如何註冊相關密鑰？ （鍵值觀察）

Question

我創建了一個簡單的可可應用程序。在MainMenu.xib中，我添加了一個NSDatePicker和一個NSTextField。這兩個對象都具有綁定到App Delegate屬性的值。我期望當用戶在NSDatePicker中更改日期時，NSTextField將被更新。這沒有發生。這是在App代表：

// 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 

更新代碼與dateFromPicker觀察員和一些的NSLog聲明：

@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 

這裏是日誌：

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

我知道這是不完全你在問什麼，但...

爲什麼你不把NSTextField綁定到NSDate屬性a nd在Interface Builder中添加一個格式化程序？