json - 在 Flutter 上从 Youtube api 解码 Json

Question

我调用 Youtube API 并获取此 Json:

     "kind": "youtube#videoListResponse",
 "etag": "\"XI7nbFXulYBIpL0ayR_gDh3eu1k/s7-xmHXpuqQxYzDp_wxhm59K4LE\"",
 "pageInfo": {
  "totalResults": 1,
  "resultsPerPage": 1
 },
 "items": [
  {
   "kind": "youtube#video",
   "etag": "\"XI7nbFXulYBIpL0ayR_gDh3eu1k/pajQ7iBy-7A0V_owifxkw-Kbw-Y\"",
   "id": "7lCDEYXw3mM",
   "snippet": {
    "publishedAt": "2012-06-20T23:12:38.000Z",
    "channelId": "UC_x5XG1OV2P6uZZ5FSM9Ttw",
    "title": "Google I/O 101: Q&A On Using Google APIs",
    "description": "Antonio Fuentes speaks to us and takes questions on working with Google APIs and OAuth 2.0.",
    "thumbnails": {
     "default": {
      "url": "https://i.ytimg.com/vi/7lCDEYXw3mM/default.jpg",
      "width": 120,
      "height": 90
     },
     "medium": {
      "url": "https://i.ytimg.com/vi/7lCDEYXw3mM/mqdefault.jpg",
      "width": 320,
      "height": 180
     },
     "high": {
      "url": "https://i.ytimg.com/vi/7lCDEYXw3mM/hqdefault.jpg",
      "width": 480,
      "height": 360
     }

但现在我想解析它并只得到 3 个节点:

1. 职位
2. 描述
3. 默认缩略图的 url

确实我得到了 Json 响应并且可以在日志中看到它，但是每次尝试解析它时都失败了。

这是我的代码:

final response = await http.get(
    'https://www.googleapis.com/youtube/v3/videos?id=HEREGOESMYAPIKEY&part=snippet&id=T0Jqdjbed40');

final parsed = json.decode(response.body).cast<Map<String, dynamic>>();
String title  = parsed['items']['snippet']['title'];
print(title);
String description  = parsed['items']['snippet']['description'];
print(description);
String thumbnail  = parsed['items']['snippet']['thumbnails']['default']['url'];
print(thumbnail);

Answer 1

您尝试的操作不适用于 Dart，这不是 javascript。 Dart 有非常强大的类型系统，这很棒。您正在尝试为定义为 String 的变量赋值，但您定义的响应是动态的，因此 Dart 无法验证赋值。 items 也是数组，没有这样的关键 items->snippet。

执行此操作的正确方法是创建模型定义，它将处理反序列化并提供方便的方式来访问您感兴趣的属性。

class YoutubeResponse {

  String kind;
  String etag;
  String nextPageToken;

  String regionCode;
  List<Item> items;

  YoutubeResponse(
      {this.kind,
      this.etag,
      this.nextPageToken,
      this.regionCode,
      this.items});

  Map<String, dynamic> toJson() => {
        'kind': kind,
        'etag': etag,
        'nextPageToken': nextPageToken,
        'regionCode': regionCode,
        'items': items,
      };

  factory YoutubeResponse.fromJSON(Map<String, dynamic> YoutubeResponseJson) {

    var list = YoutubeResponseJson['items'] as List;
    List<Item> itemsList = list.map((i) => Item.fromJSON(i)).toList();

    return new YoutubeResponse(
        kind: YoutubeResponseJson['kind'],
        etag: YoutubeResponseJson['etag'],
        nextPageToken: YoutubeResponseJson['nextPageToken'],
        regionCode: YoutubeResponseJson['regionCode'],
        mPageInfo: pageInfo.fromJSON(YoutubeResponseJson['pageInfo']),
        items: itemsList);
  }

}

class Item {
  String kind;
  String etag;
  Id id;
  Snippet snippet;

  Item({this.kind, this.etag, this.id, this.snippet});

  Map<String, dynamic> toJson() => {
        'kind': kind,
        'etag': etag,
        'id': id,
        'snippet': snippet,
      };

  factory Item.fromJSON(Map<String, dynamic> ItemJson) {
    return Item(
      kind: ItemJson['kind'],
      etag: ItemJson['etag'],
      id: Id.fromJSON(ItemJson['id']),
      snippet: Snippet.fromJSON(ItemJson['snippet']),
    );
  }
}

class Snippet {
  String publishedAt;
  String channelId;
  String title;
  String description;
  Thumbnails thumbnails;
  String channelTitle;
  String liveBroadcastContent;

  Snippet(
      {this.publishedAt,
      this.channelId,
      this.title,
      this.description,
      this.thumbnails,
      this.channelTitle,
      this.liveBroadcastContent});


  Map<String, dynamic> toJson() => {
        'publishedAt': publishedAt,
        'channelId': channelId,
        'title': title,
        'description': description,
        'thumbnails': thumbnails,
        'channelTitle': channelTitle,
        'liveBroadcastContent': liveBroadcastContent,
      };

  factory Snippet.fromJSON(Map<String, dynamic> SnippetJson) {


    return Snippet(
      publishedAt: SnippetJson['publishedAt'],
      channelId: SnippetJson['channelId'],
      title: SnippetJson['title'],
      description: SnippetJson['description'],
      thumbnails:  Thumbnails.fromJSON(SnippetJson['thumbnails']) ,
      channelTitle: SnippetJson['channelTitle'],
      liveBroadcastContent: SnippetJson['liveBroadcastContent'],
    );
  }
}

class Medium {
  int height;
  int width;
  String url;

  Medium({this.height, this.width, this.url});

  Map<String, dynamic> toJson() => {
        'height': height,
        'width': width,
        'url': url,
      };

  factory Medium.fromJSON(Map<String, dynamic> MediumJson) {
    return Medium(
      height: MediumJson['height'],
      width: MediumJson['width'],
      url: MediumJson['url'],
    );
  }

}

class High {
  int height;
  int width;
  String url;

  High({this.height, this.width, this.url});

  Map<String, dynamic> toJson() => {
        'height': height,
        'width': width,
        'url': url,
      };

  factory High.fromJSON(Map<String, dynamic> HighJson) {
    return High(
      height: HighJson['height'],
      width: HighJson['width'],
      url: HighJson['url'],
    );
  }

}

class Default {
  int height;
  int width;
  String url;

  Default({this.height, this.width, this.url});

  Map<String, dynamic> toJson() => {
        'height': height,
        'width': width,
        'url': url,
      };

  factory Default.fromJSON(Map<String, dynamic> defaultJson) {
    return Default(
      height: defaultJson['height'],
      width: defaultJson['width'],
      url: defaultJson['url'],
    );
  }

}

class Thumbnails {
  Default mDefault;
  Medium medium;
  High high;

  Thumbnails({this.mDefault, this.medium, this.high});

  var data = JsonEncoder().convert("");

  Map<String, dynamic> toJson() => {
        'default': mDefault,
        'medium': medium,
        'high': high,
      };

  factory Thumbnails.fromJSON(Map<String, dynamic> ThumbnailsJson) {
    return Thumbnails(
      mDefault: Default.fromJSON(ThumbnailsJson['default']),
      medium: Medium.fromJSON(ThumbnailsJson['medium']),
      high: High.fromJSON(ThumbnailsJson['high']),
    );
  }
}

既然我们已经描述了 JSON 的格式，我们希望它很容易解析:

YoutubeResponse parsedResponse =
  YoutubeResponse.fromJSON(json.decode(response.body));

然后您可以通过 parsedResponse.items 访问项目，然后遍历它们并获取标题、描述等。