Bad Access Parent is Null - Wht is this Happening? - objective-c

Question

I understand what the error is, but in this case not what is causing it. In general use it occurs maybe 1% of the time (probably less) but I have found an extreme way to cause it which I will describe below. First, I am using an in-app purchase process I found on Ray Wenderlich's site. Below are the specific pieces of concern here:
.h:
typedef void (^RequestProductsCompletionHandler)(BOOL success, NSArray * products);
#interface IAPHelper : NSObject
- (void)requestProductsWithCompletionHandler:RequestProductsCompletionHandler)completionHandler;
#end
.m
#implementation IAPHelper
{
SKProductsRequest * _productsRequest;
RequestProductsCompletionHandler _completionHandler;
}
- (void)productsRequest:(SKProductsRequest *)request didReceiveResponse:(SKProductsResponse *)response {
NSLog(#"Loaded list of products...");
_productsRequest = nil;
NSArray * skProducts = response.products;
for (SKProduct * skProduct in skProducts) {
NSLog(#"Found product: %# %# %0.2f",
skProduct.productIdentifier,
skProduct.localizedTitle,
skProduct.price.floatValue);
}
_completionHandler(YES, skProducts); // here is where bad access occurs
_completionHandler = nil;
}
Again, 99%+ of the time this works just fine. Given how infrequent the bad access happens in regular use and it has been difficult to diagnose. However, I found an extreme way to cause the issue. The setup is "Tab 1" is a table view controller and "Tab 2" is a table view controller that uses the code from above. If I quickly switch back and forth between the two tabs I can usually cause the problem to occur anywhere from a few seconds into it to 20-30 seconds. Doesn't happen every time in this scenario but it does the vast majority. As marked above the following line gets a bad access error with Parent is Null.
_completionHandler(YES, skProducts);
To solve the issue I simple do the following:
if (_completionHandler)
{
_completionHandler(YES, skProducts);
_completionHandler = nil;
}
While that fix does work and does solve the problem I am still bothered by why this is occurring. Any thoughts as to the cause of this?
Update:
Apologies to all as I did forget to include the following in what I pasted above.
- (void)requestProductsWithCompletionHandler:(RequestProductsCompletionHandler)completionHandler {
// 1
_completionHandler = [completionHandler copy];
// 2
_productsRequest = [[SKProductsRequest alloc] initWithProductIdentifiers:_productIdentifiers];
_productsRequest.delegate = self;
[_productsRequest start];
}

Answer 1

You need to treat your completion block as any other object when you are storing it. So if you are storing your block as a variable and then using it within a different scope from where you assign it, you need to increment the reference count by either copying it or retaining it. The simple solution is to create a strong property to store your block.

Answer 2

Depending on unseen bits of code, your completion handler block might not be being assigned correctly. You need to copy a block if you intend to use it outside of the scope in which it was created.
In your interface, declare your completion handler's storage attribute as "copy".
#property (nonatomic, readwrite, copy) void (^completionHandler)(BOOL, NSArray *);
If you want to control the local variable, you can synthesize the property manually in your implementation:
#synthesize completionHandler = _completionHandler;