检查last.fm中是否存在key返回的json

I'm writing an iOS app that uses the last.fm web services to search for albums. The following code converts the returned json to an NSArray:

我正在编写一个使用last.fm网络服务搜索相册的iOS应用。以下代码将返回的json转换为NSArray：

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

This works fine when a search term returns a result, but the application crashes when no results are returned. This is because the key "album" doesn't exist in the json when no results are found.

当搜索项返回结果时，这可以正常工作，但是当没有返回结果时应用程序崩溃。这是因为当没有找到结果时，json中不存在关键“专辑”。

How to I get my application to check if the "album" key exists in the json?

如何让我的应用程序检查json中是否存在“album”键？

EDIT: Sample json returned when there are no results. Search string was 'asdfgasdfg'. Please note the character return under the albummatches key.

编辑：当没有结果时返回示例json。搜索字符串是'asdfgasdfg'。请注意相册匹配下的字符返回。

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;
};
}

EDIT 2: Sample json when results are returned:

编辑2：返回结果时的样本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;
};
}

3 个解决方案

#1

There are two ways you could go about this

有两种方法可以解决这个问题

Assuming that the "opensearch:totalResults" is in fact the correct results count then you could use this value to determine if you have results or not and then use

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您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"];
}
```

Check whether you have a string or an array before processing it

在处理之前检查是否有字符串或数组

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"];
}

#2

Check:

检查：

if([[json[@"result"][@"albummatches"] allKeys] containsObject:@"album"]==YES){

    //do your work here
}

#3

Someone posted a really good answer earlier, but it was then removed. It helped a lot with this though, and I modified it slightly to get the code below, which now works.

有人早些时候发布了一个非常好的答案，但后来被删除了。虽然它有很多帮助，但我稍微修改了它以获得下面的代码，现在可以使用了。

            NSDictionary *resultsDict = [json valueForKey:@"results"];
        if (resultsDict != nil) {
            //Checks to see if any results are returned
            NSDictionary *albumMatchesDict = [resultsDict valueForKey:@"albummatches"];
            NSString *albumMatchesString = [resultsDict valueForKey:@"albummatches"];
            NSCharacterSet *set = [NSCharacterSet whitespaceCharacterSet];
            albumMatchesString = [[albumMatchesString description] stringByTrimmingCharactersInSet:[NSCharacterSet newlineCharacterSet]];

            if ([[albumMatchesString stringByTrimmingCharactersInSet:set] length] != 0) {
                results = (NSArray *) [albumMatchesDict valueForKey:@"album"];
            }
        }

#1