Force saved file to have an extension on Android

Donald Duck

I'm creating an Android app that's supposed to save .txt files like this:

QString chosenFile = QFileDialog::getSaveFileName(this, "Save", "", "Text files (*.txt)");
if(!chosenFile.isEmpty()){    //If the user didn't press Cancel
    if(!chosenFile.contains(QRegularExpression("\\.txt$"))){    //If the chosen file name doesn't end with .txt
        chosenFile += ".txt";
    }
    QFile file(chosenFile);
    if(!file.open(QIODevice::WriteOnly)){
        qDebug() << "Something bad happened";
        return;
    }
    //Write the file contents
}

What this does is that it opens a save file dialog. If I enter a file name that ends with .txt, it saves the file without a problem, as expected.

If I enter a file name that doesn't end with .txt, I would expect it to save the file and automatically add .txt at the end of the file name. But instead it creates an empty file with the file name I entered (without .txt) and prints Something bad happened in the console.

Why does it do this and how do I fix it?

jsulm

@Donald-Duck Why don't you use https://doc.qt.io/qt-5/qstring.html#endsWith instead of using a regular expression? Did you check whether your if condition is actually hit? I mean: is "chosenFile += ".txt";" executed?

Donald Duck

@jsulm said in Force saved file to have an extension on Android:

Did you check whether your if condition is actually hit? I mean: is "chosenFile += ".txt";" executed?

Yes, I checked, and it was.

Why don't you use https://doc.qt.io/qt-5/qstring.html#endsWith instead of using a regular expression?

I just wasn't aware that it existed. Now that I am I can use it, but it doesn't solve the problem.

jsulm

@Donald-Duck If you see "Something bad happened" then it means that open() failed. You should print out https://doc.qt.io/qt-5/qiodevice.html#errorString in this case to see why it failed. So, please add proper error handling to get more information...

Donald Duck

@jsulm file.errorString() just returns "Unknown error", which isn't very helpful.

mvuori

I would run the code in debugger and see what really happens to the file name and everything else. Perhaps the problem is before that code and the stack is corrupted or similar.

IntruderExcluder

Note that on Android you cannot access files directly by absolute path if this path wasn't provided by file picker due to security reasons. So if file picker returned some/path/to/file you can operate only with that path, and some/path/to/file.txt will be another path which isn't provided as 'trusted'.

This is true only for 'public' folders, like Downloads. You have full access to application's sandbox, if it can solve your problem.

Donald Duck

@IntruderExcluder Thanks, that explains why I get the error. Is there a way to get around it, for example by having the file picker add the extension automatically?

IntruderExcluder

Not sure if it possible with Qt. I think you should dig into Android docs and make sure if it possible by doing with Java, then you prabably need to write some Java code and connect it via JNI. This might help you.