我创建了一个简单的 Cocoa 应用程序。在 MainMenu.xib 我添加了一个 NSDatePicker 和一个 NSTextField。这两个对象都有到 App Delegate 属性的值绑定。我希望当用户更改 NSDatePicker 中的日期时，将更新 NSTextField。这不会发生。这是应用程序代表：
// 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
使用 dateFromPicker 和一些 NSLog 语句的观察者更新代码：
@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
这是日志：
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.