NSBlockOperation EXC_BAD_ACCESS

Question

我創建了一個實現了幾個方法的類。這些方法由另一個類調用，並通過NSBlockOperation進行管理。

我NSBlockOperation工作正常，我有問題，當我試圖評估的變量：

EXC_BAD_ACCESS

我已經做了很多研究在互聯網上和this是最接近我的問題之一。我試圖做sugerito，但你遇到同樣的問題。

你有什麼建議嗎？

編輯：

這是堆棧跟蹤：

2015-05-09 15:24:45.976 OutParameters[12326:743087] Stack trace : (
    0 OutParameters      0x000000010e5d6602 -[ListOperation _method1:] + 194 
    1 OutParameters      0x000000010e5d646f __25-[ListOperation method1:]_block_invoke + 95 
    2 Foundation       0x000000010e74257f __NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 7 
    3 Foundation       0x000000010e6830b2 -[NSBlockOperation main] + 98 
    4 Foundation       0x000000010e665774 -[__NSOperationInternal _start:] + 645 
    5 Foundation       0x000000010e665383 __NSOQSchedule_f + 184 
    6 libdispatch.dylib     0x00000001113f4614 _dispatch_client_callout + 8 
    7 libdispatch.dylib     0x00000001113db6a7 _dispatch_queue_drain + 2176 
    8 libdispatch.dylib     0x00000001113dacc0 _dispatch_queue_invoke + 235 
    9 libdispatch.dylib     0x00000001113de3b9 _dispatch_root_queue_drain + 1359 
    10 libdispatch.dylib     0x00000001113dfb17 _dispatch_worker_thread3 + 111 
    11 libsystem_pthread.dylib    0x0000000111761637 _pthread_wqthread + 729 
    12 libsystem_pthread.dylib    0x000000011175f40d start_wqthread + 13 
) 

---

修改後的代碼：

- (IBAction)testCallMethod:(id)sender { 
    NSString * output; 
    [self.listOperationObj method1:&output]; 
    NSLog(@"Output: %@", output); 
} 

而且

@interface ListOperation : NSObject 

-(void)method1:(NSString**)output; 

@end 

#define MAX_OPERATIONS 10 
//define a log-level 
static int logLevel = CSLOG_LEVEL_INFO; 
@interface ListOperation() 
// Tail used to synchronize the methods 
@property NSOperationQueue *queue; 
@end 

#pragma mark - Public methods 

@implementation ListOperation 

- (id)init { 
    self = [super init]; 
    if(self) { 
     _queue = [NSOperationQueue new]; 
     if(_queue) { 
      [_queue setMaxConcurrentOperationCount:1]; 
     }else { 
      NSLog(@"TokenMgr creation failed: error creating operation queue"); 
      self = nil; 
     } 
    } 
    return self; 
} 

-(void)method1:(NSString *__autoreleasing *)output{ 
    LOGFSTART 
    if([self _isQueueFull] == FALSE) { 
     WEAK 
     NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{ 
      STRONG 
      [strongSelf _method1:output]; 
     }]; 
     [self.queue addOperation:operation]; 
     [operation waitUntilFinished]; 
    } 
    else { 
     LOGE(@"TokenMgr's queue is full, aborting operation"); 
    } 
    LOGFEND 
} 

#pragma mark - private methods 

-(void)_method1:(NSString *__autoreleasing *)output{ 
    std::string testString = "try put string"; 
    *output = [NSString stringWithUTF8String:testString.c_str()]; 
} 

- (BOOL) _isQueueFull { 
    return self.queue.operationCount > MAX_OPERATIONS; 
} 

@end 

如果我反覆按下按鈕，此更改會導致同樣的錯誤。

Answer 1

直接的問題與塊無關。你的代碼片段說：

- (IBAction)testCallMethod:(id)sender { 
    NSString *__autoreleasing * output; 
    [self.listOperationObj method1:output]; 
    NSLog(@"Output: %@", *output); 
} 

這是行不通的，因爲output不會指向有效的內存地址，而當您嘗試取消引用與*output = ...未初始化的指針，它會崩潰。

相反， 應該是：

- (IBAction)testCallMethod:(id)sender { 
    NSString *output; 
    [self.listOperationObj method1:&output]; 
    NSLog(@"Output: %@", output); 
} 

現在output引用一個真正NSString *指針，你可以用一個對象的引用填充。

---

還有第二個，更深層次的問題，即利用* __autoreleasing *引用了裏面的操作實例化一個對象。操作擁有自己的自動釋放池，因此您在與testCallMethod中使用該對象時發生競爭。

取而代之，通常使用完成塊將數據傳回給調用者。因此：

- (IBAction)testCallMethod:(id)sender { 
    [self.listOperationObj method2:^(NSString *output) { 
     NSLog(@"Output: %@", output); 
    }]; 
} 

和

- (void)method2:(void (^)(NSString *))completionHandler { 
    LOGFSTART 
    if([self _isQueueFull] == FALSE) { 
     WEAK 
     NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{ 
      STRONG 
      [strongSelf _method2:completionHandler]; 
     }]; 
     [self.queue addOperation:operation]; 
     // [operation waitUntilFinished]; // not needed any more 
    } 
    else { 
     LOGE(@"TokenMgr's queue is full, aborting operation"); 
    } 
    LOGFEND 
} 

-(void)_method2:(void (^)(NSString *))completionHandler { 
    std::string testString = "try put string"; 
    NSString *output = [NSString stringWithUTF8String:testString.c_str()]; 
    completionHandler(output); 
} 

注意，順便說一下，這也解決了你的榜樣另一個問題，那你有事實把一個waitUntilFinished電話在那裏。你永遠不應該從主線程中調用waitUntilFinished。如果你使用完成塊，就像上面那樣，那就不再需要了。