KVO的简单用法

// Created by wjn on 15/9/30.

// Copyright © 2015年 wlm. All rights reserved.

//

#import "ViewController.h"

@interface
ViewController ()

@property (nonatomic,
retain) NSString *string;

@end

@implementation ViewController

- (void)dealloc {

//
切记一定要在完成之后移除掉 - ARC一样

[self
removeObserver:self
forKeyPath:@"string"];

[super
dealloc];
}

- (void)viewDidLoad {

[super
viewDidLoad];

// Do any additional setup after loading the view, typically from a nib.

UIButton *button = [UIButton
buttonWithType:UIButtonTypeCustom];
button.frame =
CGRectMake(0,
20, self.view.frame.size.width,
40);

[button setTitle:@"KVO"
forState:UIControlStateNormal];

[button setBackgroundColor:[UIColor
greenColor]];

[button
addTarget:self
action:@selector(buttonDidPress:)
forControlEvents:UIControlEventTouchUpInside];

[self.view
addSubview:button];

// KVO
一个类监听自己的属性（成员变量）的变化

//
参数1：观察者

//
参数2：要观察的对象

//
参数3：一旦发生改变，观察的结果是取新值还是旧值

//
参数4：一旦发生改变，可以传递的内容

[self
addObserver:self
forKeyPath:@"string"
options:NSKeyValueObservingOptionNew
context:@"BOOM"];
}

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

//
系统的方法

//
当观察到的属性改变之后，自动调用这个方法

NSLog(@"%@", keyPath);

NSLog(@"%@", object);

NSLog(@"%@", change);

NSLog(@"%@", context);
}

//-(NSMutableArray *)mutableArray {

// if (!_mutableArray) {

// self.mutableArray = [NSMutableArray arrayWithObject:@"a"];

// }

// return _mutableArray;

//}

- (void)buttonDidPress:(UIButton *)sender {

// [self.mutableArray addObject:@"a"];

[self
setValue:@"aa"
forKey:@"string"];
}

- (void)didReceiveMemoryWarning {

[super
didReceiveMemoryWarning];

// Dispose of any resources that can be recreated.
}

@end