onDraw optimization, how to increase perfomace?

Question

I'm trying to draw on the my .png image, but it works very slow. If I'm drawing on the imageView without .png file it works fine. How can I solve this? As I understand I need to prevent "A LOT of drawings all the time", how can I realise this?

code:

public class Draw
    extends Activity{

DrawingView dv ;
private Paint       mPaint;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    dv = new DrawingView(this);
    setContentView(dv);
    mPaint = new Paint();
    mPaint.setAntiAlias(true);
    mPaint.setDither(true);
    mPaint.setColor(Color.GREEN);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setStrokeJoin(Paint.Join.ROUND);
    mPaint.setStrokeCap(Paint.Cap.ROUND);
    mPaint.setStrokeWidth(12);
}

public class DrawingView extends View {

    public int width;
    public  int height;
    private Bitmap  mBitmap;
    private Canvas  mCanvas;
    private Path    mPath;
    private Paint   mBitmapPaint;
    Context context;
    private Paint circlePaint;
    private Path circlePath;

    public DrawingView(Context c) {
        super(c);
        context=c;
        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);
        circlePaint = new Paint();
        circlePath = new Path();
        circlePaint.setAntiAlias(true);
        circlePaint.setColor(Color.BLUE);
        circlePaint.setStyle(Paint.Style.STROKE);
        circlePaint.setStrokeJoin(Paint.Join.MITER);
        circlePaint.setStrokeWidth(4f);


    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);

        mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mBitmap);

    }
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        Bitmap bm = BitmapFactory.decodeFile(Environment.getExternalStorageDirectory() + "/MANUAL/workflow" + "/img.png");
        int targetWidth  = bm.getWidth() / 1;
        int targetHeight = bm.getHeight() / 1;
        Matrix matrix = new Matrix();
        matrix.postScale(0.7f, 0.65f);
        Bitmap size = Bitmap.createBitmap(bm, 0, 0, targetWidth, targetHeight, matrix, true);
        Paint paint = new Paint();
        paint.setColor(Color.RED);
        canvas.drawBitmap(size, 0, 0, paint);

        canvas.drawBitmap( mBitmap, 0, 0, mBitmapPaint);
        canvas.drawPath( mPath,  mPaint);
        canvas.drawPath( circlePath,  circlePaint);
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }
    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
            mX = x;
            mY = y;

            circlePath.reset();
            circlePath.addCircle(mX, mY, 30, Path.Direction.CW);
        }
    }
    private void touch_up() {
        mPath.lineTo(mX, mY);
        circlePath.reset();
        // commit the path to our offscreen
        mCanvas.drawPath(mPath,  mPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
        }
        return true;
    }
}

Answer 1

Your onDraw() method should look like this:

@Override
protected void onDraw(Canvas canvas) {

    paint.setColor(Color.RED);
    canvas.drawBitmap(size, 0, 0, paint);
    canvas.drawBitmap( mBitmap, 0, 0, mBitmapPaint);
    canvas.drawPath( mPath,  mPaint);
    canvas.drawPath( circlePath,  circlePaint);
}

All Bitmap decoding and resizing should be done outside (in the constructor or only when the view is resized), there is no need to decode and resize it every time when the result will be same every time.