iOS Key Value Observing Xcode Project Problems

Question

I'm now learning key value observing, have a very simple KVO project have a little problems that dont't print property new value changes when observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context method called. That's very weird, hope someone help me find where problems occurred, i'm very appreciated that!

Here is my source code:

ViewController.h

#import <UIKit/UIKit.h>

@class Person;

@interface ViewController : UIViewController

@property (nonatomic, strong) Person *person;

@end

ViewController.m

#import "ViewController.h"
#import "Person.h"

@interface ViewController ()

@end

@implementation ViewController

@synthesize person;

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.person = [[Person alloc] init];
    [self changeName];
    [self.person addObserver:self
             forKeyPath:@"fullName"
                options:NSKeyValueObservingOptionNew
                context:NULL];
}

- (void)changeName
{
    self.person.fullName = @"Andy";
}

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if ([keyPath isEqualToString:@"fullName"])
    {
        NSLog(@"%@", change);
        NSString *string = [change objectForKey:NSKeyValueChangeNewKey];
        NSLog(@"%@", string);
    }
}

- (void)dealloc
{
    [self.person removeObserver:self forKeyPath:@"fullName"];
}

@end

Person.h

#import <Foundation/Foundation.h>

@interface Person : NSObject

@property (nonatomic, strong) NSString *fullName;

@end

Person.m

#import "Person.h"

@implementation Person

@synthesize fullName = _fullName;

@end

score 1 · Accepted Answer · answered May 28 '13 at 16:17

1

You're changing the name before you're adding the observer. Swap this:

[self changeName];
[self.person addObserver:self
         forKeyPath:@"fullName"
            options:NSKeyValueObservingOptionNew
            context:NULL];

...for this:

[self.person addObserver:self
         forKeyPath:@"fullName"
            options:NSKeyValueObservingOptionNew
            context:NULL];
[self changeName];

answered May 28 '13 at 16:17

Jim

72,985
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101
108

Now i want to register dependent keys which is "fullName" in class Person. i have created another class which is Person2, it have two properties is:@"firstName", @"lastName", and "fullName" value dependent either "firstName" and "lastName" changes. I try to implement it, but don't print any values in xcode console, i want to send you the source project, can you help me look into it? Thanks. – Andy_24 May 29 '13 at 11:45
1

If you have a new question, post it as a new question. – Jim May 29 '13 at 11:58
I just added a few lines code on this project test dependent keys, i send you project, can you help me look it? – Andy_24 May 29 '13 at 12:08

score 1 · Answer 2 · answered May 28 '13 at 16:17

1

Have a look at the order of your calls,

You are calling changeName before you are registering as observer.

Move the changeName call below the addObserver method and see what happens.

answered May 28 '13 at 16:17

Abizern

146,289
39
203
257

Now i want to register dependent keys which is "fullName" in class Person. i have created another class which is Person2, it have two properties is:@"firstName", @"lastName", and "fullName" value dependent either "firstName" and "lastName" changes. I try to implement it, but don't print any values in xcode console, i want to send you the source project, can you help me look into it? Thanks. – Andy_24 May 29 '13 at 11:49
@Andy_24 you want to implement `+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key` as explained in the [KVO Protocol Reference](http://developer.apple.com/library/iOS/#documentation/Cocoa/Reference/Foundation/Protocols/NSKeyValueCoding_Protocol/Reference/Reference.html#//apple_ref/occ/cat/NSKeyValueCoding). If you still don't understand, then ask another question. – Abizern May 29 '13 at 14:50

iOS Key Value Observing Xcode Project Problems

2 Answers2