EXC_BAD_ACCESS on performSelectorInBackground - ios

Question

I want to call this method in background,
-(void)downloadImage_3:(NSString* )Path AtIndex:(int)i
I am calling in this way but it crashes and shows EXC_BAD_ACCESS
[self performSelectorInBackground:#selector(downloadImage_3:AtIndex:) withObject:[NSArray arrayWithObjects:#"http://www.google.com",i, nil]];
How to call downloadImage_3: method in background ?
where i am doing mistake ?

Answer 1

try this
[self performSelectorInBackground:#selector(downloadImage_3:AtIndex:) withObject:[NSArray arrayWithObjects:#"http://www.google.com",i, nil] afterDelay:15.0];
Or try this
NSString* number = [NSString stringWithFormat:#"%d",i];
NSArray* arrayValues = [[NSArray alloc] initWithObjects:[[msg_array objectAtIndex:i] valueForKey:#"Merchant_SmallImage"],number, nil];
NSArray* arrayKeys = [[NSArray alloc] initWithObjects:#"Path",#"Index",nil];
NSDictionary* dic = [[NSDictionary alloc] initWithObjects:arrayValues forKeys:arrayKeys];
[self performSelectorInBackground:#selector(downloadImage_3:) withObject:dic];
Define downloadImage_3 function like this:
-(void)downloadImage_3:(NSDictionary *)dic
{
NSString *path = [dic valueForKey:#"Path"];
int i = [[dic valueForKey:#"Index"] intValue];
//Your code
}

Answer 2

You can not call parameterized function in #selector. Create NSDictionary and then pass that dictionary in withObject:
int atIndex = 2; // whatever index you want to pass
NSArray *arr = [NSArray arrayWithObjects:obj1,obj2,nil];
NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:atIndex],#"AtIndex",arr,#"ImagesUrl", nil];
[self performSelectorInBackground:#selector(downloadImage_3:) withObject:dictionary];
Define downloadImage_3 function like this:
-(void)downloadImage_3:(NSDictionary *)dic {
// use dic and write your code
}

Answer 3

You can't use performSelectorInBackground:withObject: for selectors that take multiple arguments. The suggestions other answers give sort of work, but they all assume you can manipulate the method you are calling, which is not always possible (or even a good idea for clarity).
I recommend using NSInvocation instead, as it allows multiple arguments, or alternatively using GCD to dispatch a block to a background queue (or any queue other than the main for that matter).
Here's an example usage of NSInvocation
NSMethodSignature *sig = [[self class] instanceMethodSignatureForSelector:#selector(downloadImage_3:AtIndex:)];
NSInvocation *inv = [NSInvocation invocationWithMethodSignature:sig];
[inv setTarget:self];
[inv setSelector:#selector(downloadImage_3:AtIndex:)];
[inv setArgument:(__bridge void *)#"http://www.google.com" atIndex:2];
NSUInteger i = 1;
[inv setArgument:&i atIndex:3];
[inv performSelectorInBackground:#selector(invoke) withObject:nil];
It's worth double checking, I wrote the code in the browser so I might've missed something the compiler will pick up.
As additional note, you should really re-think your method naming conventions, a much more standard way to name methods would have the method be named -(void)downloadImage3:(NSString* )path atIndex:(int)i. Notice how atIndex begins with lowercase and how there is no underscore (which just looks weird in the middle of a method name). Also it might be worth noting that using NSUInteger is preferred for the index, as NSArray in general works with NSUIntegers (both should work, but there are cases in which int might not suffice).