Develop Reference

How to call onDraw(Canvas canvas) function of CustomView in Android?

I generate an ellipse and I try to reshape with respect to sweepAngle_speed that u can see below. This sweepAngle_speed come from MainActivity.java. In MainActivity.java, I create a seekbar and I use an algorithm between value of seekbar and sweepAngle_speed, therefore I expected a change in filled area in my ellipse. onDraw function is not called directly, so I use invalidate function in my getLog function which is created by me. However I cannot call onDraw function anyway. When I run the code, onDraw function is called directly by system 3 times, however when I change seekbar value, I do not call onDraw function anyway. My first question is that How onDraw function is called directly by system ? The second one is how I can call onDraw function during system is working. Thanks.
CustomView.java
public CustomView(Context context, #Nullable AttributeSet attrs) {
super(context);
m_Context = context;
getLog();
// create the Paint and set its color
}``
#Override
protected void onDraw(Canvas canvas) {
//c=canvas;
//super.onDraw(c);
Paint p1 = new Paint();
RectF rectF = new RectF(-750, 0, 750, 720);
//p1.setColor(Color.parseColor("#34ebe2"));
p1.setShader(new LinearGradient(0, -360, 0, getHeight(), Color.CYAN, Color.BLUE, Shader.TileMode.MIRROR));
Log.d(TAG, "CANVAS: onDraw içine girdi ve Speed angle: " + sweepAngle_speed);
canvas.drawArc(rectF, 90, -sweepAngle_speed, true, p1);
}
public void getLog () {
paint = new Paint();
paint.setColor(Color.BLUE);
Log.d(TAG, "Speed geldi buraya ve invalidate yaptı");
setWillNotDraw(false);
//this.invalidate();
this.invalidate();
}
}

You can't call onDraw directly. You can use the invalidate method which will redraw it
You can make a function inside CustomView class e.g:
public void setSpeed(int sweepAngle_speed){
this. sweepAngle_speed = sweepAngle_speed;
invalidate(); // This invalidate will call onDraw and draw your view again
}

If you extend ViewGroup so you need to call setWillNotDraw(false) in the constructor of your ViewGroup

Canvas DrawLine Is Invisible

I am trying to draw a single line in Android using canvas
My class :
public class LineDrawer extends View {
public LineDrawer(Context context) {
super(context);
}
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
Paint paint = new Paint();
paint.setColor(Color.BLACK);
paint.setStyle(Paint.Style.FILL_AND_STROKE);
paint.setStrokeWidth(10);
float left = 20;
float top = 20;
float right = 50;
float bottom = 100;
canvas.drawLine(left, top, right, bottom, paint);
}
}
My Main Activity :
public class MainActivity extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
LineDrawer lineDrawer = new LineDrawer(this);
setContentView(R.layout.activity_Main);
}
}
I cannot find where is the problem , I try all the solutions in the internet but nothing happen , still a blank activity..
Should I import some code ?

lineDrawer is created but not added anywhere. Just creating a view is not enough, you need to add it to the current displayed views to be taken into account and drawn. You have two options:
Add it to your XML layout. You will have to add the following constructor to your custom view.
public LineDrawer(Context context, AttributeSet attrs) {
super(context, attrs);
}
Use addView(). Anyway, given how simple is your example, I'll use first (common) method.
As an additional comment, the Paint paint object should be created on view initialization, as is a costly operation. See in the original documentation for more information about this.

Drawing over a view and all it's children

I'm trying to apply a visual effect to a viewgroup. My idea is to grab a bitmap of the viewgroup, shrink it down, expand it back up, and draw it over the viewgroup to give it a blocky, low quality effect.
I've got most of the way there using this code:
public class Blocker {
private static final float RESAMPLE_QUALITY = 0.66f; // less than 1, lower = worse quality
public static void block(Canvas canvas, Bitmap bitmap_old) {
block(canvas, bitmap_old, RESAMPLE_QUALITY);
}
public static void block(Canvas canvas, Bitmap bitmap_old, float quality) {
Bitmap bitmap_new = Bitmap.createScaledBitmap(bitmap_old, Math.round(bitmap_old.getWidth() * RESAMPLE_QUALITY), Math.round(bitmap_old.getHeight() * RESAMPLE_QUALITY), true);
Rect from = new Rect(0, 0, bitmap_new.getWidth(), bitmap_new.getHeight());
RectF to = new RectF(0, 0, bitmap_old.getWidth(), bitmap_old.getHeight());
canvas.drawBitmap(bitmap_new, from, to, null);
}
}
I simply pass in the canvas to draw on and a bitmap of what needs to be scaled down+up and it works well.
public class BlockedLinearLayout extends LinearLayout {
private static final String TAG = BlockedLinearLayout.class.getSimpleName();
public BlockedLinearLayout(Context context, AttributeSet attrs) {
super(context, attrs);
applyCustomAttributes(context, attrs);
setup();
}
public BlockedLinearLayout(Context context) {
super(context);
setup();
}
private void setup() {
this.setDrawingCacheEnabled(true);
}
#Override
public void draw(Canvas canvas) {
super.draw(canvas);
// block(canvas); If I call this here, it works but no updates
}
#Override
public void onDraw(Canvas canvas) {
// block(canvas); If I call this here, draws behind children, still no updates
}
private void block(Canvas canvas) {
Blocker.block(canvas, this.getDrawingCache());
}
}
The problem I'm having is in my viewgroup. If I run the block method in the viewgroup's draw, it draws over everything but doesn't ever update when child views change. I've traced function calls with Log, and the draw method seems to be running, but nothing changes.
I've also tried implementing this in onDraw. This draws the bitmap behind all the children views, and again they aren't updating.
Can anyone explain how I would go about fixing this?

