iOS how to use delegate method from within block - ios

Question

I am using AFNetworking for request/response tasks of my application. In one of my handler classes, when an HTTP request succeeds, I must perform a call to delegate method in order to update the UI (note that I specifically do not want to use NSNotificationCenter). However, the compiler complains about the delegate not being declared. I must keep overlooking it somehow and make the same mistake. Here is the simple version of my problematic code:
//MYHandler.h
#protocol MYDelegate <NSObject>
-(void) myDelegateMethod;
#end
#interface MYHandler : NSObject
#property (strong, nonatomic) id<MYDelegate> delegate;
#end
//MYHandler.m
#implementation MYHandler
-(void) doSomething
{
//Create and configure request
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
operation.responseSerializer = [AFJSONResponseSerializer serializer];
[operation setCompletionBlockWithSuccess: successBlock failure:failureBlock];
[operation start];
}
//Success and Failure
typedef void(^SuccessBlock)(AFHTTPRequestOperation *operation, id responseObject);
typedef void(^FailureBlock)(AFHTTPRequestOperation *operation, NSError *error);
SuccessBlock successBlock = ^(AFHTTPRequestOperation *operation, id responseObject) {
[_delegate myDelegateMethod];
//^this says _delegate is an undeclared identifier
};
FailureBlock failureBlock = ^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(#"%#", error);
};
#end
//MyViewController.h
#interface MyViewController : UIViewController <MYDelegate>
#end
//MyViewController.m
-(void) myDelegateMethod
{
//
}

Answer 1

You are outside of any method. It's like you're trying to do this:
//MYHandler.m
#implementation MYHandler
-(void) doSomething
{
//Create and configure request
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
operation.responseSerializer = [AFJSONResponseSerializer serializer];
[operation setCompletionBlockWithSuccess: successBlock failure:failureBlock];
[operation start];
}
[_delegate myDelegateMethod];
#end
Clearly this won't work.
You could instead create methods that return those blocks:
#implementation MYHandler
typedef void(^SuccessBlock)(AFHTTPRequestOperation *operation, id responseObject);
typedef void(^FailureBlock)(AFHTTPRequestOperation *operation, NSError *error);
- (void(^)(AFHTTPRequestOperation *operation, id responseObject))success
{
return ^(AFHTTPRequestOperation *operation, id responseObject) {
[_delegate myDelegateMethod];
};
}
- (void(^)(AFHTTPRequestOperation *operation, NSError *error))failure
{
return ^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(#"%#", error);
};
}
-(void) doSomething
{
SuccessBlock successBlock = [self success];
FailureBlock failureBlock = [self failure];
//Create and configure request
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
operation.responseSerializer = [AFJSONResponseSerializer serializer];
[operation setCompletionBlockWithSuccess: successBlock failure:failureBlock];
[operation start];
}
#end

Answer 2

You are trying to access the delegate variable from outside the scope of the class. Try this:
//MYHandler.m
-(void) doSomething
{
//Success and Failure
typedef void(^SuccessBlock)(AFHTTPRequestOperation *operation, id responseObject);
typedef void(^FailureBlock)(AFHTTPRequestOperation *operation, NSError *error);
SuccessBlock successBlock = ^(AFHTTPRequestOperation *operation, id responseObject) {
[_delegate myDelegateMethod];
//^this says _delegate is an undeclared identifier
};
FailureBlock failureBlock = ^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(#"%#", error);
};
//Create and configure request
AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
operation.responseSerializer = [AFJSONResponseSerializer serializer];
[operation setCompletionBlockWithSuccess: successBlock failure:failureBlock];
[operation start];
}

Answer 3

In your case the parameter you call _delegate is not in the scope and can not be used at all. To solve this you might want to declare the block itself where this parameter is in the scope. For instance declare it in the method where you pass the block to the request method.
Since this procedure is actually more common then yours where you declare the block outside the class implementation I can assume there is a reason you moved it outside. If this is for some reason mandatory you could use a macro #define successBlock ^(AFHTTPRequestOperation... which will at compile time insert the code for you. Doing so will insert the scope where you use the block as well.
Why this happens is you need to think of block just the same as C/C++ functions, if you want to use the _delegate you also need to insert it as an input parameter next to AFHTTPRequestOperation *operation, id responseObject in one of your cases.
Why this actually works without an explicit parameter declaration is because the compiler already kind of does that for you but still, it does need that object to be in the scope.