Reputation: 10274
I'm having a hard time manually triggering a property update with key-value-observing. Here's a contrived example to illustrate my problem:
Bar.h
#import <Foundation/Foundation.h>
@interface Bar : NSObject
{
NSString *test;
}
@property (nonatomic, retain) NSString *test;
-(void) updateTest1;
-(void) updateTest2;
@end
Bar.m
#import "Bar.h"
@implementation Bar
@synthesize test;
-(id) init
{
if (self = [super init])
{
self.test = @"initial";
}
return self;
}
-(void) updateTest1
{
self.test = @"updating 1";
}
-(void) updateTest2
{
NSString *updateString = @"updating 2";
[updateString retain];
test = updateString;
[self didChangeValueForKey:@"test"];
}
@end
Foo.h
#import <Foundation/Foundation.h>
@interface Foo : NSObject
@end
Foo.m
#import "Foo.h"
@implementation Foo
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
NSLog(@"something changed!");
}
@end
main.m
#import <Foundation/Foundation.h>
#import "Foo.h"
#import "Bar.h"
int main (int argc, const char * argv[])
{
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
Bar *bar = [Bar new];
Foo *foo = [Foo new];
[bar addObserver:foo forKeyPath:@"test" options:0 context:nil];
[bar updateTest1];
[bar updateTest2];
[pool drain];
return 0;
}
The program returns:
2011-08-09 03:57:52.630 Temp[5159:707] something changed!
Program ended with exit code: 0
Why is didChangeValueForKey:
not triggering the observer's observeValueForKeyPath:ofObject:change:context:
event? Does this method not work like I think it does?
Upvotes: 2
Views: 1827
Reputation: 9198
It is not triggering the notification because you forgot the corresponding
[self willChangeValueForKey:@"test"];
that must always be paired with didChangeValueForKey:
Upvotes: 6
Reputation: 25692
NSKeyValueObservingOptions
These constants are passed to addObserver:forKeyPath:options:context: and determine the values that are returned as part of the change dictionary passed to an observeValueForKeyPath:ofObject:change:context:. You can pass 0 if you require no change dictionary values.
enum {
NSKeyValueObservingOptionNew = 0x01,
NSKeyValueObservingOptionOld = 0x02,
NSKeyValueObservingOptionInitial = 0x04,
NSKeyValueObservingOptionPrior = 0x08
}; typedef NSUInteger NSKeyValueObservingOptions;
try like this
[bar addObserver:foo forKeyPath:@"test" options:NSKeyValueObservingOptionNew context:nil];
change this code
-(void) updateTest2
{
NSString *updateString = @"updating 2";
[updateString retain];
test = updateString;
[self didChangeValueForKey:@"test"];
}
to
-(void) updateTest2
{
self.test = @"updating 2"//note here
[self didChangeValueForKey:@"test"];
}
Upvotes: 1