Working with the Low-Level API
The low-level API for MIDlets is composed of the Canvas and Graphics classes (we will discuss the GameCanvas class in the next article). The Canvas class is abstract; you must create your own canvases to write/draw on by extending this class and providing an implementation for the paint(Graphics g) method, in which the actual drawing on a device is done. The Canvas and Graphics classes work together to provide low-level control over a device.
Let's start with a simple canvas. Listing 7 shows an example canvas that draws a black square in the middle of the device screen.
package com.j2me.part2;
import javax.microedition.lcdui.Canvas;
import javax.microedition.midlet.MIDlet;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Graphics;
public class CanvasExample
extends MIDlet {
Canvas myCanvas;
public CanvasExample() {
myCanvas = new MyCanvas();
}
public void startApp() {
Display display = Display.getDisplay(this);
// remember, Canvas is a Displayable so it can
// be set on the display like Screen elements
display.setCurrent(myCanvas);
// force repaint of the canvas
myCanvas.repaint();
}
public void pauseApp() {
}
public void destroyApp(boolean unconditional) {
}
}
class MyCanvas extends Canvas {
public void paint(Graphics g) {
// create a 20x20 black square in the center
g.setColor(0x000000); // make sure it is black
g.fillRect(
getWidth()/2 - 10,
getHeight()/2 - 10,
20, 20);
}
}
Listing 7. Creating and displaying a Canvas
The class MyCanvas extends Canvas and overrides the paint() method. Although this method is called as soon as the canvas is made the current displayable element (by setCurrent(myCanvas)), it is a good idea to call the repaint() method on this canvas soon afterwards. The paint() method accepts a Graphics object, which provides methods for drawing 2D objects on the device screen. For example, in Listing 7, a black square is created in the middle of the screen using this Graphics object. Notice that before drawing the square, using the fillRect() method, the current color of the Graphics object is set to black by using the method g.setColor(). This is not necessary, as the default color is black, but this illustrates how to change it if you wanted to do so.
If you run this listing, the output on the emulator will be as shown in Figure 8.
Figure 8. Drawing a single square in the middle of a Canvas
Notice the highlighted portion at the top in Figure 8. Even though the MIDlet is running, the AMS still displays the previous screen. This is because in the paint() method, the previous screen was not cleared away, and the square was drawn on the existing surface. To clear the screen, you can add the following code in the paint() method, before the square is drawn.
g.setColor(0xffffff);
// sets the drawing color to white
g.fillRect(0, 0, getWidth(), getHeight());
// creates a fill rect which is the size of the screen
Note that the getWidth() and getHeight() methods return the size of the display screen as the initial canvas, which is the whole display screen. Although the size of this canvas cannot be changed, you can change the size and location of the clip area in which the actual rendering operations are done. A clip area, in Graphics, is the area on which the drawing operations are conducted. The Graphics class provides the method setClip(int x, int y, int width, int height) to change this clip area, which in an initial canvas is the whole screen, with the top left corner as the origin (0, 0). Thus, if you use the method getClipWidth() (or getClipHeight()) on the Graphics object passed to the paint method in Listing 7, it returns a value equal to the value returned by the getWidth() (or getHeight()) method of the Canvas.
The Graphics object can be used to render not only squares and rectangles, but arcs, lines, characters, images, and text, as well. For example, to draw the text "Hello World" on top of the square in Listing 7, you can add the following code before or after the square is drawn:
g.drawString("Hello World", getWidth()/2, getHeight()/2 - 10,
Graphics.HCENTER | Graphics.BASELINE);
This will result in the screen shown in Figure 9.
Figure 9. Drawing text using the Graphics object
Text, characters, and images are positioned using the concept of anchor points. The full syntax of the drawString() method is drawstring(String text, int x, int y, int anchor). The anchor positioning around the x, y coordinates is specified by bitwise ORing of two constants. One constant specifies the horizontal space (LEFT, HCENTER, RIGHT) and the other specifies the vertical space (TOP, BASELINE, BOTTOM). Thus, to draw the "Hello World" text on top of the square, the anchor's horizontal space needs to be centered around the middle of the canvas (getWidth()/2) and hence, I have used the Graphics.HCENTER constant. Similarly, the vertical space is specified by using the BASELINE constant around the top of the square (getHeight()/2 - 10). You can also use the special value of 0 for the anchor, which is equivalent to TOP | LEFT.
Images are similarly drawn and positioned on the screen. You can create off-screen images by using the static createImage(int width, int height) method of the Image class. You can get a Graphics object associated with this image by using the getGraphics() method. This method can only be called on images that are mutable. An image loaded from the file system, or over the network, is considered an immutable image, and any attempt to get a Graphics object on such an image will result in an IllegalStateException at runtime.
Using anchor points with images is similar to using them with text and characters. Images allow an additional constant for the vertical space, specified by Graphics.VCENTER. Also, since there is no concept of a baseline for an image, using the BASELINE constant will throw an exception if used with an image.
