add sample usage to readme-lib.md in session 3

Session-3/readme-lib.md

@@ -26,8 +26,167 @@ cd ubuscribble/
 python3 setup.py install --user
 ```
 
-5. run
-```sh
-BenApp
+5. usage 
+```python
+#!/usr/bin/python3 
+
+from PyQt5.QtWidgets import QAction, QMenu, QMainWindow, QApplication, QMessageBox, QFileDialog, QInputDialog
+from PyQt5.QtGui import QImage, QImageWriter
+from PyQt5.QtCore import QDir
+
+from ubuscribble.scribble import Scribble
+
+class MainWindow(QMainWindow):
+    def __init__(self):
+        super(MainWindow, self).__init__()
+
+        self.saveAsActs = []
+
+        self.scribble= Scribble()
+        self.setCentralWidget(self.scribble)
+
+        self.createActions()
+        self.createMenus()
+
+        self.setWindowTitle("Scribble")
+        self.resize(500, 500)
+
+    def closeEvent(self, event):
+        if self.maybeSave():
+            event.accept()
+        else:
+            event.ignore()
+
+    def open(self):
+        if self.maybeSave():
+            fileName, _ = QFileDialog.getOpenFileName(self, "Open File",
+                    QDir.currentPath())
+            if fileName:
+                self.scribble.openImage(fileName)
+
+    def save(self):
+        action = self.sender()
+        fileFormat = action.data()
+        self.saveFile(fileFormat)
+
+    def penColor(self):
+        newColor = QColorDialog.getColor(self.scribble.penColor())
+        if newColor.isValid():
+            self.scribble.setPenColor(newColor)
+
+    def penWidth(self):
+        newWidth, ok = QInputDialog.getInt(self, "Scribble",
+                "Select pen width:", self.scribble.penWidth(), 1, 50, 1)
+        if ok:
+            self.scribble.setPenWidth(newWidth)
+
+    def about(self):
+        QMessageBox.about(self, "About Scribble",
+                "<p>The <b>Scribble</b> example shows how to use "
+                "QMainWindow as the base widget for an application, and how "
+                "to reimplement some of QWidget's event handlers to receive "
+                "the events generated for the application's widgets:</p>"
+                "<p> We reimplement the mouse event handlers to facilitate "
+                "drawing, the paint event handler to update the application "
+                "and the resize event handler to optimize the application's "
+                "appearance. In addition we reimplement the close event "
+                "handler to intercept the close events before terminating "
+                "the application.</p>"
+                "<p> The example also demonstrates how to use QPainter to "
+                "draw an image in real time, as well as to repaint "
+                "widgets.</p>")
+
+    def createActions(self):
+        self.openAct = QAction("&Open...", self, shortcut="Ctrl+O",
+                triggered=self.open)
+
+        for format in QImageWriter.supportedImageFormats():
+            format = str(format)
+
+            text = format.upper() + "..."
+
+            action = QAction(text, self, triggered=self.save)
+            action.setData(format)
+            self.saveAsActs.append(action)
+
+        self.printAct = QAction("&Print...", self,
+                triggered=self.scribble.print_)
+
+        self.exitAct = QAction("E&xit", self, shortcut="Ctrl+Q",
+                triggered=self.close)
+
+        self.penColorAct = QAction("&Pen Color...", self,
+                triggered=self.penColor)
+
+        self.penWidthAct = QAction("Pen &Width...", self,
+                triggered=self.penWidth)
+
+        self.clearScreenAct = QAction("&Clear Screen", self, shortcut="Ctrl+L",
+                triggered=self.scribble.clearImage)
+
+        self.aboutAct = QAction("&About", self, triggered=self.about)
+
+        self.aboutQtAct = QAction("About &Qt", self,
+                triggered=QApplication.instance().aboutQt)
+
+    def createMenus(self):
+        self.saveAsMenu = QMenu("&Save As", self)
+        for action in self.saveAsActs:
+            self.saveAsMenu.addAction(action)
+
+        fileMenu = QMenu("&File", self)
+        fileMenu.addAction(self.openAct)
+        fileMenu.addMenu(self.saveAsMenu)
+        fileMenu.addAction(self.printAct)
+        fileMenu.addSeparator()
+        fileMenu.addAction(self.exitAct)
+
+        optionMenu = QMenu("&Options", self)
+        optionMenu.addAction(self.penColorAct)
+        optionMenu.addAction(self.penWidthAct)
+        optionMenu.addSeparator()
+        optionMenu.addAction(self.clearScreenAct)
+
+        helpMenu = QMenu("&Help", self)
+        helpMenu.addAction(self.aboutAct)
+        helpMenu.addAction(self.aboutQtAct)
+
+        self.menuBar().addMenu(fileMenu)
+        self.menuBar().addMenu(optionMenu)
+        self.menuBar().addMenu(helpMenu)
+
+    def maybeSave(self):
+        if self.scribble.isModified():
+            ret = QMessageBox.warning(self, "Scribble",
+                        "The image has been modified.\n"
+                        "Do you want to save your changes?",
+                        QMessageBox.Save | QMessageBox.Discard |
+                        QMessageBox.Cancel)
+            if ret == QMessageBox.Save:
+                return self.saveFile('png')
+            elif ret == QMessageBox.Cancel:
+                return False
+
+        return True
+
+    def saveFile(self, fileFormat):
+        initialPath = QDir.currentPath() + '/untitled.' + fileFormat
+
+        fileName, _ = QFileDialog.getSaveFileName(self, "Save As", initialPath,
+                "%s Files (*.%s);;All Files (*)" % (fileFormat.upper(), fileFormat))
+        if fileName:
+            return self.scribble.saveImage(fileName, fileFormat)
+
+        return False
+
+
+if __name__ == '__main__':
+
+    import sys
+
+    app = QApplication(sys.argv)
+    window = MainWindow()
+    window.show()
+sys.exit(app.exec_())
 ```