Tutorial 4: A simple GUI

This tutorial will be implemented only in C++.

We have seen how to draw on the frame, but now we will create a simple GUI. We will place a Label, a Textfield and two Buttons on the Frame. When you click the first Button we will display a Messagebox with the text of the Textfield. The second Button will close the frame.

We have to program the following for this example:

• Initialize the JVM.
• Create and initialize a frame, a label, a textfield and two buttons.
• Place the control elements on the frame.
• A callback function for the window events of the frame.
• Check if the user wanted to close the frame (in the window listener callback function) and close it then.
• A callback function for the buttons to display the messagebox, when the first button was pressed and to close the frame, when the second button was pressed.

#include "grubc/awt/frame.h"
#include "grubc/awt/label.h"
#include "grubc/awt/textfield.h"
#include "grubc/awt/button.h"
#include "grubc/swing/messagebox.h"

/* Declare the needed variables. */
Jvm jvm;
Frame frame;
Label label;
TextField textfield;
Button buttonmsgbox;
Button buttonexit;

jboolean exitprg = JNI_FALSE;


/* Declare the needed listener callback function. */
void windowListener(jint event, Window window);
void actionListener(Component source);


int main(int argc, char *argv[]) {
   WindowListener wl; /* WindowListener Object. */
   ActionListener albuttonmsgbox; /* ActionListener Object. */
   ActionListener albuttonexit; /* ActionListener Object. */

   initJvm(&jvm, "./grubc.jar"); /* Initialize the JVM. */

   createFrame(&frame); /* Create the frame. */
   createLabel(&label); /* Create the label. */
   createTextField(&textfield); /* Create the textfield. */
   createButton(&buttonmsgbox); /* Create the first button. */
   createButton(&buttonexit); /* Create the second button. */

   /* Register the callback functions as listeners. */
   wl = frame.addWindowListener(&windowListener);
   albuttonmsgbox = buttonmsgbox.addActionListener(&actionListener);
   albuttonexit = buttonexit.addActionListener(&actionListener);

   /* Set the title and state of the frame. */
   frame.setTitle("Tutorial 4");
   frame.setSize(600, 300);

   /* Set the labels text, position and size. */
   label.setText("Text:");
   label.setSize(30, 10);
   label.setLocation(50, 50);

   /* Set the textfields size and location. */
   textfield.setSize(250, 25);
   textfield.setLocation(80, 45);

   /* Set the first buttons text, size and location. */
   buttonmsgbox.setLabel("MessageBox");
   buttonmsgbox.setSize(100, 25);
   buttonmsgbox.setLocation(350, 40);

   /* Set the second buttons text, size and location. */
   buttonexit.setLabel("Exit");
   buttonexit.setSize(50, 25);
   buttonexit.setLocation(450, 40);

   /* Add the control elements to the frame. */
   frame.addComponent(label.getObject());
   frame.addComponent(textfield.getObject());
   frame.addComponent(buttonmsgbox.getObject());
   frame.addComponent(buttonexit.getObject());

   frame.show(); /* Display the frame. */
   
   /* Wait until exitprg is true. */
   WAIT_UNTIL_TRUE(exitprg);

   frame.close(); /* Close the frame. */

   /* Deregister the callback functions. */
   frame.removeWindowListener(wl);
   buttonmsgbox.removeActionListener(albuttonmsgbox);
   buttonexit.removeActionListener(albuttonexit);

   /* Cleanup */
   deleteLabel(&label);
   deleteButton(&buttonmsgbox);
   deleteButton(&buttonexit);
   deleteTextField(&textfield);
   deleteFrame(&frame);

   return 0;
}

void windowListener(jint event, Window window) {
   if (frame.isSameWindow(window)) {
      switch (event) {
         case CB_WINDOW_OPENED:
            break;
         case CB_WINDOW_CLOSING:
            exitprg = JNI_TRUE; /* Frame should be closed. */
            break;
         case CB_WINDOW_CLOSED:
            break;
         case CB_WINDOW_ICONIFIED:
            break;
         case CB_WINDOW_DEICONIFIED:
            break;
         case CB_WINDOW_ACTIVATED:
            break;
         case CB_WINDOW_DEACTIVATED:
            break;
      }
   }
}

void actionListener(Component source) {
   /* Check if the source was the messagebox button or the exit button??? */
   if (isSameObject(source, buttonmsgbox.getObject())) {
      char *text = textfield.getText(); /* Get the text of the Textfield. */
      showModalMessageDialog(&frame, text); /* Show the Messagebox. */
      free(text); /* Free the text. */
   } else if (isSameObject(source, buttonexit.getObject())) {
      exitprg = JNI_TRUE;
   }
}

When you execute it you will get a window which looks like this:

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