How to detect browser's protocol handlers?

Question

I have created a custom URL protocol handler.

http://

mailto://

custom://

I have registered a WinForms application to respond accordingly. This all works great.

But I would like to be able to gracefully handle the case where the user doesn't have the custom URL protocol handler installed, yet.

In order to be able to do this I need to be able to detect the browser's registered protocol handlers, I would assume from JavaScript. But I have been unable to find a way to poll for the information. I am hoping to find a solution to this problem.

Thanks for any ideas you might be able to share.

user_6144 · Answer

This would be a very, very hacky way to do this... but would this work?

Put the link in as normal...
But attach an onclick handler to it, that sets a timer and adds an onblur handler for the window
(in theory) if the browser handles the link (application X) will load stealing the focus from the window...
If the onblur event fires, clear the timer...
Otherwise in 3-5seconds let your timeout fire... and notify the user "Hmm, looks like you don't have the Mega Uber Cool Application installed... would you like to install it now? (Ok) (Cancel)"

Far from bulletproof... but it might help?

user_186202 · Answer

What seams the most easy solution is to ask the user the first time.

Using a Javascript confirm dialog per example:

You need this software to be able to read this link. Did you install it ?

if yes: create a cookie to not ask next time; return false and the link applies
if false: window.location.href = '/downloadpage/'

user_200565 · Answer

I'm trying to do something similar and I just discovered a trick that works with Firefox. If you combine it with the trick for IE you can have one that works on both main browsers (I'm not sure if it works in Safari and I know it doesn't work in Chrome)

if (navigator.appName=="Microsoft Internet Explorer" && document.getElementById("testprotocollink").protocolLong=="Unknown Protocol") {
    alert("No handler registered");
} else {
    try {
        window.location = "custom://stuff";
    } catch(err) {
        if (err.toString().search("NS_ERROR_UNKNOWN_PROTOCOL") != -1) {
            alert("No handler registered");
        }
    }
}

In order for this to work you also need to have a hidden link somewhere on the page, like this:

testprotocollink

It's a bit hacky but it works. The Firefox version unfortunately still pops up the default alert that comes up when you try to visit a link with an unknown protocol, but it will run your code after the alert is dismissed.

user_84473 · Answer

This is not a trivial task; one option might be to use signed code, which you could leverage to access the registry and/or filesystem (please note that this is a very expensive option). There is also no unified API or specification for code signing, so you would be required to generate specific code for each target browser. A support nightmare.

Also, I know that Steam, the gaming content delivery system, doesn't seem to have this problem solved either.

How to detect browser's protocol handlers?

13 Answers