result of custom image view is perfect, but cell not recalculate height.</p>
  
  <p>some of system UI like as UILable, UITextView has own instrinsicContentSize implement,which cooperate well with cell</p>
  
  <p>so, the question is how to send information to cell, and let it know subview content size has change and recalculate height</p>
  
  <p>thanks</p>