## UIView Drawing Model

### `-[UIView setNeedsDisplay]`

You have some kind of custom `UIView` which implements some awesome drawing in `-[UIView drawRect:]`. In order to have UIKit draw it for you, you send the `-[UIView setNeedsDisplay]` to the view object to queue it up for drawing.

### `-[CALayer setNeedsDisplay]`

         *------------*           *------------*
         |            |           |            |
         |   UIView   |  ------>  |   CALayer  |
         |            |           |            |
         *------------*           *------------*
    -[UIView setNeedsDisplay] -[CALayer setNeedsDisplay]

The `UIView` object has another, special object backing it, a `CALayer`. When its convenient for the rendering system, it'll ask the view's `CALayer` to display by sending it the `-[CALayer display]` message.

Now, the `CALayer` creates a region of memory called a "Backing Store," a bitmap that stores the result of all the rendering operations.

         *------------*           *------------*          ..............
         |            |           |            |          .            .
         |   UIView   |  ------>  |   CALayer  |  ----->  .   Backing  .
         |            |           |            |          .    Store   .
         *------------*           *------------*          ..............
                                -[CALayer display]

This backing store image can be displayed on screen repeatedly until the view needs some different drawing (e.g., for a highlighted state). At that point, the process would start over by sending the view the `-[UIView setNeedsDisplay]` message.

For a better walk through, check out the in-depth aside on UIView drawing in the WWDC 2012 presentation, [Building Concurrent User Interfaces on iOS][wwdc2012ui] by [Andy Matuschak][amtwitter] (video requires an ADC login).

[wwdc2012ui]: https://developer.apple.com/videos/wwdc/2012/
[amtwitter]: http://twitter.com/andy_matuschak