Implementing a method taking a block to use as callback

You can call a block like a regular function:

BOOL finished = ...;
if (completion) {
    completion(finished);
}

So that means implementing a complete block function using your example would look like this:

+ (void)myMethod:(UIView *)exampleView completion:(void (^)(BOOL finished))completion {
    if (completion) {
        completion(finished);
    }
}

I would highly recommend that you read up on Blocks to understand what is happening.

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