تنفيذ نمط جسر متوافق مع KVO/Bindings في الكاكاو

Question

أحاول تنفيذ جسر كائن بسيط في الكاكاو حيث يعمل كائن الجسر بمثابة قطرة متوافقة مع kvo/bindings لبعض مثيلات NSObject التعسفية الأخرى.

هذه هي مشكلتي (مزيد من التفاصيل في الكود أدناه):

يعمل كائن الجسر كنقطة إدخال لكائن شخص، مع استدعاء خاصية NSString* اسم وخاصية العنوان* عنوان.يعمل الارتباط بـ "اسم" أو "عنوان" مسار المفتاح للجسر بشكل جيد.تبدأ المشكلة عند ربط بعض الكائنات بمسار المفتاح "address.street" الخاص بالجسر ويتم تعيين كائن عنوان جديد لكائن الشخص عنوان ملكية.وينتج عن ذلك استثناءات متعلقة بـ KVO والتي تبدو كما يلي:

Cannot remove an observer <NSKeyValueObservance 0x126b00> for the key path "street" from <Address 0x12f1d0> because it is not registered as an observer

يحدث هذا على الرغم من أن الجسر يلاحظ التغيير في خاصية "العنوان" ويصدر صف willChangeValueForKeyPath/didChangeValueForKeyPath.

الكود أدناه ينتج المشكلة.إنه كود الهدف-c مستقل بذاته والذي يمكن حفظه في ملف "BridgeDemo.m" وتشغيله مع

gcc -o test BridgeDemo.m -framework AppKit -framework Foundation; ./test

إذا كنت تعرف حلاً لهذه المشكلة أو يمكنك أن تقدم لي طريقة أفضل لحل نفس المشكلة، فأنت تجعلني أ جداً مبرمج سعيد!

بريدج ديمو.م:

#import <Foundation/Foundation.h>
#import <AppKit/AppKit.h>

/* --- Address ----------------------------------------- */

@interface Address : NSObject {
    NSString* street;
    NSNumber* zipCode;
    NSString* city;
}

@property(retain) NSString* street;
@property(retain) NSNumber* zipCode;
@property(retain) NSString* city;

@end

@implementation Address

@synthesize street, zipCode, city;

-(id)init {
    if( !( self = [super init] ) ) { return nil; }

    self.street  = @"Elm Street";
    self.zipCode = @"12345";
    self.city    = @"Crashington";

    return self;
}

-(void) modifyStreet {
    self.street = @"Main Street";
}

-(void)dealloc { 
    [street release]; [zipCode release]; [city release]; 
    [super dealloc];
}
@end

/* --- Person ----------------------------------------- */

@interface Person : NSObject {
    NSString* name;
    Address* address;
}
@property(retain) NSString* name;
@property(retain) Address* address;
@end

@implementation Person

@synthesize address, name;

-(id)init {
    if( !( self = [super init] ) ) { return nil; }

    self.name = @"Tom";
    self.address = [[Address new] autorelease];

    return self;
}

- (void)modifyAddress {
    Address* a = [[Address new] autorelease];
    a.street  = @"Jump Street";
    a.zipCode = @"54321";
    a.city    = @"Memleakville";
    self.address = a;
}

- (void)dealloc { [address release]; [name release]; [super dealloc]; }

@end

/* --- Bridge ----------------------------------------- */

@interface Bridge : NSObject {
    NSMutableDictionary* observedKeys;
    NSObject* obj;
}

@property(retain) NSObject* obj;

@end

@implementation Bridge

@synthesize obj;

- (id)init {
    if( !( self = [super init] ) ) { return nil; }
    observedKeys = [NSMutableDictionary new];
    return self;
}
- (void)forwardInvocation:(NSInvocation*)inv {
    [inv invokeWithTarget:obj];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    return [obj methodSignatureForSelector:aSelector];
}

- (void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    NSLog( @">>>> Detected Change in keyPath: %@", keyPath );
    [self willChangeValueForKey:keyPath];
    [self didChangeValueForKey:keyPath];    
}

-(id)valueForUndefinedKey:(NSString*)key {
    /* Register an observer for the key, if not already done */
    if( ![observedKeys objectForKey:key] ) {
        [observedKeys setObject:[NSNumber numberWithBool:YES] forKey:key];
        [obj addObserver:self forKeyPath:key options:NSKeyValueObservingOptionNew context:nil];
    } 
    return [obj valueForKey:key];
}

