Qt Tutorial 4 : Paint App
I received a bunch of questions about the Qt Paint App Example, so I decided to do a tutorial in which I explain every line of code. This paint app allows for opening, saving and printing images. You can also draw on the screen using different colors and pen widths. The are a bunch of dialogs covered as well along with numerous other topics. I provide all of the heavily commented code below for the finished app.
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Code from the Video
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// ---------- Qt Tutorial 4 ---------- If you get this error "Undefined symbols for architecture x86_64" Go to /Users/derekbanas/Qt/5.11.0/clang_64/mkspecs/macx-clang/qmake.conf and change QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.11 to QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.12 // ---------- mainwindow.h ---------- #ifndef MAINWINDOW_H #define MAINWINDOW_H #include <QList> #include <QMainWindow> // ScribbleArea used to paint the image class ScribbleArea; class MainWindow : public QMainWindow { // Declares our class as a QObject which is the base class // for all Qt objects // QObjects handle events Q_OBJECT public: MainWindow(); protected: // Function used to close an event void closeEvent(QCloseEvent *event) override; // The events that can be triggered private slots: void open(); void save(); void penColor(); void penWidth(); void about(); private: // Will tie user actions to functions void createActions(); void createMenus(); // Will check if changes have occurred since last save bool maybeSave(); // Opens the Save dialog and saves bool saveFile(const QByteArray &fileFormat); // What we'll draw on ScribbleArea *scribbleArea; // The menu widgets QMenu *saveAsMenu; QMenu *fileMenu; QMenu *optionMenu; QMenu *helpMenu; // All the actions that can occur QAction *openAct; // Actions tied to specific file formats QList<QAction *> saveAsActs; QAction *exitAct; QAction *penColorAct; QAction *penWidthAct; QAction *printAct; QAction *clearScreenAct; QAction *aboutAct; QAction *aboutQtAct; }; #endif // ---------- END mainwindow.h ---------- // ---------- scribblearea.h ---------- #ifndef SCRIBBLEAREA_H #define SCRIBBLEAREA_H #include <QColor> #include <QImage> #include <QPoint> #include <QWidget> class ScribbleArea : public QWidget { // Declares our class as a QObject which is the base class // for all Qt objects // QObjects handle events Q_OBJECT public: ScribbleArea(QWidget *parent = 0); // Handles all events bool openImage(const QString &fileName); bool saveImage(const QString &fileName, const char *fileFormat); void setPenColor(const QColor &newColor); void setPenWidth(int newWidth); // Has the image been modified since last save bool isModified() const { return modified; } QColor penColor() const { return myPenColor; } int penWidth() const { return myPenWidth; } public slots: // Events to handle void clearImage(); void print(); protected: void mousePressEvent(QMouseEvent *event) override; void mouseMoveEvent(QMouseEvent *event) override; void mouseReleaseEvent(QMouseEvent *event) override; // Updates the scribble area where we are painting void paintEvent(QPaintEvent *event) override; // Makes sure the area we are drawing on remains // as large as the widget void resizeEvent(QResizeEvent *event) override; private: void drawLineTo(const QPoint &endPoint); void resizeImage(QImage *image, const QSize &newSize); // Will be marked true or false depending on if // we have saved after a change bool modified; // Marked true or false depending on if the user // is drawing bool scribbling; // Holds the current pen width & color int myPenWidth; QColor myPenColor; // Stores the image being drawn QImage image; // Stores the location at the current mouse event QPoint lastPoint; }; #endif // ---------- END scribblearea.h ---------- // ---------- main.cpp ---------- #include "mainwindow.h" #include <QApplication> int main(int argc, char *argv[]) { // The main application QApplication app(argc, argv); // Create and