结合之前某智付项目 用到了监测 程序是否isLogin 登录状态
在AppDelegate中 添加观察者
[[MobileBankSession sharedInstance] addObserver:self forKeyPath:@"isLogin" options:NSKeyValueObservingOptionNew context:nil];
当MobileBankSession（网络类）类中的isLogin值变化的时候会调用
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context{
做一些登出isLogin ＝ NO；的操作
比如显示侧边栏
}
小例子：
#import "ViewController.h"#import "Walker.h"@interface ViewController (){ UILabel *label; Walker *walker;}@end@implementation ViewController- (void)viewDidLoad { [super viewDidLoad]; walker = [[Walker alloc] initWithName:@"Boss" age:45];
//关键代码 添加观察者
[walker addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew context:nil];
label = [[UILabel alloc] init];
label.frame = CGRectMake(10, 80, 200, 60); label.text = [NSString stringWithFormat:@"姓名：%@,年龄：%ld",walker.name,walker.age]; label.backgroundColor = [UIColor grayColor]; [self.view addSubview:label]; UIButton *button = [[UIButton alloc] init]; button.frame = CGRectMake(100, 200, 80, 80); button.backgroundColor = [UIColor grayColor]; [button setTitle:@"年龄增加5岁" forState:UIControlStateNormal]; [button addTarget:self action:@selector(btnAction) forControlEvents:UIControlEventTouchUpInside]; [self.view addSubview:button];}
//关键代码
/* KVO function， 只要object的keyPath属性发生变化，就会调用此函数*/- (void)observeValueForKeyPath:(nullable NSString *)keyPath ofObject:(nullable id)object change:(nullable NSDictionary*)change context:(nullable void *)context;
{
if ([keyPath isEqualToString:@"age"] && object == walker){
label.text = [NSString stringWithFormat:@"姓名：%@,年龄：%ld",walker.name,walker.age];
}
}
- (void)btnAction{
walker.age += 5;
}
- (void)viewWillDisappear:(BOOL)animated{
[super viewWillDisappear:animated];
[walker removeObserver:self forKeyPath:@"age"];
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
@end
点击了按钮之后，系统会调用 observeValueForKeyPath ：函数，这个函数应该也是回调函数。在该函数内部做UI更新。我们以这种轻量级的方式达到了目的。
