Can I implement custom logic for visiting links in Colly?

Yes, you can implement custom logic for visiting links in Colly. Colly is a flexible web scraping framework for Go, which allows developers to customize many aspects of their web scraping tasks, including how and which links are followed during the scraping process.

To implement custom logic for visiting links, you can use the OnHTML callback function to selectively determine which links to visit based on your specific criteria. You can parse the HTML of a page, inspect the links, and then use the Request.Visit method to visit only the links that match your requirements.

Here's a basic example in Go to illustrate how you can implement custom logic for following links with Colly:

package main

import (
    "fmt"
    "log"
    "net/url"

    "github.com/gocolly/colly/v2"
)

func main() {
    // Initialize the collector
    c := colly.NewCollector()

    // OnHTML callback with custom logic for visiting links
    c.OnHTML("a[href]", func(e *colly.HTMLElement) {
        // Extract the link URL
        link := e.Attr("href")

        // Parse the URL (to handle relative URLs and such)
        parsedLink, err := url.Parse(link)
        if err != nil {
            log.Printf("Error parsing URL: %s", err)
            return
        }

        // Implement your custom logic here. For example, visit only if the link contains "example"
        if parsedLink.Host == "example.com" {
            absoluteURL := e.Request.AbsoluteURL(link)
            fmt.Printf("Visiting: %s\n", absoluteURL)
            e.Request.Visit(absoluteURL)
        }
    })

    // Start scraping on an example page
    c.Visit("http://example.com")
}

In this example, the OnHTML function is used to look for all a elements with an href attribute (all links). For each link, it parses the URL and applies a custom logic where it only visits the link if the host is example.com. You could extend this logic to check for other attributes, such as the presence of certain words in the URL path or query parameters.

Remember to handle relative and absolute URLs correctly. The e.Request.AbsoluteURL function is used to convert a relative URL to an absolute one, ensuring that the Visit method receives a proper URL.

You can also use colly.URLFilters to define regular expressions that URLs must match before being visited or the AllowedDomains attribute to restrict the domains that the collector can visit. However, for more complex logic, using the OnHTML callback as shown allows for greater flexibility and custom behavior.