How to use canvas in your android apps - Part 2

Wed, 04/28/2010 - 08:01 | by jozsi

SDK Version:

In the second part of this tutorial series, I will show you what you can do with canvas and paint.

If you complete the first part, your onDraw method in Panel class is similar as this:

        @Override
        public void onDraw(Canvas canvas) {
                
                Paint paint = new Paint();
                
 
                Bitmap kangoo = BitmapFactory.decodeResource(getResources(),
                                R.drawable.kangoo);
                canvas.drawColor(Color.BLACK);
                canvas.drawBitmap(kangoo, 130, 10, null);
                
        }
         

We will use the the Paint type object this time. You can use it to draw lines, circles and text on your canvas. For example this two lines will draw a red circle for you.

paint.setColor(Color.RED);
canvas.drawCircle(20, 50, 25, paint);

A blue circle with antialiasing turned on:

        paint.setAntiAlias(true);
        paint.setColor(Color.BLUE);
        canvas.drawCircle(60, 50, 25, paint);

Now we will create some triangles. Create a Path object to store our triangle's segments. Use .offset to draw in many locations.

                        paint.setStyle(Paint.Style.FILL_AND_STROKE);
                        paint.setStrokeWidth(2);
                        paint.setColor(Color.GREEN);
                        Path path = new Path();
                        path.moveTo(4, -10);
                        path.lineTo(20, 0);
                        path.lineTo(-9, 0);
                        path.close();
                        path.offset(60, 40);
                        canvas.drawPath(path, paint);
                        path.offset(90, 100);
                        canvas.drawPath(path, paint);
                        path.offset(80, 150);
                        canvas.drawPath(path, paint);

Now draw a text using FILL style:

                        paint.setStyle(Paint.Style.FILL);
                        paint.setAntiAlias(true);
                        paint.setTextSize(20);
                        canvas.drawText("Hello Android! Fill...", 50, 230, 
 
paint);

You can rotate the text, and you can draw it where you want:

        int x = 75;
        int y = 185;
        paint.setColor(Color.GRAY);
        paint.setTextSize(25);
        String rotatedtext = "Rotated helloandroid :)";

Draw bounding rect before rotating text:

Rect rect = new Rect();
paint.getTextBounds(rotatedtext, 0, rotatedtext.length(), rect);
canvas.translate(x, y);
paint.setStyle(Paint.Style.FILL);
 
canvas.drawText("Rotated helloandroid :)", 0, 0, paint);
paint.setStyle(Paint.Style.STROKE);
canvas.drawRect(rect, paint);
 
canvas.translate(-x, -y);
 
                        
paint.setColor(Color.RED);
canvas.rotate(-45, x + rect.exactCenterX(),y + rect.exactCenterY());
paint.setStyle(Paint.Style.FILL);
canvas.drawText(rotatedtext, x, y, paint);
                        

Now draw a draw a thick dashed line in angle:

DashPathEffect dashPath = new DashPathEffect(new float[]{10,40}, 1);
paint.setPathEffect(dashPath);
paint.setStrokeWidth(8);
canvas.drawLine(0, 60 , 320, 300, paint);

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