Custom completion block for my own method

Question

I have just discovered completion blocks:

 completion:^(BOOL finished){


                     }];

What do I need to do to have my own method take a completion block?

https://stackoverflow.com/a/62671217/1023074 – Sunil Targe Jul 01 '20 at 06:55 — Sunil Targe, Jul 01 '20 at 06:55

score 234 · Accepted Answer · answered May 01 '13 at 18:36

234

1) Define your own completion block,

typedef void(^myCompletion)(BOOL);

2) Create a method which takes your completion block as a parameter,

-(void) myMethod:(myCompletion) compblock{
    //do stuff
    compblock(YES);
}

3)This is how you use it,

[self myMethod:^(BOOL finished) {
    if(finished){
        NSLog(@"success");
    }
}];

enter image description here

answered May 01 '13 at 18:36

Thilina Chamath Hewagama

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compclock(YES) is the same as return compclock = YES ? – user2206906 May 01 '13 at 18:52
Yes, You got it, we can put compclock(YES)/compclock(NO) depending on the requirement, we can return NO for failure completions here – Thilina Chamath Hewagama May 01 '13 at 18:55
track the returned value: if(finished) NSLog(@"success");else NSLog(@"failure"); – Thilina Chamath Hewagama May 01 '13 at 18:58
2

If you're a C/C++ programmer, then I find it very easy to remember the syntax of block declaration using a pointer to function syntax. The only thing you need to do is replace "*" in C/C++ function-pointer with "^" ...thought it might help someone in future :) – Viren Mar 18 '14 at 04:20
For more than one parameter in the block, use a comma to separate. E.g. `completion:(void (^)(BOOL success, NSString* videoPath))` – Stan James Oct 27 '14 at 21:36
What if my method is prepareForSegue ?!?! any possible way to use it for that? Like if user wants to go to the next page and I need to show waiting animation once the prepareForSegue starts preparing and do compBlock(YES) inside prepareForSegue method? – Reza.Ab Feb 16 '18 at 23:48

score 25 · Answer 2 · answered May 01 '13 at 18:36

You define the block as a custom type:

typedef void (^ButtonCompletionBlock)(int buttonIndex);

Then use it as an argument to a method:

+ (SomeButtonView*)buttonViewWithTitle:(NSString *)title 
                          cancelAction:(ButtonCompletionBlock)cancelBlock
                      completionAction:(ButtonCompletionBlock)completionBlock

When calling this in code it is just like any other block:

[SomeButtonView buttonViewWithTitle:@"Title"
                       cancelAction:^(int buttonIndex) {
                             NSLog(@"User cancelled");
                   } 
                     completionAction:^(int buttonIndex) {
                             NSLog(@"User tapped index %i", buttonIndex);
                   }];

If it comes time to trigger the block, simply call completionBlock() (where completionBlock is the name of your local copy of the block).

score 2 · Answer 3 · edited May 23 '17 at 12:18

2

Block variables are similar in syntax to function pointers in C.

Because the syntax is ugly they are often typedefed, however they can also be declared normally.

typedef void (^MyFunc)(BOOL finished);

- (void)myMethod:(MyFunc)func
{
}

See this answer for non typedef:

Declare a block method parameter without using a typedef

edited May 23 '17 at 12:18

Community

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1

answered May 01 '13 at 18:34

Justin Meiners

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50
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ok thanks i'll check it out, it is a little bit complicated to me where do i have the typedef ? – user2206906 May 01 '13 at 18:38
1

You can put the typedef in the header file, before the `@interface`. – Chris Nolet Jun 07 '15 at 22:07

Custom completion block for my own method

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