如何在Swift3中获取Json包的内容（unwrap Json package）

我想在这个帖子里面总结两个问题：一个是通过ReST获取Json的语句，这个比较简单

另一个是如何将Json包一层一层转存为字典，并最终通过keyword: String获得所需内容的方法

废话不说，来看代码

首先是服务器返回的Json形式，这里以youtube API为例，因为google做的Json包很规范（都是 String: String 或者 String: Int）

youtube API 的 请求 URL 形式（我这里仅获取关键字搜索视频列表）是

https://www.googleapis.com/youtube/v3/search?part=snippet&q=视频关键字&type=video&key=你的API Key

其中，视频关键字比如 minecraft

API Key 形如 AIzaSyDDqTGpVR7jxeozoOEjH6SLaRdw0YY-HPQ

其返回的Json包形式为

{
 "kind": "youtube#searchListResponse",
 "etag": ""uQc-MPTsstrHkQcRXL3IWLmeNsM/dJyYeiv8CbvQiayof_7MqRQOSi8"",
 "nextPageToken": "CAUQAA",
 "regionCode": "AU",
 "pageInfo": {
  "totalResults": 1000000,
  "resultsPerPage": 5
 },
 "items": [
  {
   "kind": "youtube#searchResult",
   "etag": ""uQc-MPTsstrHkQcRXL3IWLmeNsM/ey6kUbVRRdmcLIPpklkbtxDp_-o"",
   "id": {
    "kind": "youtube#video",
    "videoId": "XgdgO5UzdR4"
   },
   "snippet": {
    "publishedAt": "2016-11-27T12:00:01.000Z",
    "channelId": "UCUVa51UA_690sEKyRbHb-5A",
    "title": "5 Werewolves Caught on Camera & Spotted In Real Life!",
    "description": "5 Werewolves Caught on Tape & Spotted In Real Life! Description: Myths and stories of humans with the ability to transform into animals exist across all human ...",
    "thumbnails": {
     "default": {
      "url": "https://i.ytimg.com/vi/XgdgO5UzdR4/default.jpg",
      "width": 120,
      "height": 90
     },
     "medium": {
      "url": "https://i.ytimg.com/vi/XgdgO5UzdR4/mqdefault.jpg",
      "width": 320,
      "height": 180
     },
     "high": {
      "url": "https://i.ytimg.com/vi/XgdgO5UzdR4/hqdefault.jpg",
      "width": 480,
      "height": 360
     }
    },
    "channelTitle": "Top 5s Finest",
    "liveBroadcastContent": "none"
   }
  },
  {
   "kind": "youtube#searchResult",
   "etag": ""uQc-MPTsstrHkQcRXL3IWLmeNsM/LKJrjJEelN2h1v58M533Dwv_fjs"",
   "id": {
    "kind": "youtube#video",
    "videoId": "KUS0454UUDM"
   },
   "snippet": {
    "publishedAt": "2016-08-03T10:16:33.000Z",
    "channelId": "UC0rzsIrAxF4kCsALP6J2EsA",
    "title": "MAIN-MAIN: WEREWOLF",
    "description": "Yang mau liat tutorial cara mainnya, klik di sini: https://youtu.be/ShGULqRjLMk.",
    "thumbnails": {
     "default": {
      "url": "https://i.ytimg.com/vi/KUS0454UUDM/default.jpg",
      "width": 120,
      "height": 90
     },
     "medium": {
      "url": "https://i.ytimg.com/vi/KUS0454UUDM/mqdefault.jpg",
      "width": 320,
      "height": 180
     },
     "high": {
      "url": "https://i.ytimg.com/vi/KUS0454UUDM/hqdefault.jpg",
      "width": 480,
      "height": 360
     }
    },
    "channelTitle": "Raditya Dika",
    "liveBroadcastContent": "none"
   }
  },
  {
   "kind": "youtube#searchResult",
   "etag": ""uQc-MPTsstrHkQcRXL3IWLmeNsM/8qIl76CAeB__wn-Sd_g4pik1XT0"",
   "id": {
    "kind": "youtube#video",
    "videoId": "fScsIQ8wgNo"
   },
   "snippet": {
    "publishedAt": "2016-10-02T22:21:35.000Z",
    "channelId": "UCNUohh38MTkgY4xKpFeVGmQ",
    "title": "Werewolf Scenes - My Favorite Transformation HD",
    "description": "Werewolf Scenes - My Favorite Transformation HD.",
    "thumbnails": {
     "default": {
      "url": "https://i.ytimg.com/vi/fScsIQ8wgNo/default.jpg",
      "width": 120,
      "height": 90
     },
     "medium": {
      "url": "https://i.ytimg.com/vi/fScsIQ8wgNo/mqdefault.jpg",
      "width": 320,
      "height": 180
     },
     "high": {
      "url": "https://i.ytimg.com/vi/fScsIQ8wgNo/hqdefault.jpg",