- (void)dealloc {
    for( NSString* key in [observedKeys allKeys] ) {
        [obj removeObserver:self forKeyPath:key];
    }
    [obj release];
    [observedKeys release];
    [super dealloc];
}

@end

/* --- MyObserver ------------------------------------ */

@interface MyObserver : NSObject {
    Address* address;
    NSString* street;
}

@property(retain) Address* address;
@property(retain) NSString* street;
@end

@implementation MyObserver

@synthesize street, address;

-(void)dealloc { [street release]; [super dealloc]; }

@end


/* This works fine */
void testBindingToAddress() {
    NSLog( @"Testing Binding to 'address' --------------" );
     NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    Bridge* b = [[Bridge new] autorelease];
    b.obj = [Person new];
    MyObserver* o = [[MyObserver new] autorelease];
    [o bind:@"address" toObject:b withKeyPath:@"address"
        options:nil];
    NSLog( @"Before modifyStreet: %@", o.address.street );    
    [[b valueForKey:@"address"] performSelector:@selector(modifyStreet)];
    NSLog( @"After modifyStreet: %@", o.address.street );        

    [b performSelector:@selector(modifyAddress)];
    NSLog( @"After modifyAdress:  %@", o.address.street );

    [pool drain];   
}

/* This produces an exception */
void testBindingToStreet() {
    NSLog( @"Testing Binding to 'address.street' --------------" );    
     NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    Bridge* b = [[Bridge new] autorelease];
    b.obj = [Person new];
    MyObserver* o = [[MyObserver new] autorelease];
    [o bind:@"street" toObject:b withKeyPath:@"address.street"
        options:nil];

    NSLog( @"Before modifyStreet: %@", o.street );    
    [[b valueForKey:@"address"] performSelector:@selector(modifyStreet)];
    NSLog( @"After modifyStreet: %@", o.street );        

    [b performSelector:@selector(modifyAddress)];
    NSLog( @"After modifyAdress:  %@", o.street );

    [pool drain];   
}

/* --- main() ------------------------------------ */
int main (int argc, const char * argv[]) {
    testBindingToAddress();
    testBindingToStreet();    
    return 0;
}

Answer 1

هنا هي المشكلة:

[self willChangeValueForKey:keyPath];<--- وإضافة نفسه إلى القيمة الجديدة.

من خلال عدم توفير القيمة الجديدة فإنك تحرم المراقب من فرصة إلغاء الاشتراك.

إليك نسخة مخترقة تعمل وتوضح المشكلة.

/* --- Bridge ----------------------------------------- */
....
.....

@interface Bridge : NSObject {
    NSMutableDictionary* observedKeys;
    NSObject* obj;

    //**** Dictionary for old values just before we send the didChangeValue notification.
    NSMutableDictionary * oldValues;
}

...
.....

- (id)init {
    if( !( self = [super init] ) ) { return nil; }
    observedKeys = [NSMutableDictionary new];
    //************* Initialize the new dictionary
    oldValues = [NSMutableDictionary new];
    return self;
}
....
.....

- (void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    NSLog( @">>>> Detected Change in keyPath: %@", keyPath );
    // ****  Cache the old value before telling everyone its going to change. 
    [oldValues setValue:[change valueForKey:NSKeyValueChangeOldKey] forKey:keyPath];
    [self willChangeValueForKey:keyPath];
    // **** Simulate the change by removing the old value.
    [oldValues removeObjectForKey:keyPath];
    // **** Now when we say we did change the value, we are not lying.
    [self didChangeValueForKey:keyPath];    
}

-(id)valueForUndefinedKey:(NSString*)key {
    // **** Important part, return oldvalue if it exists
    id oldValue;
    if(oldValue = [oldValues valueForKey:key]){
        return oldValue;
    }
    /* Register an observer for the key, if not already done */     
    if( ![observedKeys objectForKey:key] ) {
        [observedKeys setObject:[NSNumber numberWithBool:YES] forKey:key];
        NSLog(@"adding observer for:%@", key);
        [obj addObserver:self forKeyPath:key options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];
    } 
    return [obj valueForKey:key];
}
....
......