经验拾遗之KVO

一看就懂demo下载，地址：github

  iOS开发，肯定免不了要接触苹果的黑魔法：kvo，翻译过来有点拗口叫键值观察，如果是我翻译的话会选择叫“望咩望”，哈哈。其实kvo本质来说就是针对一个对象的变动进行操作，注意，是针对变动。如果app中有需求是针对某对象变动需要进行对应的操作的话，kvo是理想的实现技术。下面，来实践一下。

1.创建一个类，重写- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary*)change context:(void *)context方法：

@implementation AppStatusWatcher

- (void)observeValueForKeyPath:(NSString*)keyPath ofObject:(id)object change:(NSDictionary*)change context:(void*)context {

NSLog(@"Property '%@' of object '%@' changed: %@ context: %@",keyPath,object,change,context);

}

@end

2.在app delegate中处理，定义了一个appStatus属性，当变动时，观察者watcher就会接收到通知，并且变动前后的值都可以选择传递过来：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

self.watcher = [AppStatusWatcher new];

[self addObserver:self.watcher forKeyPath:@"appStatus" options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:@"望咩望"];

self.appStatus = @"launch";

return YES;

}

3.在app的不同阶段，改变appStatus的值：

- (void)applicationWillResignActive:(UIApplication *)application {

self.appStatus = @"inactive";

}

- (void)applicationDidEnterBackground:(UIApplication *)application {

self.appStatus = @"background";

}

- (void)applicationWillEnterForeground:(UIApplication *)application {

self.appStatus = @"inactive";

}

- (void)applicationDidBecomeActive:(UIApplication *)application {

self.appStatus = @"active";

}

- (void)applicationWillTerminate:(UIApplication *)application {

self.appStatus = @"terminate";

}

4.appstatus的值一改变，watcher就会接收到通知，执行自定义操作，实际运行效果：

2016-03-21 17:59:23.663 KVO[5767:13407361] Property 'appStatus' of object '' changed: {    kind = 1;    new = launch;    old = "";} context: 望咩望

2016-03-21 17:59:23.668 KVO[5767:13407361] Property 'appStatus' of object '' changed: {    kind = 1;    new = active;    old = launch;} context: 望咩望

2016-03-21 17:59:28.249 KVO[5767:13407361] Property 'appStatus' of object '' changed: {    kind = 1;    new = inactive;    old = active;} context: 望咩望

2016-03-21 17:59:28.881 KVO[5767:13407361] Property 'appStatus' of object '' changed: {   kind = 1;     new = background;    old = inactive;} context: 望咩望