如何使用Golang解码Reddit的RSS？ [英] How to decode Reddit's RSS using Golang?

问题描述

我一直在使用Go的XML包，因此看不到以下代码有什么问题。

I've been playing about with Go's XML package and cannot see what is wrong with the following code.

package main

import (
    "encoding/xml"
    "fmt"
    "net/http"
) 

type Channel struct {
    Items Item
}

type Item struct {
    Title       string `xml:"title"`
    Link        string `xml:"link"`
    Description string `xml:"description"`
}

func main() {

    var items = new(Channel)
    res, err := http.Get("http://www.reddit.com/r/google.xml")

    if err != nil {
        fmt.Printf("Error: %v\n", err)
    } else {
        decoded := xml.NewDecoder(res.Body)

        err = decoded.Decode(items)

        if err != nil {
            fmt.Printf("Error: %v\n", err)
        }

        fmt.Printf("Title: %s\n", items.Items.Title)
    }
}

上面的代码运行无误，并打印到终端：

The above code runs without any errors and prints to the terminal:

Title:

该结构似乎是空的，但我看不到为什么它没有填充XML数据。

The struct seems empty but I can't see why it isn't getting populated with the XML data.

推荐答案

您的程序已接近完成，但需要指定更多上下文来匹配XML文档。

Your program comes close, but needs to specify just a little bit more context to match the XML document.

您需要修改字段标签，以帮助引导XML绑定通过
Channel 结构传递到 Item 结构：

You need to revise your field tags to help guide the XML binding down through your Channel structure to your Item structure:

type Channel struct {
    Items []Item `xml:"channel>item"`
}

type Item struct {
    Title       string `xml:"title"`
    Link        string `xml:"link"`
    Description string `xml:"description"`
}

每个 encoding / xml.Unmarshal（） ，此处适用第七个项目符号：

Per the documentation for encoding/xml.Unmarshal(), the seventh bullet item applies here:

如果XML元素包含名称与$ b $相匹配的子元素b标记格式为 a或 a> b> c的前缀，解组
将XML结构查找具有给定名称
的元素，并将最里面的元素映射到该结构
字段。以>开头的标记等效于以字段名开头的
，后跟>。

If the XML element contains a sub-element whose name matches the prefix of a tag formatted as "a" or "a>b>c", unmarshal will descend into the XML structure looking for elements with the given names, and will map the innermost elements to that struct field. A tag starting with ">" is equivalent to one starting with the field name followed by ">".

您的情况下，您希望遍历顶级< rss> 元素的< channel> 元素查找每个< item> 元素。不过请注意，我们实际上不需要-指定 Channel 结构应遍历顶级 < rss> 元素，方法是将 Items 字段的标签写为

In your case, you're looking to descend through the top-level <rss> element's <channel> elements to find each <item> element. Note, though, that we don't need to—an in fact can't—specify that the Channel struct should burrow through the top-level <rss> element by writing the Items field's tag as

`xml:"rss>channel>item"`

该上下文是隐式的;提供给 Unmarshall（）的结构已经映射到顶级XML元素。

That context is implicit; the struct supplied to Unmarshall() already maps to the top-level XML element.

请注意，您的 Channel 结构的 Items 字段应为slice_of- Item ，而不仅仅是单个 Item 。

Note too that your Channel struct's Items field should be of type slice-of-Item, not just a single Item.

您提到您很难使该建议生效。这是一个完整的清单，我发现它可以像人们期望的那样工作：

You mentioned that you're having trouble getting the proposal to work. Here's a complete listing that I find works as one would expect:

package main

import (
    "encoding/xml"
    "fmt"
    "net/http"
    "os"
) 

type Channel struct {
    Items []Item `xml:"channel>item"`
}

type Item struct {
    Title       string `xml:"title"`
    Link        string `xml:"link"`
    Description string `xml:"description"`
}

func main() {
    if res, err := http.Get("http://www.reddit.com/r/google.xml"); err != nil {
        fmt.Println("Error retrieving resource:", err)
        os.Exit(1)
    } else {
        channel := Channel{}
        if err := xml.NewDecoder(res.Body).Decode(&channel); err != nil {
            fmt.Println("Error:", err)
            os.Exit(1)
        } else if len(channel.Items) != 0 {
            item := channel.Items[0]
            fmt.Println("First title:", item.Title)
            fmt.Println("First link:", item.Link)
            fmt.Println("First description:", item.Description)
        }
    }
}

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