Cannot call custom draw method from another class in Android

Question

its my first time to ask a question here. I have this problem with my android app. I would like to create different draw methods inside my custom view class. There are three buttons corresponding to 3 different shapes. When a button is pressed it will draw its shape. But the app crashes when I try to call the custom draw from the MainActivity for me to test it.

MainActivity

import com.example.shapes.view.ShapesView;

public class MainActivity extends Activity {

ShapesView shapesview;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    shapesview = (ShapesView) findViewById(R.id.ShapesViewID);
    shapesview.DrawRectangle();
}

ShapesView

public class ShapesView extends View{

Canvas canvas;

public ShapesView(Context context, AttributeSet attrs) {
    super(context, attrs);
    // TODO Auto-generated constructor stub
}

@Override
protected void onDraw(Canvas canvas) {
    // TODO Auto-generated method stub
    super.onDraw(canvas);
    this.canvas = canvas;
}

public void DrawRectangle() {
    Paint mypaint = new Paint();
    mypaint.setColor(Color.BLUE);
    canvas.drawRect(30, 30, 200, 200, mypaint);
}
}

My XML layout file

<com.example.shapes.view.ShapesView android:id="@+id/ShapesViewID" android:layout_width="fill_parent" android:layout_height="fill_parent" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />`

Please do help! Thank you very much!

Answer 1

Look at method's name its onDraw() and not getCanvas(). The documentation also doesn't makes any claims about the Canvas provided.

After onDraw() is done, that canvas may be disposed, its bitmap/buffer may be re-cycled, who knows.

So, it is not safe to use the Canvas outside of this method. Draw what you want, but only inside onDraw() method.

If you want to trigger the View to re-draw, at some other time, call invalidate().

Example:

View class to render any shape:

public class ShapeView extends View {

    private Paint mPaint;
    private ShapeRenderer mRenderer;

    public ShapeView(Context context) {
        super(context);
        init();
    }

    public ShapeView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public ShapeView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    private void init(){
        mPaint = new Paint();
    }

    public void setPaintColor(int color){
        mPaint.setColor(color);
    }

    public void setPaintStrokeWidth(float width){
        mPaint.setStrokeWidth(width);
    }

    public void setRenderer(ShapeRenderer renderer) {
        mRenderer = renderer;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        if(mRenderer != null){
            mRenderer.drawShape(canvas,mPaint);
        }
    }

    public static interface ShapeRenderer{
        public void drawShape(Canvas canvas, Paint paint);
    }
}

A Class that draws a rectangle:

public class RectRenderer implements ShapeView.ShapeRenderer {
    @Override
    public void drawShape(Canvas canvas, Paint paint) {
        canvas.drawRect(0,0,100,100,paint);
    }
}

Draw a shape at runtime:

 myShapeView.setPaintColor(Color.GREEN);
 myShapeView.setPaintStroke(5f);
 myShapeView.setRenderer(new RectRenderer());
 myShapeView.invalidate();

Answer 2

If I understand your question you need a custom view (a button) which will draw itself differently based on some event. If that is the case then you need to respect the android's guidelines about the view drawing take a look here.

Now one possible solution for your case is to set some kind of flag about the state of your view and then use that flag when you are ready to draw. For example you can do this:

public class ShapesView extends View{

public int state = 0;

public ShapesView(Context context, AttributeSet attrs) {
    super(context, attrs);
    // TODO Auto-generated constructor stub
}

@Override
protected void onDraw(Canvas canvas) {
// TODO Auto-generated method stub
super.onDraw(canvas);
if (state == 1) {
    Paint mypaint = new Paint();
    mypaint.setColor(Color.BLUE);
    canvas.drawRect(30, 30, 200, 200, mypaint);
}
}

and then whenever you need to draw your view from an activity you can use the following:

myview.state = 1;
myview.invalidate();

In your code what you are doing is that you call a views function during the onCreate of your activity which in turn tries to use a null canvas because the onDraw method of your view has not be called during that time. Furthermore as others have pointed out you must not use a canvas object outside the onDraw method.

Hope this helps...

Answer 3

I changed your code with following.

public class ShapesView extends View {

    Paint mypaint;

    public ShapesView(Context context, AttributeSet attrs) {
        super(context, attrs);
        mypaint = new Paint();
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        mypaint.setColor(Color.BLUE);
        canvas.drawRect(30, 30, 200, 200, mypaint);
    }
}

When you want to draw, call shapesview.invalidate();

Answer 4

I think you must redesign your component, so it can draw itself in onDraw method. If you need some class that must contain Canvas, you can try to do something like this.

class Drawer {
  private Canvas canvas;

  public Drawer(Canvas canvas) {
    this.canvas = canvas;
  }

  public void DrawRectangle() {
    Paint mypaint = new Paint();
    mypaint.setColor(Color.BLUE);
    canvas.drawRect(30, 30, 200, 200, mypaint);
  }
}

On yours custom view you can do something like this.

public class ShapesView extends View{

  public ShapesView(Context context, AttributeSet attrs) {
    super(context, attrs);
  }

  @Override
  protected void onDraw(Canvas canvas) {
    // TODO Auto-generated method stub
    super.onDraw(canvas);
    Drawer drawer = new Drawer(canvas);
    drawer.DrawRectangle();
  }

  public void DrawRectangle() {
    Paint mypaint = new Paint();
    mypaint.setColor(Color.BLUE);
    canvas.drawRect(30, 30, 200, 200, mypaint);
  }
}