使用Decodable返回的空模型解析JSON

我正在尝试加载本地JSON文件并使用符合Decodable协议的模型进行解析。

JSON文件：

[
{
    "body": {},
    "header": {
        "returnCode": "200",
        "returnMessage": "Successfully Received",
    }
}
]

响应消息模型：

struct ResponseMessage: Decodable {

    struct header: Decodable {
        let returnCode: String
        let returnMessage: String
    }
}

模拟API的实现：

let url = Bundle.main.url(forResource: "MockJSONData", withExtension: "json")!
            do {
                let data = try Data(contentsOf: url)
                let teams = try JSONDecoder().decode(ResponseMessage.self, from: data)
                print(teams)
            } catch {
                print(error)
            }

但是响应消息为此返回空数据。

感谢您的帮助和建议！

谢谢

更新ResponseMessage和Header类型如下，

struct ResponseMessage: Decodable {
    var header: Header
}


struct Header: Decodable {
    let returnCode: String
    let returnMessage: String
}

而decode像这样的，

do {
    let data = try Data(contentsOf: url)
    let teams = try JSONDecoder().decode([ResponseMessage].self, from: data)
    print(teams.first!.header.returnMessage)
} catch {
    print(error)
}