open the main window MainWindow window; window.show(); // Display the main window return app.exec(); } // ---------- END main.cpp ---------- // ---------- mainwindow.cpp ---------- #include <QtWidgets> #include "mainwindow.h" #include "scribblearea.h" // MainWindow constructor MainWindow::MainWindow() { // Create the ScribbleArea widget and make it // the central widget scribbleArea = new ScribbleArea; setCentralWidget(scribbleArea); // Create actions and menus createActions(); createMenus(); // Set the title setWindowTitle(tr("Scribble")); // Size the app resize(500, 500); } // User tried to close the app void MainWindow::closeEvent(QCloseEvent *event) { // If they try to close maybeSave() returns true // if no changes have been made and the app closes if (maybeSave()) { event->accept(); } else { // If there have been changes ignore the event event->ignore(); } } // Check if the current image has been changed and then // open a dialog to open a file void MainWindow::open() { // Check if changes have been made since last save // maybeSave() returns true if no changes have been made if (maybeSave()) { // Get the file to open from a dialog // tr sets the window title to Open File // QDir opens the current dirctory QString fileName = QFileDialog::getOpenFileName(this, tr("Open File"), QDir::currentPath()); // If we have a file name load the image and place // it in the scribbleArea if (!fileName.isEmpty()) scribbleArea->openImage(fileName); } } // Called when the user clicks Save As in the menu void MainWindow::save() { // A QAction represents the action of the user clicking QAction *action = qobject_cast<QAction *>(sender()); // Stores the array of bytes of the users data QByteArray fileFormat = action->data().toByteArray(); // Pass it to be saved saveFile(fileFormat); } // Opens a dialog to change the pen color void MainWindow::penColor() { // Store the chosen color from the dialog QColor newColor = QColorDialog::getColor(scribbleArea->penColor()); // If a valid color set it if (newColor.isValid()) scribbleArea->setPenColor(newColor); } // Opens a dialog that allows the user to change the pen width void MainWindow::penWidth() { // Stores button value bool ok; // tr("Scribble") is the title // the next tr is the text to display // Get the current pen width // Define the min, max, step and ok button int newWidth = QInputDialog::getInt(this, tr("Scribble"), tr("Select pen width:"), scribbleArea->penWidth(), 1, 50, 1, &ok); // Change the pen width if (ok) scribbleArea->setPenWidth(newWidth); } // Open an about dialog void MainWindow::about() { // Window title and text to display QMessageBox::about(this, tr("About Scribble"), tr("<p>The <b>Scribble</b> example is awesome</p>")); } // Define menu actions that call functions void MainWindow::createActions() { // Create the action tied to the menu openAct = new QAction(tr("&Open..."), this); // Define the associated shortcut key openAct->setShortcuts(QKeySequence::Open); // Tie the action to MainWindow::open() connect(openAct, SIGNAL(triggered()), this, SLOT(open())); // Get a list of the supported file formats // QImageWriter is used to write images to files foreach (QByteArray format, QImageWriter::supportedImageFormats()) { QString text = tr("%1...").arg(QString(format).toUpper()); // Create an action for each file format QAction *action = new QAction(text, this); // Set an action for each file format action->setData(format); // When clicked call MainWindow::save() connect(action, SIGNAL(triggered()), this, SLOT(save())); // Attach each file format option menu item to Save As saveAsActs.append(action); } // Create print action and tie to MainWindow::print() printAct = new QAction(tr("&Print..."), this); connect(printAct, SIGNAL(triggered()), scribbleArea, SLOT(print())); // Create exit action and tie to MainWindow::close() exitAct = new QAction(tr("E&xit"), this); exitAct->setShortcuts(QKeySequence::Quit); connect(exitAct, SIGNAL(triggered()), this, SLOT(close())); // Create pen color action and tie to MainWindow::penColor() penColorAct = new QAction(tr("&Pen Color..."), this); connect(penColorAct, SIGNAL(triggered()), this, SLOT(penColor())); // Create pen width action and tie to MainWindow::penWidth() penWidthAct = new QAction(tr("Pen &Width..."), this); connect(penWidthAct, SIGNAL(triggered()), this, SLOT(penWidth())); // Create clear screen action and tie to MainWindow::clearImage() clearScreenAct = new QAction(tr("&Clear Screen"), this); clearScreenAct->setShortcut(tr("Ctrl+L")); connect(clearScreenAct, SIGNAL(triggered()), scribbleArea, SLOT(clearImage())); // Create about action and tie to MainWindow::about() aboutAct = new QAction(tr("&About"), this); connect(aboutAct, SIGNAL(triggered()), this, SLOT(about())); // Create about Qt action and tie to MainWindow::aboutQt() aboutQtAct = new QAction(tr("About &Qt"), this); connect(aboutQtAct, SIGNAL(triggered()), qApp, SLOT(aboutQt())); } // Create the menubar void MainWindow::createMenus() { // Create Save As option and the list of file types saveAsMenu = new QMenu(tr("&Save As"), this); foreach (QAction *action, saveAsActs) saveAsMenu->addAction(action); // Attach all actions to File fileMenu = new QMenu(tr("&File"), this); fileMenu->addAction(openAct); fileMenu->addMenu(saveAsMenu); fileMenu->addAction(printAct); fileMenu->addSeparator(); fileMenu->addAction(exitAct); // Attach all actions to Options optionMenu = new QMenu(tr("&Options"), this); optionMenu->addAction(penColorAct); optionMenu->addAction(penWidthAct); optionMenu->addSeparator(); optionMenu->addAction(clearScreenAct); // Attach all actions to Help helpMenu = new QMenu(tr("&Help"), this); helpMenu->addAction(aboutAct); helpMenu->addAction(aboutQtAct); // Add menu items to the menubar menuBar()->addMenu(fileMenu); menuBar()->addMenu(optionMenu); menuBar()->addMenu(helpMenu); } bool MainWindow::maybeSave() { // Check for changes since last save if (scribbleArea->isModified()) { QMessageBox::StandardButton ret; // Scribble is the title // Add text and the buttons ret = QMessageBox::warning(this, tr("Scribble"), tr("The image has been modified.\n" "Do you want to save your changes?"), QMessageBox::Save | QMessageBox::Discard | QMessageBox::Cancel); // If save button clicked call for file to be saved if (ret == QMessageBox::Save) { return saveFile("png"); // If cancel do nothing } else if (ret == QMessageBox::Cancel) { return false; } } return true; } bool MainWindow::saveFile(const QByteArray &fileFormat) { // Define path, name and default file type QString initialPath = QDir::currentPath() + "/untitled." + fileFormat; // Get selected file from dialog // Add the proper file formats and extensions QString fileName = QFileDialog::getSaveFileName(this, tr("Save As"), initialPath, tr("%1 Files (*.%2);;All Files (*)") .arg(QString::fromLatin1(fileFormat.toUpper())) .arg(QString::fromLatin1(fileFormat))); // If no file do nothing if (fileName.isEmpty()) { return false; } else { // Call for the file to be saved return scribbleArea->saveImage(fileName, fileFormat.constData()); } } // ---------- END mainwindow.cpp ---------- // ---------- scribblearea.cpp ---------- #include <QtWidgets> #if defined(QT_PRINTSUPPORT_LIB) #include <QtPrintSupport/qtprintsupportglobal.h> #if QT_CONFIG(printdialog) #include <QPrinter> #include <QPrintDialog> #endif #endif #include "scribblearea.h" ScribbleArea::ScribbleArea(QWidget *parent) : QWidget(parent) { // Roots the widget to the top left even if resized setAttribute(Qt::WA_StaticContents); // Set defaults for the monitored variables modified = false; scribbling = false; myPenWidth = 1; myPenColor = Qt::blue; } // Used to load the image and place it in the widget bool ScribbleArea::openImage(const QString &fileName) { // Holds the image QImage loadedImage; // If the image wasn't loaded leave this function if (!loadedImage.load(fileName)) return false; QSize newSize = loadedImage.size().expandedTo(size()); resizeImage(&loadedImage, newSize); image = loadedImage; modified = false; update(); return true; } // Save the current image bool ScribbleArea::saveImage(const QString &fileName, const char *fileFormat) { // Created to hold the image QImage visibleImage = image; resizeImage(&visibleImage, size()); if (visibleImage.save(fileName, fileFormat)) { modified = false; return true; } else { return false; } } // Used to change the pen color void ScribbleArea::setPenColor(const QColor &newColor) { myPenColor = newColor; } // Used to change the pen width void ScribbleArea::setPenWidth(int newWidth) { myPenWidth = newWidth; } // Color the image area with white void ScribbleArea::clearImage() { image.fill(qRgb(255, 255, 255)); modified = true; update(); } // If a mouse button is pressed check if it was the // left button and if so store the current position // Set that we are currently drawing void ScribbleArea::mousePressEvent(QMouseEvent *event) { if (event->button() == Qt::LeftButton) { lastPoint = event->pos(); scribbling = true; } } // When the mouse moves if the left button is clicked // we call the drawline function which draws a line // from the last position to the current void ScribbleArea::mouseMoveEvent(QMouseEvent *event) { if ((event->buttons() & Qt::LeftButton) && scribbling) drawLineTo(event->pos()); } // If the button is released we set variables to stop drawing void ScribbleArea::mouseReleaseEvent(QMouseEvent *event) { if (event->button() == Qt::LeftButton && scribbling) { drawLineTo(event->pos()); scribbling = false; } } // QPainter provides functions to draw on the widget // The QPaintEvent is sent to widgets that need to // update themselves void ScribbleArea::paintEvent(QPaintEvent *event) { QPainter painter(this); // Returns the rectangle that needs to be updated QRect dirtyRect = event->rect(); // Draws the rectangle where the image needs to // be updated painter.drawImage(dirtyRect, image, dirtyRect); } // Resize the image to slightly larger then the main window // to cut down on the need to resize the image void ScribbleArea::resizeEvent(QResizeEvent *event) { if (width() > image.width() || height() > image.height()) { int newWidth = qMax(width() + 128, image.width()); int newHeight = qMax(height() + 128, image.height()); resizeImage(&image, QSize(newWidth, newHeight)); update(); } QWidget::resizeEvent(event); } void ScribbleArea::drawLineTo(const QPoint &endPoint) { // Used to draw on the widget QPainter painter(&image); // Set the current settings for the pen painter.setPen(QPen(myPenColor, myPenWidth, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin)); // Draw a line from the last registered point to the current painter.drawLine(lastPoint, endPoint); // Set that the image hasn't been saved modified = true; int rad = (myPenWidth / 2) + 2; // Call to update the rectangular space where we drew update(QRect(lastPoint, endPoint).normalized() .adjusted(-rad, -rad, +rad, +rad)); // Update the last position where we left off drawing lastPoint = endPoint; } // When the app is resized create a new image using // the changes made to the image void ScribbleArea::resizeImage(QImage *image, const QSize &newSize) { // Check if we need to redraw the image if (image->size() == newSize) return; // Create a new image to display and fill it with white QImage newImage(newSize, QImage::Format_RGB32); newImage.fill(qRgb(255, 255, 255)); // Draw the image QPainter painter(&newImage); painter.drawImage(QPoint(0, 0), *image); *image = newImage; } // Print the image void ScribbleArea::print() { // Check for print dialog availability #if QT_CONFIG(printdialog) // Can be used to print QPrinter printer(QPrinter::HighResolution); // Open printer dialog and print if asked QPrintDialog printDialog(&printer, this); if (printDialog.exec() == QDialog::Accepted) { QPainter painter(&printer); QRect rect = painter.viewport(); QSize size = image.size(); size.scale(rect.size(), Qt::KeepAspectRatio); painter.setViewport(rect.x(), rect.y(), size.width(), size.height()); painter.setWindow(image.rect()); painter.drawImage(0, 0, image); } #endif // QT_CONFIG(printdialog) } // ---------- END scribblearea.cpp ---------- |
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