iOS KVO崩溃全情景列举+解决方案分析

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****** 19.10.30 更新

被观察者在销毁前，要移除所有的观察者，iOS10以下会崩溃，iOS11以上不会崩溃

先上结果

崩溃原因总结

1、observe忘记写监听回调方法 observeValueForKeyPath
2、add和remove次数不匹配
3、监听者和被监听者dealloc之前没有remove（其实也原因2，但是监听者和被监听者的生命周期不同）

KVO 是iOS开发着常用的键值模式，但是使用不好经常会带来崩溃。在复杂的业务逻辑中遇到KVO相关的偶发崩溃，原因寻找起来比较困难（必现的的还好），下面将由简到繁介绍KVO的崩溃情形和崩溃原因。情形六有助于更好的了解kvo的崩溃。

情型一、obsever没有实现observeValueForKeyPath方法

- (void)viewDidLoad {
    [super viewDidLoad];

    // Do any additional setup after loading the view, typically from a nib
    
    [self.titleLabel addObserver:self forKeyPath:@"backgroundColor" options:NSKeyValueObservingOptionNew context:nil];
    _titleLabel.backgroundColor = [UIColor blueColor];
}

屏幕快照43.40.png

情型二、没有移除KVO 比如observer是VC在pop的时候没有在dealloc中removeKVO

//- (void)dealloc{
//    [_titleLabel removeObserver:self //forKeyPath:@"backgroundColor"];
//}

- (void)viewDidLoad {
    [super viewDidLoad];

    // Do any additional setup after loading the view, typically from a nib
    
    [self.titleLabel addObserver:self forKeyPath:@"backgroundColor" options:NSKeyValueObservingOptionNew context:nil];
    _titleLabel.backgroundColor = [UIColor blueColor];
}
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    
    if ([keyPath isEqualToString:@"backgroundColor"]) {
        NSLog(@"颜色改变了");
    }else{
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
    
}

屏幕快照44.png

情型三、多次移除KVO

- (void)dealloc{
    //第二次remove
    [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
}

- (void)viewDidLoad {
    [super viewDidLoad];

    [_titleLabel addObserver:self forKeyPath:@"backgroundColor" options:NSKeyValueObservingOptionNew context:nil];
    _titleLabel.backgroundColor = [UIColor blueColor];
    
    //第一次remove
    [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];

}

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

    if ([keyPath isEqualToString:@"backgroundColor"]) {
        NSLog(@"颜色改变了");
    }else{
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

屏幕快照50.png

情型四、多次添加相同KVO但是remove次数不同（如果add 和remove相匹配不会崩溃 例子中在dealloc中remove两次）

- (void)dealloc{
    [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
    // [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [_titleLabel addObserver:self forKeyPath:@"backgroundColor" options:NSKeyValueObservingOptionNew context:nil];
}

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

    if ([keyPath isEqualToString:@"backgroundColor"]) {
        NSLog(@"颜色改变了");
    }else{
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

屏幕快照.36.png

情形六 监听者和被监听者的生命周期不同

- (void)dealloc{
    [_titleLabel removeObserver:_titleView forKeyPath:@"backgroundColor"];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [_titleLabel addObserver:_titleView forKeyPath:@"backgroundColor" options:NSKeyValueObservingOptionNew context:nil];
    _titleLabel.backgroundColor = [UIColor blueColor];

///全都注释不会崩溃  任意打开下面一个都会崩溃  这也是一些分业务逻辑复杂的功能，会有不易查出崩溃的原因， 监听对象和被监听对象的释放时机没有掌握好就会导致类似的崩溃。
// 比如对AVPlayerItem 的监听，在切换AVPlayerItem 若果逻辑不严谨可能会导致kvo的崩溃。
    //_titleLabel = nil;
    // _titleView = nil;

}

///_titleView 自定义的实现 observeValueForKeyPath 方法
#import "ObserView.h"

@implementation ObserView

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
    
    if ([keyPath isEqualToString:@"backgroundColor"]) {
        NSLog(@"颜色改变了");
    }else{
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

崩溃原因总结

1、忘记写监听回调方法 observeValueForKeyPath
2、add和remove次数不匹配
3、监听者和被监听者dealloc之前没有remove

解决方案分析

方案一 cc_addObserver 代码在下面

情形一会崩溃
- (void)dealloc{
    [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
    [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
}

- (void)viewDidLoad {
    [super viewDidLoad];
  
    [_titleLabel cc_addObserver:self forKeyPath:@"backgroundColor"];
    _titleLabel.backgroundColor = [UIColor blueColor];

}

情形二、不会崩溃

- (void)dealloc{
    [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    [_titleLabel cc_addObserver:self forKeyPath:@"backgroundColor"];
    _titleLabel.backgroundColor = [UIColor blueColor];
    _titleLabel = nil;

}

cc_addObserver 只能防治生命周期不匹配的崩溃形式，不能防止add remove 不匹配的情形

方案二 使用JJException

情形一、不会崩溃

- (void)dealloc{
    [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
    //[_titleLabel removeObserver:self 
}

