Implement a Custom Protocol on Mac

[ChrisW67]

I think @stefanwoe is asking a related but different question.

How do you make an arbitrary browser on Mac open your application and pass the URL as argument when you give the browser a URL like myapp://some/arguments? On Windows this is done with registry magic (and offerings to the deity of your choice).

This is not the same thing as making a custom scheme for use from within a Qt program, which is what QDesktopServices does in a non-persistent fashion.

[stefanwoe]

@ChrisW67 Yes, thats what i need.

[SGaist]

@ChrisW67 that's also what I understood.

Even if iOS is mentioned in the documentation, AFAIR, the same applies for macOS with the changes you have to do to the info.plist file through the CFBundleURLSchemes entry.

[stefanwoe]

@SGaist Thats what i tried, but so far no success.

[SGaist]

@stefanwoe can you share a minimal compilable sample application so we can investigate ?

[stefanwoe]

@SGaist if i had this i would not have asked this question.

[stefanwoe]

@SGaist But you may have a look at the link i posted:
www.xmldatabases.org/WK/blog/1154_Handling_URL_schemes_in_Cocoa.item (very old...)

[SGaist]

@stefanwoe said in Implement a Custom Protocol on Mac:

@SGaist if i had this i would not have asked this question.

The goal is not that you provide a working solution. Having a minimal project will allow us to check your issue and help you debug it.

[stefanwoe]

@SGaist This can be done with any sample application. i.e. the MDI Example https://doc.qt.io/qt-6.2/qtwidgets-mainwindows-mdi-example.html.
If we us a URL in a browser such us

mayprotocol://mydata

A handler within this application shall be called.
AND: if the application is not running it shall be started. That works fine on windows (no programming in the required - its just called with a command line parameter) see the link in my question), but i dont get it to work on the mac.

[stefanwoe]

After spending several days in total with this I now had success and the custom protocol works as expected.

Basically its correct, that only a CFBundleURLTypes entry in Info.plist is needed but there are some caveats that were so hard to find out:

1. Updating Info.plist in the QCreator project will not immediately update the Info.plist file in the created application boundle. This remains unchanged until a full rebuild is made.
2. But even if we modify the file inside the Application boundle (MyApp.app/Contents/Info.plist) a modification in this file will not always be noticed by the macOS after you once start the application. I have not found the details of this. I created a sample project in XCode and there all changes were registered immediately. Also if you download a dmg file this is registered immediately.
3. The Key to get all this to work during development is the lsregister utility described here:
   https://eclecticlight.co/2019/03/25/lsregister-a-valuable-undocumented-command-for-launchservices/

Command to remove all custom protocols:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain u -domain s -domain l -v

If you do that, as soon as you start your application, the custom protocol defined in MyApp.app/Contents/Info.plist will be registered

To find if a protocol is registered:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -dump | grep myprotocol

Where myprotocol is the name of the protocol searched. You may pipe the dump to less or a file to see which application is registered for the protocol. The dump returns a huge amount of data.

Also you can tell lsregister to add the protocol(s) from a boundle:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -R -f pathname

The linked article states "You shouldn’t ever have to do this" - but this seems invalid to me.

Or tell lsregister to forget about a application:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -R -f -u pathname
code_text

where pathname is the path and name of the app.

So the whole process is:

1. Add CFBundleURLTypes to your Info.plist

<key>CFBundleURLTypes</key>
<array>
   <dict>
     <key>CFBundleTypeRole</key>
     <string>Viewer</string>
     <key>CFBundleURLName</key>
     <string>com.mydomain</string>
     <key>CFBundleURLSchemes</key>
     <array>
         <string>myProtocol</string>
     </array>
   </dict>
</array>

And then take care about the registering as described above.

If the protocol IS registered on the Mac, then you'll get a nice clean QEvent::FileOpen in the event callback of your application class derived from QApplication:

class CMyApplication : public QApplication
{
public:
    CMyApplication(int &argc, char **argv)
        : QApplication(argc, argv)
    {
    }

    bool event(QEvent *event) override
    {
        if (event->type() == QEvent::FileOpen) {
            QFileOpenEvent *openEvent = static_cast<QFileOpenEvent *>(event);
            QString urlString = openEvent->url().toString();
                // Processs urlString ...
        }
        return QApplication::event(event);
    }
};

Now you can open a browser and type in the address bar:

myProtocol://?param1=value1&param2=value2

And the browser will ask you if you want to start the registered application:

If you accept this, openEvent->url().toString() will return the complete string you typed into your browser. If your Application is not running it will be started by the macOS.

[SGaist]

Thanks for the thorough feedback !

[stefanwoe]

The Implementation to handle a custom Protocol in the Application does no more work in Qt 6.8.2 as described above (only the C++ programming part, the other stuff - Info.plist etc. remains unchanged)
The correct way to implement such a handler is to use

void QDesktopServices::setUrlHandler()

See https://doc.qt.io/qt-6/qdesktopservices.html#setUrlHandler
This paragraph also describes how to set Info.plist etc...
This most likely also would have worked in 6.5.3 - why did I never find it?

For Windows all of this works quite differently. Registering works through well known registry entries:

[HKEY_CLASSES_ROOT\myprotocol]
"URL Protocol"=""
[HKEY_CLASSES_ROOT\myprotocol\shell]
[HKEY_CLASSES_ROOT\myprotocol\shell\open]
[HKEY_CLASSES_ROOT\myprotocol\shell\open\command] 
@="\"path to application.exe" \"%1\"

This will just start a new instance of the application with the custom protocols URL as first parameter %1. Its the applications responsibility to handle that correctly (i.e. call a existing instance via a QLocalSocket etc.