Javascript console.log() in an iOS UIWebView

Question

When writing a iPhone / iPad app with a UIWebView, the console isn\'t visible. this excellent answer shows how to trap errors, but I would like to use the console.log() as w

Answer 1

Tried Leslie Godwin's fix but was getting this error:

'objectForKeyedSubscript' is unavailable: use subscripting

For Swift 2.2, here's what worked for me:

You will need to import JavaScriptCore for this code to compile:

import JavaScriptCore

if let context = webView.valueForKeyPath("documentView.webView.mainFrame.javaScriptContext") {
    context.evaluateScript("var console = { log: function(message) { _consoleLog(message) } }")
    let consoleLog: @convention(block) String -> Void = { message in
        print("javascript_log: " + message)
    }
    context.setObject(unsafeBitCast(consoleLog, AnyObject.self), forKeyedSubscript: "_consoleLog")
}

Then in your javascript code, calling console.log("_your_log_") will print in Xcode console.

Better yet, add this code as an extension to UIWebView:

import JavaScriptCore

extension UIWebView {
    public func hijackConsoleLog() {
        if let context = valueForKeyPath("documentView.webView.mainFrame.javaScriptContext") {
            context.evaluateScript("var console = { log: function(message) { _consoleLog(message) } }")
            let consoleLog: @convention(block) String -> Void = { message in
                print("javascript_log: " + message)
            }
            context.setObject(unsafeBitCast(consoleLog, AnyObject.self), forKeyedSubscript: "_consoleLog")
        }
    }
}

And then call this method during your UIWebView initialization step:

let webView = UIWebView(frame: CGRectZero)
webView.hijackConsoleLog()