OC Syntax: KVC and KVO, ockvckvo

OC Syntax: KVC and KVO, ockvckvo

1. Key-Value Coding (KVC)

KVC refers to NSKeyValueCoding, an informal Protocol that provides a mechanism to indirectly access object attributes. KVO is one of the key technologies based on KVC.

An object has certain attributes. For example, a Person object has a name and an address attribute. In KVC, the Person object has a value corresponding to the key of its name and address. The key is only a string, and its corresponding value can be any type of object. At the most basic level, KVC has two methods: one is to set the key value, and the other is to get the key value. Example:

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void changeName(Person *p, NSString *newName){     // using the KVC accessor (getter) method    NSString *originalName = [p valueForKey:@"name"];     // using the KVC  accessor (setter) method.    [p setValue:newName forKey:@"name"];     NSLog(@"Changed %@'s name to: %@", originalName, newName); }

Now, if the Person has another key spouse (spouse) and the spouse key value is another Person object, you can write it like this using KVC:

 

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void logMarriage(Person *p){     // just using the accessor again, same as example above    NSString *personsName = [p valueForKey:@"name"];     // this line is different, because it is using    // a "key path" instead of a normal "key"    NSString *spousesName = [p valueForKeyPath:@"spouse.name"];     NSLog(@"%@ is happily married to %@", personsName, spousesName); }

Key and key pat must be distinguished. A key can obtain a value from an object, while a key path can separate multiple keys by the dot ".", for example:

[p valueForKeyPath:@"spouse.name"];

This is equivalent ......

[[p valueForKey:@"spouse"] valueForKey:@"name"];

Well, the above is the basic knowledge of KVC. Let's take a look at KVO.

Key-Value Observing (KVO)

Key-Value Observing (KVO) is built on KVC. It can observe the change of the key path Value of an object. For example, to observe the address change of a person object using code, the following three methods are implemented:

• WatchPersonForChangeOfAddress: Observed
• ObserveValueForKeyPath: ofObject: change: context: called when the value of the observed key path changes.
• Dealloc stop observation

 

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static NSString *const KVO_CONTEXT_ADDRESS_CHANGED = @"KVO_CONTEXT_ADDRESS_CHANGED" @implementation PersonWatcher -(void) watchPersonForChangeOfAddress:(Person *)p{     // this begins the observing    [p addObserver:self        forKeyPath:@"address"           options:0           context:KVO_CONTEXT_ADDRESS_CHANGED];     // keep a record of all the people being observed,    // because we need to stop observing them in dealloc    [m_observedPeople addObject:p];} // whenever an observed key path changes, this method will be called- (void)observeValueForKeyPath:(NSString *)keyPath                      ofObject:(id)object                        change:(NSDictionary *)change                       context:(void *)context {    // use the context to make sure this is a change in the address,    // because we may also be observing other things    if(context == KVO_CONTEXT_ADDRESS_CHANGED) {        NSString *name = [object valueForKey:@"name"];        NSString *address = [object valueForKey:@"address"];        NSLog(@"%@ has a new address: %@", name, address);    }} -(void) dealloc;{     // must stop observing everything before this object is    // deallocated, otherwise it will cause crashes    for(Person *p in m_observedPeople){        [p removeObserver:self forKeyPath:@"address"];    }     [m_observedPeople release];    m_observedPeople = nil;     [super dealloc]; } -(id) init;{    if(self = [super init]){        m_observedPeople = [NSMutableArray new];    }     return self;} @end

This is the role of KVO. It observes the object value through the key path and will receive a notification when the value changes.