      "width": 480,
      "height": 360
     }
    },
    "channelTitle": "ENJOY PLEASE",
    "liveBroadcastContent": "none"
   }
  },
  {
   "kind": "youtube#searchResult",
   "etag": ""uQc-MPTsstrHkQcRXL3IWLmeNsM/SByO_v4iInfFZ5GUe2xJUYXIWh4"",
   "id": {
    "kind": "youtube#video",
    "videoId": "ShGULqRjLMk"
   },
   "snippet": {
    "publishedAt": "2016-08-03T10:13:33.000Z",
    "channelId": "UC0rzsIrAxF4kCsALP6J2EsA",
    "title": "TUTORIAL: MAIN WEREWOLF",
    "description": "Ini adalah video tutorial untuk main werewolf. Untuk melihat video permainannya, bisa klik di sini: https://youtu.be/KUS0454UUDM.",
    "thumbnails": {
     "default": {
      "url": "https://i.ytimg.com/vi/ShGULqRjLMk/default.jpg",
      "width": 120,
      "height": 90
     },
     "medium": {
      "url": "https://i.ytimg.com/vi/ShGULqRjLMk/mqdefault.jpg",
      "width": 320,
      "height": 180
     },
     "high": {
      "url": "https://i.ytimg.com/vi/ShGULqRjLMk/hqdefault.jpg",
      "width": 480,
      "height": 360
     }
    },
    "channelTitle": "Raditya Dika",
    "liveBroadcastContent": "none"
   }
  },
  {
   "kind": "youtube#searchResult",
   "etag": ""uQc-MPTsstrHkQcRXL3IWLmeNsM/NjscIZ4D3C-FDPsnBKe-j5tslrE"",
   "id": {
    "kind": "youtube#video",
    "videoId": "MI1Yi2AUVAc"
   },
   "snippet": {
    "publishedAt": "2016-09-09T07:27:23.000Z",
    "channelId": "UCNUohh38MTkgY4xKpFeVGmQ",
    "title": "Vampire vs Werewolf Fight Scene HD - Van Helsing - Vampire vs Lycan Wolf",
    "description": "Vampire vs Werewolf Fight Scene HD - Van Helsing - Vampire vs Lycan Wolf.",
    "thumbnails": {
     "default": {
      "url": "https://i.ytimg.com/vi/MI1Yi2AUVAc/default.jpg",
      "width": 120,
      "height": 90
     },
     "medium": {
      "url": "https://i.ytimg.com/vi/MI1Yi2AUVAc/mqdefault.jpg",
      "width": 320,
      "height": 180
     },
     "high": {
      "url": "https://i.ytimg.com/vi/MI1Yi2AUVAc/hqdefault.jpg",
      "width": 480,
      "height": 360
     }
    },
    "channelTitle": "ENJOY PLEASE",
    "liveBroadcastContent": "none"
   }
  }
 ]
}
---------------------------------------------------------------------------------------------------------------------
第 0 层 
{  }
第一层
{  
   "kind":"youtube#searchListResponse",
   "etag":""uQc-MPTsstrHkQcRXL3IWLmeNsM/g4lxZZLl2WXJPiISOtol-Vf33d8"",
   "nextPageToken":"CAUQAA",
   "regionCode":"US",
   "pageInfo":{  },
   "items":[  ]
}
第二层
对于 pageInfo 
{  
      "totalResults":1000000,
      "resultsPerPage":5
}
对于 items （注意那个中括号）
[  [
      {  
         "kind":"youtube#searchResult",
         "etag":""uQc-MPTsstrHkQcRXL3IWLmeNsM/ey6kUbVRRdmcLIPpklkbtxDp_-o"",
         "id":{  },
         "snippet":{  }
      },
      {  
         "kind":"youtube#searchResult",
         "etag":""uQc-MPTsstrHkQcRXL3IWLmeNsM/LKJrjJEelN2h1v58M533Dwv_fjs"",
         "id":{  },
         "snippet":{  }
      },
      {  
         "kind":"youtube#searchResult",
         "etag":""uQc-MPTsstrHkQcRXL3IWLmeNsM/8qIl76CAeB__wn-Sd_g4pik1XT0"",
         "id":{  },
         "snippet":{  }
      },
      {  
         "kind":"youtube#searchResult",
         "etag":""uQc-MPTsstrHkQcRXL3IWLmeNsM/SByO_v4iInfFZ5GUe2xJUYXIWh4"",
         "id":{  },
         "snippet":{  }
      },
      {  
         "kind":"youtube#searchResult",
         "etag":""uQc-MPTsstrHkQcRXL3IWLmeNsM/NjscIZ4D3C-FDPsnBKe-j5tslrE"",
         "id":{  },
         "snippet":{  }
      }
   ]
这就是说，中括号里面的内容实际上是一个数组，这个数组有五个元素。

