在Cocoa中实现符合KVO / Bindings标准的桥接模式

Question

我正在尝试在cocoa中实现一个简单的对象桥，其中桥对象充当某个任意其他NSObject实例的kvo / bindings-compliant drop。

这是我的问题（下面代码中的更多细节）：

桥对象充当Person-Object的drop in，其中NSString *属性名为 name ，Address *属性 address 。绑定到keyPath <！>“; name <！>”;或<！> quot; address <！> quot;桥的工作很好。将某些对象绑定到keyPath <！>“address.street <！>”时，麻烦就开始了;桥的和一个新的Address-Object被设置为Person的地址属性。这导致与KVO相关的异常如下所示：

  

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

即使网桥注意到<！> quot; address <！> quot; -Property中的更改并发出了一个willChangeValueForKeyPath / didChangeValueForKeyPath元组，也会发生这种情况。

下面的代码会产生问题。它是自包含的objective-c代码，可以保存在文件<！>“BridgeDemo.m <！>”中;并使用

编译运行

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

如果你知道这个问题的解决方案，或者可以为我解决同样的问题提供更好的方法，那么你就会让我成为一个非常快乐的程序员！

BridgeDemo.m：

#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

问题在于：

[self willChangeValueForKey：keyPath]; <！> lt; ---此时实际观察者需要取消订阅街道  [self didChangeValueForKey：keyPath]; <！> lt; ---并将其自身添加到新值。

如果不提供新值，则表示您拒绝给观察者取消订阅的机会。

这是一个黑客版本，可以解决问题。

/* --- 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];
}
....
......