Objective-C block doesn't return and call it self in the block? - objective-c

Question

I'm learning to develop iOS applications and now I'm reading some Objective-C source code.
This is a method to get user profile.
+ (void)getProfile:(void (^)(NSString *message))completion {
NSDictionary *dic = #{#"module":#"profile"};
[[self defaultManager] POST:KBaseUrl parameters:dic success:^(AFHTTPRequestOperation *operation, id responseObject) {
if ([self jsonOKForResponseObject:responseObject] && [self checkLogin:responseObject]) {
[ProfileManager sharedInstance].rank = responseObject[#"Variables"][#"space"][#"group"][#"grouptitle"];
[ProfileManager sharedInstance].credit = responseObject[#"Variables"][#"space"][#"credits"];
[ProfileManager sharedInstance].gender = responseObject[#"Variables"][#"space"][#"gender"];
completion(nil);
} else {
completion(#"fail");
}
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
completion(#"fail");
}];
}
My question is about the completion block.
I suppose that the completion block returns void and receives an NSString parameter.
In the block, what does completion(nil) mean?
Does that mean the block completion calls it self and send nil as parameter?
Doesn't that conflict with the parameter's type NSString*?
I'm not quite familiar with block in ObjC. Can anyone give a hint?

Answer 1

Yes you are right. It calls itself and sends nil as a parameter and it doesn't conflict with the NSString parameter. You are just passing nil to the NSString param.
You can call the above method like:
[YourClass getProfile:^(NSString *message) {
//message will be nill if you pass completion(nil);
}];
So when you pass the nill in the completion block, the message in the above method call will be nil!
The completion block is to notify you that your method call is complete, and at this point you can let that method pass certain paramteres , and if we consider your method:
+ (void)getProfile:(void (^)(NSString *message))completion {
NSDictionary *dic = #{#"module":#"profile"};
[[self defaultManager] POST:KBaseUrl parameters:dic success:^(AFHTTPRequestOperation *operation, id responseObject) {
if ([self jsonOKForResponseObject:responseObject] && [self checkLogin:responseObject]) {
.....
completion(#"hey sucess");
} else {
completion(#"if loop failed");
}
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
completion(#"some error occured");
}];
}
and when you call the method getProfile:
[YourClass getProfile:^(NSString *message) {
//Execution will reach here when the method call for getPRofile is complete and you have a result which you just sent through the completion block as a parameter which is a string and is waiting for you to process.
//you can do more from here
if([message isEqualToString:#"hey success"]){
//do something
}
if([message isEqualToString:#"if loop failed"]){
//do something
}
if([message isEqualToString:#"some error occured"]){
//do something
}
}];
As per #rmaddy comment, iis always a good practice to use BOOL to indicate the status success or fail rather than depending on a string as string can get localized/changed. We shold use the string to get more description of the error.
So your block should be:
+ (void)getProfile:(void (^)(BOOL status,NSString *message))completion {
.....
completion(YES,#"hey success");
}
and you can call it like":
[YourClass getProfile:^(BOOL status, NSString *message) {
if(status){
//get the success message
}else{
//get the fail message
}
}];

Answer 2

Blocks had a lot of type like :-
As a local variable:
returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...};
As a property:
#property (nonatomic, copy, nullability) returnType (^blockName)(parameterTypes);
As a method parameter:
- (void)someMethodThatTakesABlock:(returnType (^nullability)(parameterTypes))blockName;
As an argument to a method call:
[someObject someMethodThatTakesABlock:^returnType (parameters) {...}];
As a typedef:
typedef returnType (^TypeName)(parameterTypes);
TypeName blockName = ^returnType(parameters) {...};
read more about it from here

Answer 3

Completion block does not return anything. It is just a piece of code to be executed, thats all. Though, you can give it some input where you call it so you can use the result elsewhere.
NSString *message is the input for your block so when you call your function getProfile as:
[MyClass getProfile:^(NSString *message) {
// write code to be executed when getProfile function finishes its job and sends message here.
}];
[MyClass getProfile:nil];
When used like this you're preferring not to do anything when getProfile finishes its job.
You are probably mixing your network manager's function's completion block with the one you wrote.