使用KVO监听数组的变化

以前有个需求，需要监听数组长度的变化。使用KVO直接监听数组本身，由于数组地址并没有发生改变，所以并不能监听到。试了好多方法，都没有能够实现。知道最近听的一场公开课中，才掌握了正确的监听方法

先举个错误的例子

@interface ViewController ()
@property (nonatomic, strong) NSMutableArray *array;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    [self.array addObserver:self forKeyPath:@"count" options:(NSKeyValueObservingOptionNew) context:nil];
}

这样会直接报错，NSMutableArray 没有 count的 keyPath。

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '[<__NSArrayM 0x282195770> addObserver:forKeyPath:options:context:] is not supported. Key path: count'

正确的打开方式是这样的，不多说废话，直接上代码

#import "ViewController.h"

@interface ViewController ()
@property (nonatomic, strong) NSMutableArray *array;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
 
    [self addObserver:self forKeyPath:NSStringFromSelector(@selector(array)) options:(NSKeyValueObservingOptionNew) context:nil];
}



- (void)observeValueForKeyPath:(nullable NSString *)keyPath ofObject:(nullable id)object change:(nullable NSDictionary<NSKeyValueChangeKey, id> *)change context:(nullable void *)context {
    
    /*
     typedef NS_ENUM(NSUInteger, NSKeyValueChange) {
     NSKeyValueChangeSetting = 1,
     NSKeyValueChangeInsertion = 2,
     NSKeyValueChangeRemoval = 3,
     NSKeyValueChangeReplacement = 4,
     };
     */

    
    //第一次触发是添加一条数据,kind为2，对应的是NSKeyValueChangeInsertion
    /*
     {
    indexes = "<_NSCachedIndexSet: 0x280495620>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
    kind = 2;
    new =     (
               "NewData- 1"
               );
     }
     */
    
    
    //第二次触发是修改一条数据,kind为4，对应的是NSKeyValueChangeReplacement
    /*
     {
     indexes = "<_NSCachedIndexSet: 0x280495620>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
     kind = 4;
     new =     (
     "NewData- 2"
     );
     }
     */
    
    //第三次次触发是删除一条数据,kind为3，对应的是NSKeyValueChangeRemoval
    /*
     {
     indexes = "<_NSCachedIndexSet: 0x280495620>[number of indexes: 1 (in 1 ranges), indexes: (0)]";
     kind = 3;
     }
     */
    
    
    NSLog(@"我触发了");
}




- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    static NSInteger a = 0;
    a++;
    
    //这一步特别重要
    NSMutableArray *tempArray = [self mutableArrayValueForKey:NSStringFromSelector(@selector(array))];
    if (1 == a) {//插入一条
        [tempArray addObject:[NSString stringWithFormat:@"NewData- %tu",a]];
    } else if (2 == a) {//修改一条
        tempArray[0] = [NSString stringWithFormat:@"NewData- %tu",a];
    } else if (3 == a) {
        [tempArray removeAllObjects];
    }
}


- (NSMutableArray *)array {
    if (!_array) {
        _array = [NSMutableArray array];
    }
    return _array;
}

- (void)dealloc {
    [self removeObserver:self forKeyPath:NSStringFromSelector(@selector(array))];
}

 KVO三部曲，注册监听，实现代理方法，移除监听。

在监听数组变化时，需要特别注意，

    NSMutableArray *tempArray = [self mutableArrayValueForKey:NSStringFromSelector(@selector(array))];

 一定要使用该方法复制出一个数组，而不是使用数组本身进行操作。