Objective-C block as return type

Question

When I check out the src of Underscore project, then I found that the block somehow comprehensive for me:

typedef BOOL (^UnderscoreTestBlock)(id obj);

In the class, it define this class method:

+ (UnderscoreTestBlock (^)(UnderscoreTestBlock))negate
{
    return ^UnderscoreTestBlock (UnderscoreTestBlock test) {
        return ^BOOL (id obj) {
            return !test(obj);
        };
    };
}

and how can I understand these code, Thanks!

Answer 1

I think that you got it right. negate is a class method which returns a block. That returned block takes an UnderscoreTestBlock argument and returns another UnderscoreTestBlock. Example:

UnderscoreTestBlock isANumber = ^BOOL (id obj) {
    return [obj isKindOfClass:[NSNumber class]];
};
BOOL b1 = isANumber(@123);       // YES
BOOL b2 = isANumber(@"abc");     // NO

UnderscoreTestBlock isNotANumber = [Underscore negate](isANumber);
BOOL b3 = isNotANumber(@123);   // NO
BOOL b4 = isNotANumber(@"abc"); // YES

[Underscore negate] returns a block that is called with the isANumber argument. The return value of that call is the block

^BOOL (id obj) { return !test(obj); };

where test is the isANumber argument, which is captured by the block.

---

I am sure that there are good reasons why the Underscore project defines the negate method like this, it is probably necessary for their "functional" notation. Otherwise a slightly simpler way would be to define a method

+ (UnderscoreTestBlock)negateBlock:(UnderscoreTestBlock)test
{
    return ^BOOL (id obj) {
        return !test(obj);
    };
}

which would be used like

UnderscoreTestBlock isNotANumber = [Underscore negateBlock:isANumber];