cocoa - 从属键如何注册？ (键值观察)

Question

我创建了一个简单的 Cocoa 应用程序。在 MainMenu.xib 中，我添加了一个 NSDatePicker 和一个 NSTextField。这两个对象都具有与应用程序委托(delegate)的属性的值绑定(bind)。我希望当用户更改 NSDatePicker 中的日期时， NSTextField 将被更新。这并没有发生。这是应用程序委托(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

使用 dateFromPicker 的观察者和一些 NSLog 语句更新了代码:

@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.

Answer 1

我知道这不完全是你要问的，但是......

为什么不将 NSTextField 绑定(bind)到 NSDate 属性并在 Interface Builder 中为其添加格式化程序？