title: How does a UIView/CALayer render itself without drawRect:?
body: <p>In Apple's Core Animation documentation, it says there are two rendering paths involved. From what I know, CALayer caches bitmap data of a <code>UIView</code> content. There are two ways of providing content of a CALayer. One is implementing <code>drawRect:</code> or other CALayer drawing methods, the other is setting a bitmap to the property contents of CALayer.</p>
  
  <p>Here I'm wondering, what happens behind the scene if neither of the above two things is done? I believe there is a private drawing path <code>UIView</code> uses in this situation. What is this private drawing path consists of? How does it work?</p>