Cocoa Programming for OS X: The Big Nerd Ranch Guide

For the More Curious: Flipped Views

Both PDF and PostScript use the standard Cartesian coordinate system, in which y increases as you move up the page. AppKit views follow this model by default. Thus, the origin is usually at the lower-left corner of the view.

For some types of drawing, the math is easier if the upper-left corner is the origin and y increases as you move down the page. Such a view is said to be flipped.

To flip a view, you override the flipped property to return true:

 ​ ​o​v​e​r​r​i​d​e​ ​v​a​r​ ​f​l​i​p​p​e​d​:​ ​B​o​o​l​ ​{​
 ​ ​ ​ ​ ​ ​r​e​t​u​r​n​ ​t​r​u​e​
 ​ ​}​