iOS之简单使用KVO监听属性改变

开发中我们常常需要监听到某些属性值的变化而做一些相应的事情,比如说展示筹款进度的进度条,如果进度发生改变,我们是希望进度条也会跟随变化滚动,这个时候我们就可以通过KVO监听属性的办法满足这个需求.

以下为一个简单的使用KVO的demo
demo介绍,我们现在假设在一个界面上面有一个文字属性,里面展示的是人物的年龄,我们给这个年龄添加一个监听者,如果人物年龄发生变化,我们就让展示的文字属性值跟随改变.

.h文件的nian'lin

@interface HJPerson : NSObject

/** age */
@property(nonatomic, assign)NSInteger age;

@end

控制器的.m文件

#import "ViewController.h"
#import "HJPerson.h"

@interface ViewController ()
/** person */
@property(nonatomic, strong)HJPerson * person;
/** 展示的文字 */
@property(nonatomic, strong) UILabel *personAge;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    //需求,让控制器能够监听到person年龄的变化,并且将最新的年龄显示到界面上去
    HJPerson *person = [HJPerson new];
    self.person = person;
    person.age = 10;

    UILabel *personAge = [UILabel new];
    personAge.frame = CGRectMake(100, 100, 200, 200);
    personAge.text = [NSString stringWithFormat:@"%zd",person.age];
    personAge.backgroundColor = [UIColor cyanColor];
    [self.view addSubview:personAge];
    self.personAge = personAge;
    
    [self.person addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew context:nil];
    
    
}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    self.person.age = 20;
}


-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    /**
     NSLog(@"keyPath=%@,object=%@,change=%@,context=%@",keyPath,object,change,context);
     
     keyPath=age,object=,change={
     kind = 1;
     new = 20;
     },context=(null)
     */
    NSLog(@"keyPath=%@,object=%@,change=%@,context=%@",keyPath,object,change,context);
    
    这里需要将NSNumber类型转换为字符串类型
    NSNumberFormatter* numberFormatter = [[NSNumberFormatter alloc] init];
     NSString *ageStr = [numberFormatter stringFromNumber:[change objectForKey:@"new"]];

    
    self.personAge.text = ageStr;
}

@end

效果如下

KVO简单使用.gif