Can I use the Canvas element in other functions to display on the view

Question content

I created a layout that contains some views and also contains a linearlayout for drawing graphics

main_page_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.constraintlayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/bgcolor"
    android:id="@+id/main_page_layout"
    >

....

    <linearlayout
        android:id="@+id/graphicview"
        android:layout_width="match_parent"
        android:layout_height="196dp"
        android:layout_margintop="64dp"
        android:layout_marginleft="0dp"
        android:layout_marginright="0dp"
        android:background="@color/white"
        android:orientation="horizontal"
        app:layout_constraintend_toendof="parent"
        app:layout_constraintleft_toleftof="parent"
        app:layout_constrainttop_tobottomof="@id/menulayout" />

...

</androidx.constraintlayout.widget.constraintlayout>

I'm trying to draw a graph on it using a custom class called graphplotter

graphplotter.java

public class graphplotter extends view {
    private paint paint = null;
    private bitmap bitmap;
    private canvas canvas;

 public graphplotter(context context) {
        super(context);
        this.init();
    }

    /* initiates graph plotter */
    private void init()
    {
        this.paint = new paint();
        this.paint.setcolor(color.blue);
        this.paint.setstrokewidth(5);
        this.paint.setstyle(paint.style.fill_and_stroke);

        // create a bitmap and a canvas associated with the bitmap
        this.bitmap = bitmap.createbitmap(1000, 1000, bitmap.config.argb_8888);
        this.canvas = new canvas(bitmap);

    }
    public void setcolor(int color) {
        this.paint.setcolor(color);
        invalidate(); // force a redraw
    }
    /* draws a line with a custom color */
    public void drawline(float starx, float starty, float stopx, float stopy,int color)
    {
        this.setcolor(color);
        this.canvas.drawline(starx, starty, stopx, stopy, this.paint);
    }

........

This is the activity I want to draw

mainpageactivity.java

public class MainPageActivity extends AppCompatActivity {
        private ConnectionHelper connectionHelper = null;
        private SQLQuery sqlQuery = null;
        private Person person = null;
        private TextView txtID = null;
        private RecyclerView recyclerView = null;
        private final ProductAdapter productAdapter = null;
        private AsyncTaskRunner asyncTaskRunner = null;
        private ViewHolder viewHolder = null;
        private Spinner graphicTypeSpinner = null;
        private Button menuButton = null;
        private ConstraintLayout menuView  = null;
        private List<Product> productList  = null;
        private GraphPlotter graphPlotter = null;



        @Override
        protected void onCreate(Bundle savedInstanceState){

            super.onCreate(savedInstanceState); // necessary for onCreate function
            setContentView(R.layout.main_page_layout); // R = resource , R.layout => layouts directory

            init();


            LinearLayout graphicView = (LinearLayout) findViewById(R.id.graphicView);
            graphPlotter = new GraphPlotter(this);

            graphPlotter.drawAxisX(159,159,259, Color.BLUE);

            graphicView.addView(graphPlotter);

}

           ...........
...................

But I can't see any lines or axes.

I want to draw a graphic for a product using canvas but I can't see any results, what's wrong with my class or code

Correct answer

will draw the behavior Put it in the ondraw() function

@Override
protected void onDraw(Canvas canvas) {
    drawLine(...);
    drawAxisX();
}

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