Objective-C学习笔记-KVO

1.KVO(key-value observering)是指当对象的属性值改变的时候，允许另外一个对象接受通知，其中context可以用来区分该通知是发给基类还是自己的。

#import "TestObserver.h"


@implementation TestObserver

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    NSNumber *oldValue=[change objectForKey:NSKeyValueChangeOldKey];
    NSNumber *newValue=[change objectForKey:NSKeyValueChangeNewKey];
    NSLog(@"Observed:%@ of %@ was changed from %@ to %@",keyPath,object,oldValue,newValue);
}
@end

#import <Foundation/Foundation.h>
#import "TestClass.h"
#import "TestObserver.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        TestClass *testObj=[[TestClass alloc]init];
        __unused TestObserver *testObserver=[[TestObserver alloc]init];
        
        //添加观察者
        [testObj addObserver:testObserver
                 forKeyPath:@"height"
                 options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld
                 context:nil];
        
        //两秒更新一次值
        __unused NSTimer *timer=[NSTimer scheduledTimerWithTimeInterval:2.0 target:testObj
                                                               selector:@selector(updateLastValue) userInfo:nil repeats:YES];
        [[NSRunLoop currentRunLoop] run];
    }
    return 0;
}

2.如果属性不是通过存取方法来改变值的，例如直接给实例变量赋值，_name=xxx，这个时候可以显示触发通知。

-(void) updateLastValue
{
    [self willChangeValueForKey:@"height"];
    _height++;
    [self didChangeValueForKey:@"height"];
}

3.有时候一个属性的改变会引起另外一个属性改变，我们可以通过keyPathsForValuesAffecting使某个属性触发通知时，也触发另外一个属性的改变通知，keyPathsForValuesAffecting后面为要影响的属性名，首字母大写，不需要声明。

-(void) updateLastValue
{
    [self willChangeValueForKey:@"height"];
    _height++;
    _size=_height*2;
    [self didChangeValueForKey:@"height"];
}

+(NSSet*) keyPathsForValuesAffectingSize
{
    return [NSSet setWithObject:@"height"];
}