How to make application not to run a new instance when openening a new file in Qt?

Question

For example we have a TextEditor Application. Like notepad++. We have tabs at which file content was displaying.
The default text editor in OS is set to TextEditor Application. When we open a new file application added a tab and put content to it.
How to make an application not to run a new instance when opening a new file in Qt?
Which is the best way you think?

Answer 1

The problem is how can you make a single-instance application. When you open a file the operating system will open the associated application and give it the file as a command line argument. You cannot simply delegate an 'open file' command to a running application through OS mechanism, you have to implement it by yourself.

At the AppWhirr project we used QLocalServer/Client to communicate between instances: when the AppWhirr app is executed it checks whether a QLocalServer with a fix ID is already taken or not. If not this instance of the application is the first/only running instance. If the ID is already taken it means another instance of the application is already running so this second instance will only do 2 things: send the given input arguments to the other instance through Qt's local client/server communication, and when it's successfully finish the communication it will quit (the second instance).

That's one solution for the problem, requires quite a bit of coding and I would not recommend it if you don't want to use local client/server communication for anything else, but it's a viable solution.

Another solution would be that the first instance of the application creates a text file at a fixed location and writes our the instance's ID. After this the second instance can read the text file and send a message to the specified ID. And of course the first instance have to remove the text-file when it quits and probably you have to implement some fail-safe code to remove the text-file in case the first instance crashes. This solution will use less resource than the first one but requires a fail-safe cleanup code.

Or as a third option you can use third-party solutions like @Matteo Italia suggested.