iphone - 检查 last.fm 返回的 json 中是否存在 key

Question

我正在编写一个 iOS 应用程序，它使用 last.fm 网络服务来搜索专辑。以下代码将返回的 json 转换为 NSArray:

results = (NSArray *)[[[json valueForKey:@"results"] valueForKey:@"albummatches"] valueForKey:@"album"];

这在搜索词返回结果时工作正常，但在没有返回结果时应用程序崩溃。这是因为找不到结果时，json 中不存在键“album”。

如何让我的应用程序检查 json 中是否存在“专辑”键？

编辑:没有结果时返回示例 json。搜索字符串是“asdfgasdfg”。请注意 albumbatches 键下的字符返回。

2013-03-12 14:08:00.889 MyMusicLibrary[387:1a03] {
results =     {
    "@attr" =         {
        for = asdfgasdfg;
    };
    albummatches = "\n";
    "opensearch:Query" =         {
        "#text" = "";
        role = request;
        searchTerms = asdfgasdfg;
        startPage = 1;
    };
    "opensearch:itemsPerPage" = 50;
    "opensearch:startIndex" = 0;
    "opensearch:totalResults" = 0;
};
}

编辑 2:返回结果时的示例 json:

2013-03-12 14:19:42.769 MyMusicLibrary[880:1a03] {
results =     {
    "@attr" =         {
        for = coldplay;
    };
    albummatches =         {
        album =             (
                            {
                artist = Coldplay;
                id = 1416655;
                image =                     (
                                            {
                        "#text" = "http://userserve-ak.last.fm/serve/34s/85845017.png";
                        size = small;
                    },
                                            {
                        "#text" = "http://userserve-ak.last.fm/serve/64s/85845017.png";
                        size = medium;
                    },
                                            {
                        "#text" = "http://userserve-ak.last.fm/serve/126/85845017.png";
                        size = large;
                    },
                                            {
                        "#text" = "http://userserve-ak.last.fm/serve/300x300/85845017.png";
                        size = extralarge;
                    }
                );
                mbid = "c67be398-9417-4c22-bc13-d6158029ae21";
                name = "A Rush of Blood to the Head";
                streamable = 1;
                url = "http://www.last.fm/music/Coldplay/A+Rush+of+Blood+to+the+Head";
            },
                            {
                artist = Coldplay;
                id = 1984;
                image =                     (
                                            {
                        "#text" = "http://userserve-ak.last.fm/serve/34s/87203265.png";
                        size = small;
                    },
                                            {
                        "#text" = "http://userserve-ak.last.fm/serve/64s/87203265.png";
                        size = medium;
                    },
                                            {
                        "#text" = "http://userserve-ak.last.fm/serve/126/87203265.png";
                        size = large;
                    },
                                            {
                        "#text" = "http://userserve-ak.last.fm/serve/300x300/87203265.png";
                        size = extralarge;
                    }
                );
                mbid = "8e602038-c0f2-3c2d-9068-a1a3daca493d";
                name = Parachutes;
                streamable = 1;
                url = "http://www.last.fm/music/Coldplay/Parachutes";
            }
        );
    };
    "opensearch:Query" =         {
        "#text" = "";
        role = request;
        searchTerms = coldplay;
        startPage = 1;
    };
    "opensearch:itemsPerPage" = 2;
    "opensearch:startIndex" = 0;
    "opensearch:totalResults" = 375;
};
}

Answer 1

有两种方法可以解决这个问题

1. 假设 "opensearch:totalResults" 实际上是正确的结果计数，那么您可以使用此值来确定是否有结果，然后使用

   NSArray *results = nil;
   
   if ([[json valueForKeyPath:@"results.opensearch:totalResults"] intValue] > 0) {
     results = [json valueForKeyPath:@"results.albummatches.album"];
   }
   

2. 在处理之前检查你有一个字符串还是一个数组

   id albummatches = [json valueForKeyPath:@"results.albummatches"];
   
   // is this a dictionary or a string?
   // You can either ask if it implements the interface or ask what class it is. I prefer the first
   BOOL isDictionary = [albummatches respondsToSelector:@selector(objectForKey:)];
   
   // or
   BOOL isDictionary = [albummatches isKindOfClass:[NSDictionary class]];
   
   if (isDictionary) {
     results = [albummatches objectForKey:@"album"];
   }