Go实例解析

Go语言包的加载顺序如图

可以通过如下实例详细了解

代码来源于《Go实战》
代码地址：https://github.com/goinaction/code

项目代码结构

程序架构

首先分析main文件
main.main.go

package main

import (
	"log"
	"os"

	_ "github.com/goinaction/code/chapter2/sample/matchers" // 对包做初始化操作，但是不使用包里的标志符。调用包里所有的init函数
	"github.com/goinaction/code/chapter2/sample/search"
)

// init is called prior to main.
func init() {
	// Change the device for logging to stdout.
	log.SetOutput(os.Stdout)
}

// main is the entry point for the program.
func main() {
	// Perform the search for the specified term.
	search.Run("president")
}

在执行main文件之前，会首先加载matchers、search包里的init函数和包级别的常量变量定义

加载matchers包的变量常量定义和init函数

type (
	item struct {
		XMLName     xml.Name `xml:"item"`
		PubDate     string   `xml:"pubDate"`
		Title       string   `xml:"title"`
		Description string   `xml:"description"`
		Link        string   `xml:"link"`
		GUID        string   `xml:"guid"`
		GeoRssPoint string   `xml:"georss:point"`
	}

	image struct {
		XMLName xml.Name `xml:"image"`
		URL     string   `xml:"url"`
		Title   string   `xml:"title"`
		Link    string   `xml:"link"`
	}

	channel struct {
		XMLName        xml.Name `xml:"channel"`
		Title          string   `xml:"title"`
		Description    string   `xml:"description"`
		Link           string   `xml:"link"`
		PubDate        string   `xml:"pubDate"`
		LastBuildDate  string   `xml:"lasteBuildDate"`
		TTL            string   `xml:"ttl"`
		Language       string   `xml:"language"`
		ManagingEditor string   `xml:"managingEditor"`
		WebMaster      string   `xml:"webMaster"`
		Image          image    `xml:"image"`
		Item           []item   `xml:"item"`
	}

	rssDocument struct {
		XMLName xml.Name `xml:"rss"`
		Channel channel  `xml:"channel"`
	}
)

type rssMatcher struct{}

func init() {
	var matcher rssMatcher
	search.Register("rss", matcher)
}

加载search包的常量变量定义和init函数

const dataFile = "data/data.json"

type Feed struct {
	Name string `json:"site"`
	URI  string `json:"link"`
	Type string `json:"type"`
}
type Result struct {
	Field   string
	Content string
}

type Matcher interface {
	Search(feed *Feed, searchTerm string) ([]*Result, error)
}
var matchers = make(map[string]Matcher)

type defaultMatcher struct{}

func init() {
	var matcher defaultMatcher
	Register("default", matcher)
}

在回到main函数，首先执行search.Run()的函数，

func Run(searchTerm string) {
	feeds, err := RetrieveFeeds()
	if err != nil {
		log.Fatal(err)
	}

	results := make(chan *Result)
	//添加线程等待
	var waitGroup sync.WaitGroup
	waitGroup.Add(len(feeds))

	for _, feed := range feeds {
		log.Printf("%v", *feed)
		matcher, exists := matchers[feed.Type]
		if !exists {
			matcher = matchers["default"]
		}

		//起协程是执行匹配
		go func(matcher Matcher, feed *Feed) {
			Match(matcher, feed, searchTerm, results)
			waitGroup.Done()
		}(matcher, feed)
	}

	go func() {
		waitGroup.Wait()
		close(results)
	}()
	Display(results)
}

在Run函数中，执行RetrieveFeeds（）函数

func RetrieveFeeds() ([]*Feed, error) {
	//打开配置文件
	file, err := os.Open(dataFile)
	if err != nil {
		return nil, err
	}

	defer file.Close()

	var feeds []*Feed
	//解析json格式到Feed结构体
	err = json.NewDecoder(file).Decode(&feeds)
	return feeds, err
}

执行匹配

func Match(matcher Matcher, feed *Feed, searchTerm string, results chan<- *Result) {
	searchResults, err := matcher.Search(feed, searchTerm)
	if err != nil {
		log.Println(err)
		return
	}

	//将匹配到的结果写入chan
	for _, result := range searchResults {
		results <- result
	}
}

执行搜索

func (m rssMatcher) Search(feed *search.Feed, searchTerm string) ([]*search.Result, error) {
	var results []*search.Result
	log.Printf("search Feed Type[%s] Site[%s] For URI[%s]\n", feed.Type, feed.Name, feed.URI)

	document, err := m.retrieve(feed)
	if err != nil {
		return nil, err
	}
	for _, channelItem := range document.Channel.Item {
		matched, err := regexp.MatchString(searchTerm, channelItem.Title)
		if err != nil {
			return nil, err
		}

		if matched {
			results = append(results, &search.Result{
				Field:   "Title",
				Content: channelItem.Title,
			})
		}

		matched, err = regexp.MatchString(searchTerm, channelItem.Description)
		if err != nil {
			return nil, err
		}
		if matched {
			results = append(results, &search.Result{
				Field:   "Description",
				Content: channelItem.Description,
			})
		}
	}
	return results, nil
}
func (m rssMatcher) retrieve(feed *search.Feed) (*rssDocument, error) {
	if feed.URI == "" {
		return nil, errors.New("NO rss feed uri provided")
	}

	//http获取uri返回
	resp, err := http.Get(feed.URI)
	if err != nil {
		return nil, err
	}

	defer resp.Body.Close()

	if resp.StatusCode != 200 {
		return nil, fmt.Errorf("HTTP Response Error %d\n", resp.StatusCode)
	}

	//将结果解析到xml格式的struct
	var document rssDocument
	err = xml.NewDecoder(resp.Body).Decode(&document)
	return &document, err
}

最后展示结构

func Display(results chan *Result) {
	//从chan中读取，会阻塞直到chan close
	for result := range results {
		log.Printf("%s:\n%s\n\n", result.Field, result.Content)
	}
}