如何检测浏览器的协议处理程序？

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?

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.

You say you need to detect the browser's protocol handlers - do you really?

What if you did something like what happens when you download a file from sourceforge? Let's say you want to open myapp://something. Instead of simply creating a link to it, create a link to another HTML page accessed via HTTP. Then, on that page, say that you're attempting to open the application for them. If it doesn't work, they need to install your application, which they can do by clicking on the link you'll provide. If it does work, then you're all set.

There seems to be no straightforward way via javascript to detect the presence of an installed app that has registered a protocol handler.

In the iTunes model, Apple provides urls to their servers, which then provide pages that run some javascript:

http://ax.itunes.apple.com/detection/itmsCheck.js

So the iTunes installer apparently deploys plugins for the major browsers, whose presence can then be detected.

If your plugin is installed, then you can be reasonably sure that redirecting to your app-specific url will succeed.

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/'

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:

<a id="testprotocollink" href="custom://testprotocol" style="display: none;">testprotocollink</a>

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.

You can try something like this:

function OpenCustomLink(link) {


var w = window.open(link, 'xyz', 'status=0,toolbar=0,menubar=0,height=0,width=0,top=-10,left=-10');
if(w == null) {
//Work Fine
}
else {
w.close();
if (confirm('You Need a Custom Program. Do you want to install?')) {
window.location = 'SetupCustomProtocol.exe'; //URL for installer
}
}
}

This was a recommended approach for IE by Microsoft support

http://msdn.microsoft.com/en-us/library/ms537503%28VS.85%29.aspx#related_topics

"If you have some control over the binaries being installed on a user’s machine, checking the UA in script seems like a relevant approach: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform " -- By M$ support

Every web page has access to the userAgent string and if you drop a custom post platform value, detecting this in javascript using navigator.userAgent is quite simple.

Fortunately, other major browsers like Firefox and Chrome (barring Safari :( ), do not throw "page not found" errors when a link with a custom protocol is clicked and the protocol is not installed on the users machine. IE is very unforgiving here, any trick to click in a invisible frame or trap javascript errors does not work and ends up with ugly "webpage cannot be displayed" error. The trick we use in our case is to inform users with browser specific images that clicking on the custom protocol link will open an application. And if they do not find the app opening up, they can click on an "install" page. In terms of XD this wprks way better than the ActiveX approach for IE. For FF and Chrome, just go ahead and launch the custom protocol without any detection. Let the user tell you what he sees. For Safari, :( no answers yet

HTML5 defines Custom scheme and content handlers (to my knowledge Firefox is the only implementor so far), but unfortunately there is currently no way to check if a handler already exists—it has been proposed, but there was no follow-up. This seems like a critical feature to use custom handlers effectively and we as developers should bring attention to this issue in order to get it implemented.

If you have control of the program you're trying to run (the code), one way to see if the user was successful in running the application would be to:

1. Before trying to open the custom protocol, make an AJAX request to a server script that saves the user's intent in a database (for example, save the userid and what he wanted to do).

2. Try to open the program, and pass on the intent data.

3. Have the program make a request to the server to remove the database entry (using the intent data to find the correct row).

4. Make the javascript poll the server for a while to see if the database entry is gone. If the entry is gone, you'll know the user was successful in opening the application, otherwise the entry will remain (you can remove it later with cronjob).

I have not tried this method, just thought it.

There's no great cross-browser way to do this. In IE10+ on Win8+, a new msLaunchUri api enables you to launch a protocol, like so:

navigator.msLaunchUri('skype:123456',
function()
{
alert('success');
},
function()
{
alert('failed');
}
);

If the protocol is not installed, the failure callback will fire. Otherwise, the protocol will launch and the success callback will fire.

I discuss this topic a bit further here: https://web.archive.org/web/20180308105244/https://blogs.msdn.microsoft.com/ieinternals/2011/07/13/understanding-protocols/

This topic is of recent (2021) interest; see https://github.com/fingerprintjs/external-protocol-flooding for discussion.

I was able to finally get a cross-browser (Chrome 32, Firefox 27, IE 11, Safari 6) solution working with a combination of this and a super-simple Safari extension. Much of this solution has been mentioned in one way or another in this and this other question.

Here's the script:

function launchCustomProtocol(elem, url, callback) {
var iframe, myWindow, success = false;


if (Browser.name === "Internet Explorer") {
myWindow = window.open('', '', 'width=0,height=0');
myWindow.document.write("<iframe src='" + url + "'></iframe>");


setTimeout(function () {
try {
myWindow.location.href;
success = true;
} catch (ex) {
console.log(ex);
}


if (success) {
myWindow.setTimeout('window.close()', 100);
} else {
myWindow.close();
}


callback(success);
}, 100);
} else if (Browser.name === "Firefox") {
try {
iframe = $("<iframe />");
iframe.css({"display": "none"});
iframe.appendTo("body");
iframe[0].contentWindow.location.href = url;


success = true;
} catch (ex) {
success = false;
}


iframe.remove();


callback(success);
} else if (Browser.name === "Chrome") {
elem.css({"outline": 0});
elem.attr("tabindex", "1");
elem.focus();


elem.blur(function () {
success = true;
callback(true);  // true
});


location.href = url;


setTimeout(function () {
elem.off('blur');
elem.removeAttr("tabindex");


if (!success) {
callback(false);  // false
}
}, 1000);
} else if (Browser.name === "Safari") {
if (myappinstalledflag) {
location.href = url;
success = true;
} else {
success = false;
}


callback(success);
}
}

The Safari extension was easy to implement. It consisted of a single line of injection script:

myinject.js:

window.postMessage("myappinstalled", window.location.origin);

Then in the web page JavaScript, you need to first register the message event and set a flag if the message is received:

window.addEventListener('message', function (msg) {
if (msg.data === "myappinstalled") {
myappinstalledflag = true;
}
}, false);

This assumes the application which is associated with the custom protocol will manage the installation of the Safari extension.

In all cases, if the callback returns false, you know to inform the user that the application (i.e., it's custom protocol) is not installed.

Here's another hacky answer that would require (hopefully light) modification to your application to 'phone home' on launch.

1. User clicks link, which attempts to launch the application. A unique identifier is put in the link, so that it's passed to the application when it launches. Web app shows a spinner or something of that nature.
2. Web page then starts checking for a 'application phone home' event from an app with this same unique ID.
3. When launched, your application does an HTTP post to your web app with the unique identifier, to indicate presence.
4. Either the web page sees that the application launched, eventually, or moves on with a 'please download' page.