Invalidate does not call onDraw

Question

i need to by clicking on the button redraw the picture, but after method invalidate onDraw method is not called. But the call onDraw only happens after running the application.

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    final Draw draw = new Draw(this);

    Button button = (Button) findViewById(R.id.btn);
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            draw.setA(40);
            draw.setB(300);
            draw.invalidate();
        }
    });

}
}

Here is the code Draw.java

public class Draw extends View {

private Paint mPaint;
private int a;
private int b;

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

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

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

private void init() {
    mPaint = new Paint();
    mPaint.setColor(Color.BLACK);
    mPaint.setStrokeWidth(5);
    setWillNotDraw(false);
    a = 0;
    b = 0;
}

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    mPaint.setStyle(Paint.Style.FILL);
    mPaint.setColor(Color.WHITE);
    canvas.drawPaint(mPaint);

    mPaint.setColor(Color.BLACK);
    mPaint.setAntiAlias(true);
    canvas.drawLine(80, 50, 80, 500, mPaint);
    canvas.drawLine(80, 50, 70, 85, mPaint);
    canvas.drawLine(80, 50, 90, 85, mPaint);

    canvas.drawLine(80, 500, 500, 500, mPaint);
    canvas.drawLine(500, 500, 465, 510, mPaint);
    canvas.drawLine(500, 500, 465, 490, mPaint);

    mPaint.setTextSize(35);
    mPaint.setStrokeWidth(2);
    canvas.drawText("X", 480, 540, mPaint);
    canvas.drawText("Y", 45, 80, mPaint);
    canvas.drawText("0", 70, 540, mPaint);

    drawFunction(canvas, a, b);
}

public void drawFunction(Canvas canvas, int a, int b) { 

    mPaint.setColor(Color.BLACK);
    mPaint.setAntiAlias(true);
    canvas.drawLine(80, 500, a, b, mPaint);
}

public void setA(int a) {
    this.a = a;
}

public void setB(int b) {

    this.b = b;
}
}

main.xml

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <labs.example.function.Draw
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

    </labs.example.function.Draw>
</RelativeLayout>

Thanks in advance.

create id in xml and then use findviewbyId method to get Draw view rather than creating new one — USKMobility, Apr 21 '16 at 17:49

score 5 · Accepted Answer · answered Apr 21 '16 at 17:50

Add an id

<labs.example.function.Draw
    android:id="@id/draw"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

</labs.example.function.Draw>

Replace

 final Draw draw = new Draw(this);

by

 final Draw draw = (Draw) findViewById(R.id.draw); // need to refer to the view in xml.

If you use final Draw draw = new Draw(this); you need to add that view to your view hierachy.

score -1 · Answer 2 · edited May 23 '17 at 11:52

-1

Did you try to call setWillNotDraw(false). In this post you can see that by default all ViewGroup sub-classes do not call their onDraw method.

edited May 23 '17 at 11:52

Community

1
1

answered Apr 21 '16 at 18:17

AinisSK

306
1
10

extends view not viewgroup – Raghunandan Apr 21 '16 at 18:31

Invalidate does not call onDraw

2 Answers2