Objective-C Programming: The Big Nerd Ranch Guide

Dependent properties

What if you want to observe lastTimeString instead of lastTime? Try it. In main.m, start observing lastTimeString:

 ​ ​ ​ ​ ​ ​ ​ ​_​_​u​n​u​s​e​d​ ​B​N​R​O​b​s​e​r​v​e​r​ ​*​o​b​s​e​r​v​e​r​ ​=​ ​[​[​B​N​R​O​b​s​e​r​v​e​r​ ​a​l​l​o​c​]​ ​i​n​i​t​]​;​
 ​ ​ ​ ​ ​ ​ ​ ​[​l​o​g​g​e​r​ ​a​d​d​O​b​s​e​r​v​e​r​:​o​b​s​e​r​v​e​r​
 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​f​o​r​K​e​y​P​a​t​h​:​@​"​l​a​s​t​T​i​m​e​S​t​r​i​n​g​"​
 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​o​p​t​i​o​n​s​:​N​S​K​e​y​V​a​l​u​e​O​b​s​e​r​v​i​n​g​O​p​t​i​o​n​N​e​w​ ​|​ ​N​S​K​e​y​V​a​l​u​e​O​b​s​e​r​v​i​n​g​O​p​t​i​o​n​O​l​d​
 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​c​o​n​t​e​x​t​:​n​i​l​]​;​

If you build and run the program, you will see that the notifications ...