Question

我正在阅读关于KVO的教程，示例显示当我们不使用KVO样式时值不会更新，所以我想知道：为什么，如果对象是通过“引用”传递的（使用id或Position*），所以如果我们使用指向“anObject”的指针，那么值（来自对象）是不是会更新？

所以这里是代码：在“main”中：

// Create initial position
Position* pos = [[Position alloc] init];
pos.latitude = 45.0;
pos.longitude = 12.0;
pos.altitude = 30.0;

// Create a basic tracker
ValueTracker * tracker = [ValueTracker createValueTrackerForKey:@"latitude" onObject:pos];

// Dump initial state
NSLog(@"Tracker (1): %@", tracker);

pos.latitude += 5.0;
pos.longitude += 5.0;
pos.altitude += 10.0;

NSLog(@"Tracker (2): %@", tracker); //the values are not updated

对象：

@interface Position : NSObject {

    float latitude;
    float longitude;
    float altitude;
}

@property(assign, readwrite) float latitude;
@property(assign, readwrite) float longitude;
@property(assign, readwrite) float altitude;

和函数：这里，anObject是指向Position的指针，为什么不能使用指针更新值？

+ (ValueTracker *) createValueTrackerForKey: (NSString *) aKey 
                                 onObject: (id) anObject
{
    ValueTracker * tracker = [[ValueTracker alloc] init];

    tracker.valueKey = aKey;
    tracker.source = anObject;
    tracker.value = [anObject valueForKey: tracker.valueKey];

    return tracker;
}

编辑：这是我们使用kvo系统的时候：

[tracker enableObserving];

并在ValueTracker类中：

- (void) enableObserving
{
    if ( nil != source ) {
        /* We specify in options that we want to get both old and new value
         for the property.
         */
        [source addObserver: self 
                 forKeyPath: valueKey 
                    options:(NSKeyValueObservingOptionNew |
                             NSKeyValueObservingOptionOld)
                    context: NULL];
    }
}

编辑n°2：

- (void) observeValueForKeyPath: (NSString *) keyPath 
                       ofObject: (id) object 
                         change: (NSDictionary *) change 
                        context: (void *) context
{
    NSLog(@"Value %@ changed! Here is the changeset: %@", keyPath, change);
    //NSLog(@"Dump on observation: %@", [self description]);
    value = [change valueForKey:NSKeyValueChangeNewKey];
}

Answer 1

看起来你忘了合成你的iVars了吗？

@implementation Position

@synthesize latitude, longitude, altitude;

...

@end

kvo风格：为什么不能用指针更新值？

1 个答案: