How are dependent keys registered? (Key-Value Observing) - cocoa

Question

I have created a simple Cocoa Application. In MainMenu.xib I have added an NSDatePicker and an NSTextField. Both of these objects have Value bindings to properties of the App Delegate. I expect that when the user changes the date in the NSDatePicker, the NSTextField will be updated. This isn't happening. Here is the App Delegate:
// AppDelegate.h
#import <Cocoa/Cocoa.h>
#interface AppDelegate : NSObject <NSApplicationDelegate>
#property (assign) IBOutlet NSWindow *window;
#property (nonatomic, strong) NSDate *dateFromPicker;
#property (nonatomic, readonly) NSString *dateString;
#end
// AppDelegate.m
#import "AppDelegate.h"
#implementation AppDelegate
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
}
- (NSString *)dateString
{
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
return [formatter stringFromDate:self.dateFromPicker];
}
+ (NSSet *)keyPathsForValuesAffectingDateString
{
return [NSSet setWithObject:#"dateFromPicker"];
}
#end
Updated code with an observer for dateFromPicker and some NSLog statements:
#implementation AppDelegate
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
[self addObserver:self forKeyPath:#"dateFromPicker" options:0 context:NULL];
self.dateFromPicker = [NSDate dateWithNaturalLanguageString:#"12/12/12"];
}
- (NSString *)dateString
{
NSLog(#"dateString was called.");
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
return [formatter stringFromDate:self.dateFromPicker];
}
+ (NSSet *)keyPathsForValuesAffectingDateString
{
NSLog(#"keyPathsForValuesAffectingDateString was called.");
return [NSSet setWithObject:#"dateFromPicker"];
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change: (NSDictionary *)change context:(void *)context
{
NSLog(#"dateFromPicker changed.");
}
#end
Here is the log:
2012-08-10 15:37:15.086 ... keyPathsForValuesAffectingDateString was called.
2012-08-10 15:37:15.087 ... dateString was called.
2012-08-10 15:37:15.116 ... dateFromPicker changed.
2012-08-10 15:37:15.117 ... dateString was called.
2012-08-10 15:37:19.831 ... dateFromPicker changed.
2012-08-10 15:37:19.831 ... dateString was called.

Answer 1

I know this is not exactly what you were asking, but ...
Why don't you bind the NSTextField to the NSDate property and add a formatter to it in Interface Builder?