第三层
以其中一个元素为例
对于 id
{  
            "kind":"youtube#video",
            "videoId":"XgdgO5UzdR4"
}
对于 snippet
{  
            "publishedAt":"2016-11-27T12:00:01.000Z",
            "channelId":"UCUVa51UA_690sEKyRbHb-5A",
            "title":"5 Werewolves Caught on Camera & Spotted In Real Life!",
            "description":"5 Werewolves Caught on Tape & Spotted In Real Life! Description: Myths and stories of humans with the ability to transform into animals exist across all human ...",
            "thumbnails":{  },
            "channelTitle":"Top 5s Finest",
            "liveBroadcastContent":"none"
 }
第四层
对于 thumbnails
{  
               "default":{  },
               "medium":{  },
               "high":{  }
}
第五层
对于 default 
{  
                  "url":"https://i.ytimg.com/vi/XgdgO5UzdR4/default.jpg",
                  "width":120,
                  "height":90
}
其它两个类似

至此Json 包解析完毕。
然后附上代码。首先是ReST请求部分

然后是Json解析部分

只解释Json解析部分。
我现在的目的是取出四个key值：视频的title、description、thumbnail default url 还有 videoId
过程是这样：
根据我上面的分析，从第一层开始（第 0 层不算）每层都建立[String: AnyObject]字典或数组（根据Json那一层的结构而定）。在最后一层只有String内容的时候，也可以建立[String: String]字典（也就是idDict也可以建成[String: String]的，别的不可以。）
第一层 - resultDict 这个不能建成[String: String]，因为pageInfo和items关键字对应的不是String，而是一个或多个子字典。
第二层 - 因为pageInfo里面的内容我不需要（如需要建字典），所以只有items对应的数组 -- itemsArray  （建数组因为items本身是五个元素组成的数组，在前面已经说明。）
第三层 - 对于 id -- idDict字典，这个可以建[String: AnyObject]也可以建[String: String](因为内容只有String，建AnyObject的话，就需要cast一下(as! String)。)
对于 snippet -- snippetDict字典，这个必须[String: AnyObject]
第四层 - thumbnailDict
第五层 - defaultDict，其它两个Key里面的信息不需要，如果需要，也可以建相应字典提取出来。
然后我建了一个每个元素都是[String: AnyObject]字典的Array，形如[[String: AnyObject]]，用来储存我提取出来的四个keyword对应的信息。当然，这个数组建成[[String: String]]也是可以的。当时对这个代码理解还不深，属于半迷糊状态。

然后就大功告成。