Day: June 18, 2012

If you’re animating your own widgets, you might find that the Android APIs are a bit limited. Is there an Android API to draw things directly to the screen? The answer is yes. Android offers us a class called Canvas. In this hack, we see how we can use the Canvas class to draw elements and animate them by creating a box that will bounce around screen.

First of all, let’s make sure we understand what the Canvas class is. A Canvas works for you as a pretense, or interface, to the actual surface upon which your graphics will be drawn—it holds all of your “draw” calls. Via the Canvas, your drawing is actually performed upon an underlying Bitmap, which is placed into the window.

So the Canvas class holds all the draw calls. We can create a View, override the onDraw() method, and start drawing primitives there. To make everything clearer, we’ll crate a DrawView class that will take care of drawing the box and updating its position. Since we don’t have anything else onscreen, we’ll make it the activity’s content view. Here’s the code for the Activity:

public class MainActivity extends Activity {
    private DrawView mDrawView;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Display display = getWindowManager().getDefaultDisplay(); #1
        mDrawView = new DrawView(this);
        mDrawView.height = display.getHeight();
        mDrawView.width = display.getWidth();

        setContentView(mDrawView); #2
    }
}

In #1, we use the WindowManager to get the screen width and height. These values will be used inside the DrawView to limit where to draw. In we set the DrawView as the Activity’s contentView. This means that the DrawView will take all the available space.

Let’s take a look of what’s done inside the DrawView class:

public class DrawView extends View {
    private Rectangle mRectangle;
    public int width;
    public int height;

    public DrawView(Context context) {
        super(context);

        mRectangle = new Rectangle(context, this); #1
        mRectangle.setARGB(255, 255, 0, 0);
        mRectangle.setSpeedX(3);
        mRectangle.setSpeedY(3);
    }

    @Override
    protected void onDraw(Canvas canvas) { #2
        mRectangle.move(); #3
        mRectangle.onDraw(canvas); #4
        invalidate();
    }
}

In #1, we create a Rectangle instance that will play the role of the box. The Rectangle class also knows how to draw itself to a canvas and contains all the boring logic of how to update its position to get drawn in the correct place. When the onDraw() method is called, we change the rectangle’s position (#2) and we draw it to the canvas (#3). The invalidate() call in #4 is the hack itself.

The invalidate() is a View’s method to force a view to draw. Placing it inside the onDraw() method means that onDraw() will be called as soon as the view finishes drawing itself. To put it differently, we’ll loop over the Rectangle ’s move() and onDraw() calls, creating a nice animation!

Summary

Updating views positions in the onDraw() method through the invalidate() call is an easy way to provide custom animations. If you’re planning to make a small game, using this trick is a simple way to handle the game’s main loop.

External links

• http://developer.android.com/reference/android/graphics/Canvas.html
• http://developer.android.com/guide/topics/graphics/2d-graphics.html

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