How to use WKWebView in a command line app

Question

I'm trying to write a command line tool that takes a screenshot of a given webpage using WKWebView. The problem is that WKNavigationDelegate methods aren't being called. This is what I have:

import WebKit

class Main: NSObject {
    let webView: WKWebView = WKWebView()
    func load(request: URLRequest) {
        webView.navigationDelegate = self
        webView.load(request)
    }
}

extension Main: WKNavigationDelegate {
    func webView(_ webView: WKWebView, didStartProvisionalNavigation navigation: WKNavigation!) {
        print("Did start")
    }
    func webView(_ webView: WKWebView, didCommit navigation: WKNavigation!) {
        print("Did commit")
    }
    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        print("Did finish")
    }
    func webView(_ webView: WKWebView, didFail navigation: WKNavigation!, withError error: Error) {
        print("Did fail")
    }
}

let main: Main = Main()
let input: String = CommandLine.arguments[1]
if let url: URL = URL(string: input) {
    let request: URLRequest = URLRequest(url: url)
    main.load(request: request)
} else {
    print("Invalid URL")
}

Almost all examples I've found involve using WKWebView in a view controller. My guess is that in the command line, the app exits before the webpage finishes loading, but I'm not sure how to prevent that from happening.

I did find this example of a command line tool using WKWebView. The author uses RunLoop.main.run(), which to my understanding effectively simulates the event loop of a UI app? That allows the webpage to load, but I'm looking for a different solution because I want the app to behave like a normal command line tool and exit on its own after running. For example, is there some way to use async/await with WKWebView.load() much like with URLSession?

You can use something like that https://stackoverflow.com/a/28591237/1801544 The idea, is to indeed use the `RunLoop` , but also stops when the work is done. In the `webView(_:didFinish:)` or `webView(_:didFail:withError:)` changing the "stop condition". — Larme, Dec 01 '22 at 09:37
@Larme thank you, I only saw this after figuring out how to do it with continuations as detailed in [my answer](https://stackoverflow.com/a/75446788/17096282). This would also have worked! — mingwei, Feb 14 '23 at 10:54

mingwei · Accepted Answer · 2023-02-14T11:00:51.933

I ended up solving this problem using continuation. In short, I wrap webView.load() in a continuation and then call continuation.resume() in one of the WKNavigationDelegate methods. That allows me to treat webView.load() as an async task; the continuation determines its runtime. I took this solution entirely from the example in this blog post.

Here's a barebones implementation of this solution:

import WebKit

@MainActor
class WebContainer: NSObject {
    
    lazy var webView: WKWebView = {
        let webView: WKWebView = WKWebView()
        webView.navigationDelegate = self
        return webView
    }()
        
    var continuation: UnsafeContinuation<Void, Error>?
    
    func load(request: URLRequest) async throws -> Int {
        try await withUnsafeThrowingContinuation { continuation in
            self.continuation = continuation
            webView.load(request)
        }
    }
}

extension WebContainer: WKNavigationDelegate {

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        continuation?.resume(returning: ())
    }
    
    func webView(_ webView: WKWebView, didFail navigation: WKNavigation!, withError error: Error) {
        continuation?.resume(throwing: error)
    }

    func webView(_ webView: WKWebView, didFailProvisionalNavigation navigation: WKNavigation!, withError error: Error) {
        continuation?.resume(throwing: error)
    }
}

To load a website using the above implementation, simply run:

let webContainer: WebContainer = WebContainer()
let request: URLRequest = // insert URLRequest here
try await webContainer.load(request: request)

Practically speaking this implementation doesn't handle redirects reliably as some redirects are initiated after didFinish is called. I have a partial solution to that which however runs into other problems. Since all this is out of scope for this question, if anyone's interested please refer to this other question.

How to use WKWebView in a command line app

1 Answers1