ios - 如果我使用比较对对象进行排序，如果其中一个对象为零，我该如何告诉它不进行排序？

Question

假设我有以下代码块:

[allKeys sortUsingComparator:^NSComparisonResult(id obj1, id obj2) {
    NSDictionary *firstArticle = [articles objectForKey:(NSString *)obj1];
    NSNumber *firstSortID = [firstArticle objectForKey:@"sort_id"];

    NSDictionary *secondArticle = [articles objectForKey:(NSString *)obj2];
    NSNumber *secondSortID = [secondArticle objectForKey:@"sort_id"];

    if (firstSortID == nil || secondSortID == nil) {
        return nil;
    }
    else {
        return [secondSortID compare:firstSortID];
    }
}];

我收到警告:

Incompatible pointer to integer conversion returning 'void *' from a function with result type 'NSComparisonResult' (aka 'enum NSComparisonResult')

但基本上，如果其中之一为 nil，它就会崩溃，因为比较没有意义。我如何告诉它如果它为零，不要打扰，就停下来。

Answer 1

您必须决定如何对 nil 值进行排序。他们很可能应该放在列表的末尾。你想要这样的东西:

if (firstSortID) {
    if (secondSortID) {
        // Both set, compare
        NSComparisonResult result = [firstSortID compare:secondSortID];
        if (result == NSOrderedSame) {
            // optionally do another comparison on a secondary value
            // result = ...
        }
        return result;
    } else {
        // Have 1st but not 2nd
        return NSOrderedAscending;
    }
} else {
    if (secondSortID) {
       // Have 2nd but not 1st
       return NSOrderedDescending;
    } else {
       // Both nil, treat as the same
       // Another option here is to look at another value in the dictionary to be used as a secondary sort
       return NSOrderedSame;
    }
}