Develop Reference

onDraw not working properly. It only shows for one second and then dissapears

I am having problem with drawing. It only shows the elements it should draw for a second and then it is empty again. I am drawing in android surface view. Here is the code:
private void drawSurface(Canvas canvas) {
canvas.drawColor(Color.YELLOW);
for (int i = 0; i < points.size(); ++i) {
System.out.println(i);
point2 = points.get(i);
canvas.drawCircle(point2.x, point2.y, RADIUS, paint);
canvas.drawText(characters.get(i), point2.x, point2.y, tpaint);
}}
Here is where I call this method:
public void surfaceCreated(SurfaceHolder holder) {
// TODO Auto-generated method stub
addPoints();
Canvas canvas = null;
canvas = surfaceHolder.lockCanvas();
if (null != canvas) {
System.out.println("Created");
drawSurface(canvas);
surfaceHolder.unlockCanvasAndPost(canvas);
}
}

Draw multiple rectangles android canvas

I'm trying to draw 4 rectangles on the canvas so that the canvas is divided in 4 equal rectangles. With the code I now have, only the last rectangle in my code is drawn.
This is the code in my Activity:
protected void onCreate(Bundle savedInstanceState) {
...
setContentView(new MyView(this));
}
public class MyView extends View {
public MyView(Context context) {
super(context);
// TODO Auto-generated constructor stub
setFocusableInTouchMode(true);
}
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
int x = getWidth();
int y = getHeight();
Paint paintTopLeft = new Paint();
paintTopLeft.setStyle(Paint.Style.FILL);
paintTopLeft.setColor(Color.WHITE);
canvas.drawPaint(paintTopLeft);
// Use Color.parseColor to define HTML colors
paintTopLeft.setColor(Color.parseColor("#F44336"));
canvas.drawRect(0,0,x / 2,y / 2,paintTopLeft);
Paint paintTopRight = new Paint();
paintTopRight.setStyle(Paint.Style.FILL);
paintTopRight.setColor(Color.WHITE);
canvas.drawPaint(paintTopRight);
// Use Color.parseColor to define HTML colors
paintTopRight.setColor(Color.parseColor("#2196F3"));
canvas.drawRect(x / 2, 0, x, y / 2, paintTopRight);
}
}
What am I doing wrong?

Actually I see only two rectangles that are drawn with your code. But anyway, the problem is that you are calling canvas.drawPaint which clears/fills the complete canvas with that color. So you are erasing all rectangles that have been drawn already just before you draw the last one.
This code should work:
protected void onCreate(Bundle savedInstanceState) {
...
setContentView(new MyView(this));
}
public class MyView extends View {
public MyView(Context context) {
super(context);
// TODO Auto-generated constructor stub
setFocusableInTouchMode(true);
}
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
int x = getWidth();
int y = getHeight();
Paint paintTopLeft = new Paint();
paintTopLeft.setStyle(Paint.Style.FILL);
paintTopLeft.setColor(Color.WHITE);
//canvas.drawPaint(paintTopLeft); // don't do that
// Use Color.parseColor to define HTML colors
paintTopLeft.setColor(Color.parseColor("#F44336"));
canvas.drawRect(0,0,x / 2,y / 2,paintTopLeft);
Paint paintTopRight = new Paint();
paintTopRight.setStyle(Paint.Style.FILL);
paintTopRight.setColor(Color.WHITE);
// canvas.drawPaint(paintTopRight); // don't do that
// Use Color.parseColor to define HTML colors
paintTopRight.setColor(Color.parseColor("#2196F3"));
canvas.drawRect(x / 2, 0, x, y / 2, paintTopRight);
}
}

animate a Rectangle moving smoothly android view

I have multiple rectangles in the middle of the view (canvas). I want to move them all to the edges of the screen smoothly simultaneously. This is not a jump, i need to see the between positions from start to end so it appear as a smooth transition.
Whats the best way to do that?

Try this code :
public class animatedView extends View {
Bitmap i;
int x=0;
int y=0;
public animatedView(Context context) {
super(context);
// TODO Auto-generated constructor stub
i = BitmapFactory.decodeResource(getResources(), R.drawable.ic_launcher);
}
#Override
protected void onDraw(Canvas canvas) {
// TODO Auto-generated method stub
super.onDraw(canvas);
Rect bk = new Rect();
bk.set(0, 0, canvas.getWidth(), canvas.getHeight());
Paint pBk = new Paint();
pBk.setStyle(Paint.Style.FILL);
pBk.setColor(Color.BLUE);
canvas.drawRect(bk, pBk);
canvas.drawBitmap(i, x, y, pBk);
x+=10;
invalidate();
}
}
Hope it helps !