- (void)viewDidLoad {
    [super viewDidLoad];
    [_titleLabel addObserver:self forKeyPath:@"backgroundColor" options:NSKeyValueObservingOptionNew context:nil];
    _titleLabel.backgroundColor = [UIColor blueColor];
    
    _titleLabel = nil;
    //_titleView = nil;
}

情形二、会崩溃

- (void)dealloc{
    [_titleLabel removeObserver: _titleView forKeyPath:@"backgroundColor"];
    //[_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _titleLabel = [UILabel new];
    _titleLabel.backgroundColor = [UIColor redColor];
    _titleLabel.frame = CGRectMake(20, 100, 50, 50);
    [self.view addSubview:_titleLabel];
    
    _titleView = [UIView new];
    _titleView.backgroundColor = [UIColor yellowColor];
    _titleView.frame = CGRectMake(100, 100, 50, 50);
    [self.view addSubview:_titleView];
    

    [_titleLabel addObserver:_titleView forKeyPath:@"backgroundColor" options:NSKeyValueObservingOptionNew context:nil];
    //_titleLabel
    _titleLabel.backgroundColor = [UIColor blueColor];
    
    _titleLabel = nil;
    //_titleView = nil;
}

情形三、会崩溃

- (void)dealloc{
    [_titleLabel removeObserver: _titleView forKeyPath:@"backgroundColor"];
    //[_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _titleLabel = [UILabel new];
    _titleLabel.backgroundColor = [UIColor redColor];
    _titleLabel.frame = CGRectMake(20, 100, 50, 50);
    [self.view addSubview:_titleLabel];
    
    _titleView = [UIView new];
    _titleView.backgroundColor = [UIColor yellowColor];
    _titleView.frame = CGRectMake(100, 100, 50, 50);
    [self.view addSubview:_titleView];
    

    [_titleLabel addObserver:_titleView forKeyPath:@"backgroundColor" options:NSKeyValueObservingOptionNew context:nil];
    //_titleLabel
    _titleLabel.backgroundColor = [UIColor blueColor];
    
    //_titleLabel = nil;
    //_titleView = nil;

}

情形四、不会崩溃

- (void)dealloc{
    [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
    //[_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    [_titleLabel addObserver:self forKeyPath:@"backgroundColor" options:NSKeyValueObservingOptionNew context:nil];
    //_titleLabel
    _titleLabel.backgroundColor = [UIColor blueColor];
    
    //_titleLabel = nil;
    //_titleView = nil;
}

情形五、不会崩溃

- (void)dealloc{
    [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
    [_titleLabel removeObserver:self forKeyPath:@"backgroundColor"];
}

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [_titleLabel addObserver:self forKeyPath:@"backgroundColor" options:NSKeyValueObservingOptionNew context:nil];
    //_titleLabel
    _titleLabel.backgroundColor = [UIColor blueColor];
    
    //_titleLabel = nil;
    //_titleView = nil;
}

总结：

cc_addObserver 和 JJException 都不会完全解决KVO使用不严谨引发的崩溃问题，推荐使用 cc_addObserver，并且不自己写removeObserve的方法，防止监听者为nil时crash

下面是 cc_addObserver 代码

#import <Foundation/Foundation.h>

@interface NSObject (ObserverHelper)

- (void)cc_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath;

@end

#import "NSObject+ObserverHelper.h"
#import <objc/message.h>

@interface ObserverHelper : NSObject
@property (nonatomic, unsafe_unretained) id target;
@property (nonatomic, unsafe_unretained) id observer;
@property (nonatomic, strong) NSString *keyPath;
@property (nonatomic, weak) ObserverHelper *factor;
@end

@implementation ObserverHelper
- (void)dealloc {
    if ( _factor ) {
        [_target removeObserver:_observer forKeyPath:_keyPath];
    }
}
@end

@implementation NSObject (ObserverHelper)

- (void)cc_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath {
    [self addObserver:observer forKeyPath:keyPath options:NSKeyValueObservingOptionNew context:nil];
    
    ObserverHelper *helper = [ObserverHelper new];
    ObserverHelper *sub = [ObserverHelper new];
    
    sub.target = helper.target = self;
    sub.observer = helper.observer = observer;
    sub.keyPath = helper.keyPath = keyPath;
    helper.factor = sub;
    sub.factor = helper;
    
    const char *helpeKey = [[keyPath mutableCopy] UTF8String];
    const char *subKey = [[keyPath mutableCopy] UTF8String];
    // 关联属性  举例 self 和 helper 关联 当self释放的时候 helper释放 即可释放self的kvo 观察者和sub关联 当观察者释放的时候 调用sub的移除同样也能删除self的kvo   factor是同一个对象 是为防止多次移除导致的崩溃
    objc_setAssociatedObject(self, helpeKey, helper, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    objc_setAssociatedObject(observer, subKey, sub, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end