Check if object is a block - nice for assertions.

PSPDFBlockAssert.m

PSPDF_EXTERN BOOL PSPDFIsBlock(id _Nullable block) {
    static Class blockClass;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        blockClass = [^{} class];
        while ([blockClass superclass] != NSObject.class) {
            blockClass = [blockClass superclass];
        }
    });

    return [block isKindOfClass:blockClass];
}