KVO以及facebook开源库KVOController的使用

KVO,即：Key-Value Observing，当特定对象的属性被改变后，观察者会接收到通知。

假设一个View Controler 里面有一个Clock类,有一个时间data属性标示当前时间,每隔一秒变化一次, 一个UILabel, Clock的data属性不断变化后会显示显示到label上

在ViewController.m 中,,

//定义context

<span style="font-family: Arial, Helvetica, sans-serif;">static char* dateChange = "dateChange";//clock类</span>

<pre name="code" class="objc">Clock *mClock = [Clock clock];

//ViewController注册变化观察mClock的date属性的变化，从而使showLabel动态显示内容<pre name="code" class="objc">[mClock addObserver:self forKeyPath:@"date" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew context:dateChange];

//ViewController观察变化<pre name="code" class="objc">- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if ([keyPath isEqualToString:@"date"] && [object isKindOfClass:[Clock class]] && !strcmp(dateChange, context)) {
        [self showTimeWithChange:change];
    }
}
<pre name="code" class="objc">- (void)showTimeWithChange:(NSDictionary *)change
{
    NSDate *date = (NSDate *)change[NSKeyValueChangeNewKey];
    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    formatter.dateStyle = kCFDateFormatterFullStyle;
    formatter.dateFormat = @"yyyy/MM/dd HH:mm:ss";
    NSString *str = [formatter stringFromDate:date];
    _showLabel.text = str;
}

//消除变化观察<pre name="code" class="objc">[mClock removeObserver:self forKeyPath:@"date"];

是不是很简单,其实facebook一个非常安全的开源库简化了这个本来很简单的设计模式,并且具有线程安全等优点.

一句代码解决问题,但是前提是你引入三方库, #import"NSObject+FBKVOController.h"

    [self.KVOController observe:mClock keyPath:@"date" options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew block:^(id observer, id object, NSDictionary *change) {
        [self showTimeWithChange:change];
    }];