How to draw a circle in android?

I'm using this method to draw a circle in my app:
public void drawCircle(){
paint.setColor(Color.rgb(52, 73, 94));
paint.setFlags(Paint.ANTI_ALIAS_FLAG);
canvas.drawCircle(100, 200, 60, paint);
}
The problem is that the circle appears kind of oval and with pixellated edges.
I used the ANTI_ALIAS_FLAG but it didn't work.
How to draw a circle with smooth edges?
EDIT
I using a framework for games. from this book: http://www.amazon.com/Beginning-Android-Games-Mario-Zechner/dp/1430246774

Try This
#Override
public void onDraw(Canvas canvas){
super.onDraw(canvas);
canvas.drawCircle(x, y, radius, paint);
}
For a better reference on drawing custom views check out the official Android documentation
Good Luck!

#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(new MyView(this));
}
public class MyView extends View {
public MyView(Context context) {
super(context);
// TODO Auto-generated constructor stub
}
#Override
protected void onDraw(Canvas canvas) {
// TODO Auto-generated method stub
super.onDraw(canvas);
int x = getWidth();
int y = getHeight();
int radius;
radius = 100;
Paint paint = new Paint();
paint.setStyle(Paint.Style.FILL);
paint.setColor(Color.WHITE);
canvas.drawPaint(paint);
// Use Color.parseColor to define HTML colors
paint.setColor(Color.parseColor("#CD5C5C"));
canvas.drawCircle(x / 2, y / 2, radius, paint);
}
}

Android: Canvas not scaling

I have a custom View which draw some bitmaps on canvas in onDraw(). The drawing is working. Look at the code snippet:
#Override
protected void onDraw(Canvas canvas) {
// TODO Auto-generated method stub
super.onDraw(canvas);
canvas.save();
if(appInitialized) {
hsSide.draw(canvas);
scaleA.draw(canvas);
scaleB.draw(canvas);
}
canvas.scale(mScaleFactor, mScaleFactor, pivotX, pivotY);
canvas.restore();
}
I have implemented a ScaleGestureListener which scale the canvas when pinch zoom is applied on screen. The problem is that canvas is not scaling. if I put canvas.scale(mScaleFactor, mScaleFactor, pivotX, pivotY); before if(appInitialized) { it scales the canvas but after it the drawing i.e. hsSide.draw(canvas); is not scaling. It draws on its normal scale.
Here are draw methods:
hsDide.draw:
#Override
public void draw(Canvas canvas) {
// TODO Auto-generated method stub
canvas.drawBitmap(mBitmap, canvas.getWidth()/2 - mBitmap.getWidth()/2, 0, null);
}
scaleA.draw:
#Override
public void draw(Canvas canvas) {
// TODO Auto-generated method stub
canvas.drawBitmap(mBitmap, canvas.getWidth()/2 - mBitmap.getWidth()/2, canvas.getHeight()/2 - mBitmap.getHeight()/2, null);
}
scaleB.draw:
#Override
public void draw(Canvas canvas) {
// TODO Auto-generated method stub
canvas.save();
mMatrix.setRotate(currentAngle, canvas.getWidth()/2, canvas.getHeight()/2);
canvas.setMatrix(mMatrix);
canvas.drawBitmap(mBitmap, canvas.getWidth()/2 - mBitmap.getWidth()/2, canvas.getHeight()/2 - mBitmap.getHeight()/2, null);
canvas.restore();
}
Any idea where I am going wrong...???

Looking at your code infact, you should scale the canvas before drawing onto it, then restore it once drawn. So move your canvas scale as follows:
#Override
protected void onDraw(Canvas canvas) {
// TODO Auto-generated method stub
super.onDraw(canvas);
canvas.save();
canvas.scale(mScaleFactor, mScaleFactor, pivotX, pivotY); // MOVED HERE.
if(appInitialized) {
hsSide.draw(canvas);
scaleA.draw(canvas);
scaleB.draw(canvas);
}
canvas.restore();
}
You are then basing the draw size on the canvas size. So when you scale the canvas, you are still drawing it to its entire size, then resizing back to normal size. Hence not getting any effect.
What you need do is take the canvas size BEFORE scaling, then pass this to your draw method too. Then use those sizes to draw onto canvas.