How to use canvas in your android apps - Part 1

Mon, 04/26/2010 - 01:37 | by jozsi

SDK Version:

If you want to use a custom layout with a canvas in your application, this tutorial may be useful to you. We will draw a kangoo at the end of this tutorial.

There is a cool article on site Custom View - HorizontalSlider, what you can use to understand the following code:

[...]
<com.helloandroid.canvastutorial.Panel android:id="@+id/SurfaceView01" android:layout_width="wrap_content" 
 
android:layout_height="wrap_content" android:maxHeight="40dip">
</com.helloandroid.canvastutorial.Panel>
[...]

This is your main.xml.

Your main class extends Activity and similar as the following code snip:

public class Canvastutorial extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }
}

Your second class called Panel.java. This class extends SurfaceView and implements SurfaceHolder.Callback. The interface requires three additional methods we need to implement: surfaceCreated(), surfaceDestroyed(), surfaceChanged().

class Panel extends SurfaceView implements SurfaceHolder.Callback {
    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
        // TODO Auto-generated method stub
    }
 
    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        // TODO Auto-generated method stub
    }
 
    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        // TODO Auto-generated method stub
    }
}

Add this constructor to code to inicialize the panel:

       public Panel(Context context, AttributeSet attrs) {
                super(context, attrs); 
                // TODO Auto-generated constructor stub
            getHolder().addCallback(this);
            canvasthread = new CanvasThread(getHolder(), this);
            setFocusable(true);
        }

And modify the implemented methods to set your threads:

        @Override
        public void surfaceChanged(SurfaceHolder holder, int format, int width,
                        int height) {
                // TODO Auto-generated method stub
                
        }
        @Override
        public void surfaceCreated(SurfaceHolder holder) {
                // TODO Auto-generated method stub
            canvasthread.setRunning(true);
            canvasthread.start();
 
                
        }
        @Override
        public void surfaceDestroyed(SurfaceHolder holder) {
                // TODO Auto-generated method stub
                boolean retry = true;
                canvasthread.setRunning(false);
                while (retry) {
                        try {
                                canvasthread.join();
                                retry = false;
                        } catch (InterruptedException e) {
                                // we will try it again and again...
                        }
                }
 
        }

Add this class to your project to create threads, which will control the drawings. We need a constructor with the Panel and the SurfaceHolder as parameters, a setter to set the variable which keeps the thread running and we need to override the method run().:

package com.helloandroid.canvastutorial;
 
import android.graphics.Canvas;
import android.view.SurfaceHolder;
 
public class CanvasThread extends Thread {
    private SurfaceHolder _surfaceHolder;
    private Panel _panel;
    private boolean _run = false;
 
    public CanvasThread(SurfaceHolder surfaceHolder, Panel panel) {
        _surfaceHolder = surfaceHolder;
        _panel = panel;
    }
 
    public void setRunning(boolean run) {
        _run = run;
    }
 
    @Override
    public void run() {
        Canvas c;
        while (_run) {
            c = null;
            try {
                c = _surfaceHolder.lockCanvas(null);
                synchronized (_surfaceHolder) {
                    _panel.onDraw(c);
                }
            } finally {
                // do this in a finally so that if an exception is thrown
                // during the above, we don't leave the Surface in an
                // inconsistent state
                if (c != null) {
                    _surfaceHolder.unlockCanvasAndPost(c);
                }
            }
        }
    }
}

The last thing, add an onDraw method to your panel:

        @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, 10, 10, null);
                
        }
         

If you need my image, you can download from here:

Copy this image to drawable folder and refresh your project.

It will draw a kangoo. :) Next time I will show you some drawing commands. Feel free to download the source code from Here.

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Comments

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If I were to add the above

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If I were to add the above code to a program that has multiple layouts 1 main layout and horizontal and a vertical layout. How does the above code determine which layout to use? Not sure if this is able to work that way?
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Merci à l'information reçue,

Denas | Fri, 04/22/2011 - 22:08.

In this way i can use canvas

ppas | Wed, 04/20/2011 - 20:32.

In this way i can use canvas on my android smartphone, thanks a lot!

Mercedes

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If I add f.e. ImageView to

Will | Fri, 02/04/2011 - 04:29.

If I add f.e. ImageView to main.xml I can simply get to it from Canvastutorial class by
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How can I do this from Panel class?

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ask about declaration

asnl1987 | Mon, 07/26/2010 - 03:51.

I'm new for android.
when I read your code I don't understand this declaration in xml file

com.helloandroid.canvastutorial.Panel
android:id="@+id/SurfaceView01" android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:maxHeight="40dip">
com.helloandroid.canvastutorial.Panel

what does "com.helloandroid.canvastutorial.Panel " mean, I've never seen the declaration like this before?
Can you explain how the program run?

Thanks you very much

RE: ask about declaration

MatthiasW | Tue, 09/07/2010 - 08:52.

hey asbk1987

com.helloandroid.canvastutorial is your project name and the Panel is the class you want to show in that project.

greets
MatthiasW

How to use canvas in your android apps - Part 1

Comments

دردشة سورية دردشة

شات صوتي | دردشة صوتية | كلام

thanks

If I were to add the above

nono

Merci à l'information reçue,

In this way i can use canvas

You can make use of many

Thank alot for this code and

Completely I share your opini

تسريحات منتديات مكياج ازياء

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Awesome blog. I enjoyed

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If I add f.e. ImageView to

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if I test this program on a screen that has multiple layouts whe

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Thank alot for this code and

ask about declaration

RE: ask about declaration

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