KVO等具体实现步骤以及注意事项

KVO是一种设计模式,名为观察者.

addObserver:forKeyPath:options:context:

通知其他对象的方法,这个方法在NSObject中就已经申明了,也就是说任何继承自NSObject的对象都可以使用KVO.

我们来实现一个对象a值改变的时候去通知对象b.

新建两个ModelA ModelB 类.

ModelA.h + ModelA.m

#import <Foundation/Foundation.h>

@interface ModelA : NSObject

@property (nonatomic, strong) NSString *name;

@end

#import "ModelA.h"

@implementation ModelA

@end

ModelB.h + ModelB.m

#import <Foundation/Foundation.h>

@interface ModelB : NSObject

@property (nonatomic, strong) NSString *sex;

@end

#import "ModelB.h"

@implementation ModelB

-(void)observeValueForKeyPath:(NSString *)keyPath
                     ofObject:(id)object
                       change:(NSDictionary *)change
                      context:(void *)context
{
    self.sex = @"female";
}

@end

请将如下延时执行的代码添加进工程当中

- (void)delay:(int64_t)delta execute:(dispatch_block_t)block
{
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, delta * NSEC_PER_SEC),
                   dispatch_get_main_queue(), block);
}

然后再写如下的代码:

执行结果如下:

2014-05-06 17:40:35.346 FileManager[20208:60b] Application windows are expected to have a root view controller at the end of application launch
2014-05-06 17:40:37.347 FileManager[20208:60b] female

如果注释掉ModelB中的方法observeValueForKeyPath:ofObject:change:context:,运行时会导致崩溃:

如果重复移除了两次,也会导致崩溃-_-!!!!

也就是这么一层关系:

A对象要通知B对象,B对象必须实现监听的方法,否则一旦有消息发送就会导致崩溃.

A对象不想通知B对象了,需要从B对象身上移除掉通知.

要想程序不出现问题,我们需要实现3步.

(主动添加B的通知) A -------> B(不实现一个方法就崩溃)

(主动移除B的通知) A ---X--> B

(重复移除B的通知) A ---X--> B(崩溃)

用起来很恶性,还好,我们可以重复添加B的通知而不崩溃......

问题:在ARC中我们需要移除KVO的observer么?

You should explicitly remove the observer even you use ARC. Create a dealloc method and remove there..

-(void)dealloc {[[NSNotificationCenter defaultCenter] removeObserver:self];}

If you see the method you don't need to call [super dealloc]; here, only the method without super dealloc needed.

你需要非常明确的移除你的通知者,即使是在ARC中.创建一个dealloc方法然后在方法里面移除.

ARC中你不需要调用[super dealloc].

问题:ARC给一个对象添加了observer有可能会导致循环应用什么的么?

You need to call removeObserver, ARC only automates retain counts. removeObserver does not impact the retain count.

你需要调用removeObserver,ARC只是自动管理了retain counts,removeObserver并不影响retain count.(这一点我不确定,有待验证)

问题:当要通知的对象已经nil了,这个通知会自动移除吗?

Observers are not removed automatically. From the NSNotificationCenter Class Reference:

观察者不会自动移除,请查看NSNotificationCenter类的原文引述:

Important: The notification center does not retain its observers, therefore, you must ensure that you unregister observers (using removeObserver: or removeObserver:name:object:) before they are deallocated. (If you don't, you will generate a runtime error if the center sends a message to a freed object.)

很重要:通知中心并不会retain他的观察者,所以,你必须确保你那些对象销毁之前注销掉观察者,否则就会出现错误.

You should therefore call

[[NSNotificationCenter defaultCenter] removeObserver:self];

in your dealloc method if you are not 100% sure that the observer was not removed previously.

https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Protocols/NSKeyValueObserving_Protocol/Reference/Reference.html#//apple_ref/occ/instm/NSObject/addObserver%3aforKeyPath%3aoptions%3acontext%3a

addObserver:forKeyPath:options:context:

Registers anObserver to receive KVO notifications for the specified key-path relative to the receiver.

- (void)addObserver:(NSObject *)anObserver forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context

Parameters

anObserver: The object to register for KVO notifications. The observer must implement the key-value observing method observeValueForKeyPath:ofObject:change:context:.
keyPath: The key path, relative to the receiver, of the property to observe. This value must not be nil.
options: A combination of the NSKeyValueObservingOptions values that specifies what is included in observation notifications. For possible values, see “NSKeyValueObservingOptions.”
context: Arbitrary data that is passed to anObserver in observeValueForKeyPath:ofObject:change:context:.