Listing 8 shows the code snippet from the MyCanvas class paint() method that creates an off-screen image, modifies it by adding an image loaded from the file system, and draws a red line across it. Note that you will need the image duke.gif in the res folder of the CanvasExample MIDlet.
// draw a modified image
try {
// create an off screen image
Image offImg = Image.createImage(25, 19);
// get its graphics object and set its
// drawing color to red
Graphics offGrap = offImg.getGraphics();
offGrap.setColor(0xff0000);
// load an image from file system
Image dukeImg =
Image.createImage("/duke.gif");
// draw the loaded image on the off screen
// image
offGrap.drawImage(dukeImg, 0, 0, 0);
// and modify it by drawing a line across it
offGrap.drawLine(0, 0, 25, 19);
// finally, draw this modified off screen
// image on the main graphics screen
// so that it is just under the square
g.drawImage(
offImg, getWidth()/2,
getHeight()/2 + 10,
Graphics.HCENTER | Graphics.TOP);
} catch(Exception e) { e.printStackTrace(); }
Listing 8. Creating, modifying, and displaying an off-screen image on a Canvas
The resultant screen, when combined with the "Hello World" text drawn earlier, will look like Figure 10.
Figure 10. Text, a square, and a modified image drawn on a Canvas
The Canvas class provides methods to interact with the user, including predefined game actions, key events, and, if a pointing device is present, pointer events. You can even attach high-level commands to a canvas, similar to attaching commands on a high-level UI element.
Each Canvas class automatically receives key events through the invocation of the keyPressed(int keyCode), keyReleased(int keyCode), and keyRepeated(int keyCode). The default implementations of these methods are empty, but not abstract, which allows you to only override the methods that you are interested in. Similar to the key events, if a pointing device is present, pointer events are sent to the pointerDragged(int x, int y), pointerPressed(int x, int y), and pointerReleased(int x, int y) methods.
The Canvas class defines constants for key codes that are guaranteed to be present in all wireless devices. These key codes define all of the numbers (for example, KEY_NUM0, KEY_NUM1, KEY_NUM2, and so on) and the star (*) and pound (#) keys (KEY_STAR and KEY_POUND). This class makes it even easier to capture gaming events by defining some basic gaming constants. There are nine constants that are relevant to most games: UP, DOWN, LEFT, RIGHT, FIRE, GAME_A, GAME_B, GAME_C, and GAME_D. But how does a key event translate to a gaming event?
By the use of the getGameAction() method. Some devices provide a navigation control for moving around the screen, while some devices use the number keys 2, 4, 6, and 8. To find out which game action key was pressed, the Canvas class encapsulates this information and provides it in the form of the game actions. All you, as a developer, need to do is to grab the key code pressed by the user in the right method, and use the getGameAction(int keyCode) method to determine if the key pressed corresponds to a game action. As you can guess, several key codes can correspond to one game action, but a single key code may map to, at most, a single game action.
Listing 9 extends the original code from Listing 7 to add key code handling. In this listing, the square in the middle of the screen is moved around with the help of the navigation buttons.
Package com.j2me.part2;
import javax.microedition.lcdui.Canvas;
import javax.microedition.midlet.MIDlet;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Graphics;
public class CanvasExample
extends MIDlet {
Canvas myCanvas;
public CanvasExample() {
myCanvas = new MyCanvas();
}
public void startApp() {
Display display = Display.getDisplay(this);
// remember, Canvas is a Displayable so it can
// be set on the display like Screen elements
display.setCurrent(myCanvas);
// force repaint of the canvas
myCanvas.repaint();
}
public void pauseApp() {
}
public void destroyApp(boolean unconditional) {
}
}
class MyCanvas extends Canvas {
public void paint(Graphics g) {
// create a 20x20 black square in the center
// clear the screen first
g.setColor(0xffffff);
g.fillRect(0, 0, getWidth(), getHeight());
g.setColor(0x000000); // make sure it is black
// draw the square, changed to rely on instance variables
g.fillRect(x, y, 20, 20);
}
public void keyPressed(int keyCode) {
// what game action does this key map to?
int gameAction = getGameAction(keyCode);
if(gameAction == RIGHT) {
x += dx;
} else if(gameAction == LEFT) {
x -= dx;
} else if(gameAction == UP) {
y -= dy;
} else if(gameAction == DOWN) {
y += dy;
}
// make sure to repaint
repaint();
}
// starting coordinates
private int x = getWidth()/2 - 10;
private int y = getHeight()/2 - 10;
// distance to move
private int dx = 2;
private int dy = 2;
}
Listing 9. Handling key events to move the square
Notice that in this listing, the code to paint the square has been modified to rely upon instance variables. The keyPressed() method has been overridden and therefore, whenever the user presses a key, this method is invoked. The code checks if the key pressed was a game key, and based on which game key was pressed, changes the coordinates of the square accordingly. Finally, the call to repaint() in turn calls the paint() method, which moves the square on the screen as per the new coordinates.
In this article, you created the UI elements and were introduced to much of the user interface APIs for MIDlets. In the next installment, you will learn to use the Gaming API of MIDP 2.0 present in the package javax.microedition.lcdui.game.