Try this:
#Override
protected void dispatchDraw(Canvas canvas) {
// call block() here if you want to draw behind children
super.dispatchDraw(canvas);
// call block() here if you want to draw over children
}
And call destroyDrawingCache() and then, buildDrawingCache() each time you change a child.

Draw() method will work well for you.
I'm now trying to make a count time view in a circle shape, when time is passing, the view will reducing his angle. It's used to cover profile photo(a circle shape photo).

Starting with Android API 23, you can use onDrawForeground(Canvas) to draw on top of child views: https://developer.android.com/reference/android/view/View#onDrawForeground(android.graphics.Canvas)
Unlike onDraw() though, be sure to call through to the super class:
#Override
public void onDrawForeground(final Canvas canvas) {
super.onDrawForeground(canvas);
// Your code here
}

Drawing with a canvas over an ImageView

I want to create an ImageView and then draw text on top of the ImageView. I also need to be able to modify the text periodically. Currently I've created a custom view that extends ImageView. Then I overwrite onDraw() and use it to draw the text. Only problem then is that when I use my custom ImageView it doesn't draw the image, just the text.
public class BoardView extends ImageView
{
public BoardView(Context context)
{
super(context);
}
protected void onDraw(Canvas canvas)
{
Paint paint = new Paint();
setImageResource(R.drawable.board);
paint.setColor(Color.BLUE);
canvas.drawText(x.getName(), x.getX(), x.getY(), paint);
}
}

You should call the super.onDraw in your onDraw method before your draw code.

Drawing Bitmap in Android?

My project is about image processing in android.I have a bitmap that I have loaded from a resource file (an PNG image). I want to draw it. But I couldn't.
Here my code snippet:
mB = BitmapFactory.decodeResource(getResources(), R.drawable.picture);
Canvas c = new Canvas(mB);
Paint p = new Paint();
c.drawBitmap(mB,0,0,p);
it didn't work. Is the code true? .Is there any thing more that I must do?

You should use an ImageView instead and load it by
imageView.setImageResource(R.drawable.picture);
If you want to manually draw it with a Canvas, you have to use a canvas that is passed into a draw() method and implement a Custom View.
Update to add example CustomView:
public class CustomView extends View {
private Paint mPaint;
private Drawable mDrawable;
public CustomView(Context context, AttributeSet attrs) {
super(context, attrs);
mPaint = new Paint();
mDrawable = context.getResources().getDrawable(R.drawable.some_drawable);
}
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
mDrawable.draw(canvas);
}
}

There are a couple of things you are missing.
First, I think you're misunderstanding the Canvas(Bitmap b) constructor. The Bitmap passed in there is one that the Canvas will draw into. This could be just a new Bitmap that you constructed.
Second, it is recommended that you use the Canvas that is passed to you in your View's onDraw method. Presumably that View is one from your Activity, either fetched from your XML layout via findViewById or constructed and passed to setContentView in the Activity's onCreate() method.
So, you are going to have to subclass View and override the onDraw method to get your drawing done. Something like:
public class MyView extends View {
#Override
public void onDraw (Canvas c) {
Bitmap mB = BitmapFactory.decodeResource(this.getContext().getResources(), R.drawable.picture);
c.drawBitmap(mB, 0, 0, null);
}
}
Then, in your Activity, you'll need to create an instance of your new View and pass it to the Activity via setContentView:
public class MyActivity extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mv = new MyView(this);
setContentView(mv);
}
You can instead call the setContentView(View v, ViewGroup.LayoutParameters lp) overload if you want to set up the LayoutParameters.
I haven't tested any of this, but it should at least get you on the right path.