项目常见崩溃7(陆续更新)

今天介绍一个和循环引用很相似的容器类的问题.

崩溃堆栈

0  CoreFoundation!__exceptionPreprocess + 0x7c
1  libobjc.A.dylib!objc_exception_throw + 0x34
2  CoreFoundation!__NSFastEnumerationMutationHandler + 0x7c
3  MakeFriends!-[MFTransmitService onServiceDataNotification:transmitData:] [MFTransmitService.m : 52 + 0x14]
4  MakeFriends!-[TransmitCore onServiceDataNotification:] [TransmitCore.m : 74 + 0xc]
5  CoreFoundation!__CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 0x10
6  CoreFoundation!_CFXRegistrationPost + 0x18c
7  CoreFoundation!___CFXNotificationPost_block_invoke + 0x38
8  CoreFoundation!-[_CFXNotificationRegistrar find:object:observer:enumerator:] + 0x5dc
9  CoreFoundation!_CFXNotificationPost + 0x174

异常信息如下

07-17 17:28:42.601     Exception Name: NSGenericException
07-17 17:28:42.605     Exception Reason: *** Collection <__NSArrayM: 0x17425ec30> was mutated while being enumerated.

从异常信息上来看是访问一个已经被加锁的NSArrayM, 崩溃的原因在某个小伙伴的提示下, 发现并不是因为加锁了, 而是在基于NSFastEnumeration协议的NSArray在快速遍历的更改时候, 会导致NSFastEnumerationState中的mutationsPtr改变了

if (startMutations != *enumState.mutationsPtr) objc_enumerationMutation(l_collection);

当系统执行到这行代码时抛异常了. 但是纵观我的代码, 没有涉及到多线程, 不存在线程安全的问题, 下面看代码

ViewController

@interface ViewController ()

@property (nonatomic, strong) FirstViewController *obj1;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    self.testArrayM = [NSMutableArray array];
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {

    [self.testArrayM addObject:self.obj1];
    NSNumber *obj2 = @(2);
    NSNumber *obj3 = @(3);
    [self.testArrayM addObject:obj2];
    [self.testArrayM addObject:obj3];
    
    [CommonData sharedObject].commonData = self.testArrayM;
    
    while (1) {
        sleep(10);
        
        [self test1];
    }
}

- (void)test1 {
    
    for (NSObject *num in [CommonData sharedObject].commonData) {
        [self handleNum:num];
    }
}

- (void)handleNum:(NSObject *)num {
    _obj1 = nil;
    [self.obj1 test1];
}

- (NSObject *)obj1 {
    if (_obj1 == nil) {
        _obj1 = [[FirstViewController alloc] init];
    }
    return _obj1;
}

FirstViewController

@interface FirstViewController ()

@end

@implementation FirstViewController

- (void)dealloc {
    NSLog(@"FirstViewController dealloc");
    [[CommonData sharedObject].commonData removeObject:self];
}

- (void)test1 {
    NSLog(@"test1");
}

- (instancetype)init {
    if (self = [super init]) {
        [self initVc];
    }
    return self;
}

- (void)initVc {
    [[CommonData sharedObject].commonData addObject:self];
}

- (void)viewDidLoad {
    [super viewDidLoad];
}

CommonData.h

@interface CommonData : NSObject

+ (instancetype)sharedObject;

@property (nonatomic, strong) NSMutableArray *commonData;

@end

CommonData.m

@implementation CommonData

+ (instancetype)sharedObject {
    static dispatch_once_t __once;
    static CommonData * __instance = nil;
    dispatch_once(&__once, ^{
    __instance = [[CommonData alloc] init];
    });
    return __instance;
}

@end

CommonData是一个公用的数据存储类, ViewController对commonData数组进行遍历, 在遍历的时候又调用了FirstViewController的init, init里面又去操作CommonData的commonData, 就造成了对正在遍历的commonData进行了修改, 而系统在遍历的时候应该是对这个commonData进行加锁的, 这就造成了访问加锁数据崩溃.

修复方法

修改ViewController中使用方法
- (void)test1 {
    
    NSArray *temp = [[CommonData sharedObject].commonData mutableCopy];
    
    for (NSObject *num in temp) {
        [self handleNum:num];
    }
}

对commonData进行复制后, 加锁的就不是commonData而是temp, 这样再其它地方进行修改就没问题了, 这里给我们的教训是, 在遍历muteble容器的时候, 最好进行一次复制, 这样能避免容器在被遍历的时候被修改也不会有问题.

总结

造成崩溃是因为, ViewController->commonData->FirstViewController->commonData, 这样就形成了类似commonData的循环引用. 最终导致崩溃, 解决的方案也是类似破除循环引用的方式, ViewController->temp->FirstViewController->commonData.