How does Kanna handle multi-threaded or parallel scraping?

Kanna is a Swift library for XML and HTML parsing, often used in iOS and macOS development. It's a wrapper around the libxml2 library, designed to make it easy to navigate and search XML/HTML documents.

When it comes to multi-threaded or parallel scraping, Kanna itself does not inherently support or prevent multi-threading. Parallel processing in Swift is generally handled through Grand Central Dispatch (GCD) or OperationQueue, which are the primary APIs for concurrency in Swift.

When using Kanna in a multi-threaded environment, it's important to ensure that the Kanna objects are accessed in a thread-safe manner. Here are a few considerations:

1. Thread Safety: Make sure that each thread has its own instance of HTML or XMLDocument. Sharing the same Kanna document across threads without proper synchronization could lead to race conditions.

2. Grand Central Dispatch: Use DispatchQueue to perform parsing on different threads. GCD is a low-level API for managing concurrent code execution on multiple queues.

3. OperationQueue: Another option could be using OperationQueue along with Operation subclasses, which allows for more high-level abstraction over concurrency with the ability to set dependencies and manage operation completion.

Here is an example using DispatchQueue with Kanna to perform parallel scraping in Swift:

import Kanna

// Function to do some scraping work
func scrapeData(fromURL url: String) {
    // Fetch the HTML content from the URL
    if let htmlString = try? String(contentsOf: URL(string: url)!) {
        // Parse the HTML content with Kanna
        if let doc = try? HTML(html: htmlString, encoding: .utf8) {
            // Perform your scraping tasks with Kanna
            // ...
        }
    }
}

// Create a concurrent DispatchQueue
let scrapingQueue = DispatchQueue(label: "scraping.queue", attributes: .concurrent)

// URLs to scrape
let urlsToScrape = ["https://example.com/page1", "https://example.com/page2", "https://example.com/page3"]

// Dispatch scraping tasks to the queue in parallel
for url in urlsToScrape {
    scrapingQueue.async {
        scrapeData(fromURL: url)
    }
}

// If you need to wait for all tasks to complete, you can use a DispatchGroup
let group = DispatchGroup()

for url in urlsToScrape {
    group.enter()
    scrapingQueue.async {
        scrapeData(fromURL: url)
        group.leave()
    }
}

// Wait for all scraping tasks to complete
group.notify(queue: DispatchQueue.main) {
    print("All scraping tasks are completed.")
}

In the above example, each URL is scraped in a separate task dispatched to a concurrent queue. This allows for parallel scraping of the URLs. We also use a DispatchGroup to be notified when all the scraping tasks are completed.

Remember, when writing concurrent code, you must always ensure your code is thread-safe and doesn't access shared resources without proper synchronization mechanisms. With Kanna, it's usually safe as long as you are creating new instances of the parser for each thread and not mutating shared data without synchronization.