Saving a copy

Let's look back at the showImage method of MainWindow. In that method, we created an instance of QPixmap from the image and then added it to imageScene by calling imageScene->addPixmap. We didn't hold any handler of the image out of that function; hence, now we don't have a convenient way to get the QPixmap instance in the new slot, which we will implement for saveAsAction.

To solve this, we add a new private member field, QGraphicsPixmapItem *currentImage, to MainWindow to hold the return value of imageScene->addPixmap and initialize it with nullptr in the constructor of MainWindow. Then, we find the line of code in the body of MainWindow::showImage:

   imageScene->addPixmap(image);

To save the returned value, we replace this line with the following one:

   currentImage = imageScene->addPixmap(image);

Now, we are ready to create a new slot for saveAsAction. The declaration in the private slot section is straightforward, as follows:

       void saveAs();

The definition is also straightforward:

     void MainWindow::saveAs()
     {
         if (currentImage == nullptr) {
             QMessageBox::information(this, "Information", "Nothing to save.");
             return;
         }
         QFileDialog dialog(this);
         dialog.setWindowTitle("Save Image As ...");
         dialog.setFileMode(QFileDialog::AnyFile);
         dialog.setAcceptMode(QFileDialog::AcceptSave);
         dialog.setNameFilter(tr("Images (*.png *.bmp *.jpg)"));
         QStringList fileNames;
         if (dialog.exec()) {
             fileNames = dialog.selectedFiles();
             if(QRegExp(".+\\.(png|bmp|jpg)").exactMatch(fileNames.at(0))) {
                 currentImage->pixmap().save(fileNames.at(0));
             } else {
                 QMessageBox::information(this, "Information", "Save error: bad format or filename.");
             }
         }
     }

First, we check whether currentImage is nullptr. If true, it means we haven't opened any image yet. So, we open a QMessageBox to tell the user there's nothing to save. Otherwise, we create a QFileDialog, set the relevant properties for it, and open it by calling its exec method. If the user gives the dialog a filename and clicks the open button on it, we will get a list of file paths that have only one element in it as our last usage of QFileDialog. Then, we check whether the file path ends with the extensions we support using a regexp matching. If everything goes well, we get the QPixmap instance of the current image from currentImage->pixmap() and save it to the specified path. Once the slot is ready, we connect it to the signal in createActions:

   connect(saveAsAction, SIGNAL(triggered(bool)), this, SLOT(saveAs()));

To test this feature, we can open a PNG image and save it as a JPG image by giving a filename that ends with .jpg in the Save Image As... file dialog. Then, we open the new JPG image we just saved, using another image view application to check whether the image has been correctly saved.