Navigating in the folder

Now that we have completed all of the actions in relation to a single image, let's go further and navigate all the images that reside in the directory in which the current image resides, that is, prevAction and nextAction.

To know what constitutes the previous or next image, we should be aware of two things as follows:

• Which is the current one
• The order in which we count them

So, first we add a new member field, QString currentImagePath, to the MainWindow class to save the path of the current image. Then, we save the image's path while showing it in showImage by adding the following line to the method:

     currentImagePath = path;

Then, we decide to count the images in alphabetical order according to their names. With these two pieces of information, we can now determine which is the previous or next image. Let's see how we define the slot for prevAction:

     void MainWindow::prevImage()
     {
         QFileInfo current(currentImagePath);
         QDir dir = current.absoluteDir();
         QStringList nameFilters;
         nameFilters << "*.png" << "*.bmp" << "*.jpg";
         QStringList fileNames = dir.entryList(nameFilters, QDir::Files, QDir::Name);
         int idx = fileNames.indexOf(QRegExp(QRegExp::escape(current.fileName())));
         if(idx > 0) {
             showImage(dir.absoluteFilePath(fileNames.at(idx - 1)));
         } else {
             QMessageBox::information(this, "Information", "Current image is the first one.");
         }
     }

First, we get the directory in which the current image resides as an instance of QDir, and then we list the directory with name filters to ensure that only PNG, BMP, and JPG files are returned. While listing the directory, we use QDir::Name as the third argument to make sure the returned list is sorted by filename in alphabetical order. Since the current image we are viewing is also in this directory, its filename must be in the filename list. We find its index by calling indexOf on the list with a regexp, which is generated by QRegExp::escape, so that it can exactly match its filename. If the index is zero, this means the current image is the first one in this directory. A message box pops up to give the user this information. Otherwise, we show the image whose filename is at the position of index - 1 to complete the operation.

Before you test whether prevAction works, don't forget to connect the signal and the slot by adding the following line to the body of the createActions method:

   connect(prevAction, SIGNAL(triggered(bool)), this, SLOT(prevImage()));

Well, it's not too hard, so attempt the work of nextAction yourself or just read the code for it in our code repository on GitHub.