我正在尝试在 cocoa 中实现一个简单的对象桥，其中该桥对象充当其他任意NSObject实例的kvo/bindings兼容对象。
这是我的问题(下面的代码中有更多详细信息):
桥对象充当一个Person-Object的放置，具有一个名为name的NSString *属性和一个Address *属性地址。绑定(bind)到Bridge的keyPath“名称”或“地址”可以很好地工作。将某些对象绑定(bind)到网桥的keyPath“address.street”并为Person的address属性设置一个新的Address-Object时，故障开始。这导致与KVO相关的异常如下所示:
即使网桥注意到“地址”-属性中的更改并发出willChangeValueForKeyPath/didChangeValueForKeyPath元组，也会发生这种情况。
下面的代码产生了问题。它是独立的objective-c代码，可以保存在文件“BridgeDemo.m”中，并与
gcc -o test BridgeDemo.m -framework AppKit -framework Foundation; ./test
如果您知道该问题的解决方案，或者可以为我提供更好的解决同一问题的方法，那么您会让我成为非常的程序员!
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;
}
最佳答案
这是问题所在:
[self willChangeValueForKey:keyPath]; [self didChangeValueForKey:keyPath];
通过不提供新的值(value)，您将使观察者失去退订的机会。
这是一个可以正常工作并演示该问题的被黑版本。
/* --- 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];
}
....
......
关于cocoa - 用C实现符合KVO/Bindings的桥模式，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/204125/