How to render Custom View from it's instance in android?

Question

I am a web dev and very new to android dev and OOP so I might be making a mistake in how I approach this problem. This seems like a very simple problem.

So I made a custom view called GridView that contains a setter function where I can set a grid to that class which basically looks like this.

GridView.java

public class GridView extends View {
    public Grid grid;
    public GridView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public void setGrid(Grid grid) {
        this.grid = grid;
        invalidate();
    }
}

Then it renders that grid on the android canvas with the onDraw function.

---

I create an instance of this GridView in my android activity so I can use it here. This is basically how that looks.

MyActivity.java

public class MyActivity extends AppCompatActivity {

    public GridView gridView;
    public Grid grid;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        grid = new Grid(20, 20);

        gridView = new GridView(this, null);
        gridView.setGrid(grid);
        setContentView(R.layout.my_activity);
    }
}

---

I have listed that GridView in the my_activity.xml file like this:

my_activity.xml

        <view
            android:id="@+id/gridView"
            class="com.dev.myapp.GridView"
            android:layout_width="match_parent"

            android:layout_height="0dp"
            android:layout_weight="4"
            android:background="@android:color/transparent"
            android:fadingEdge="vertical"
            android:gravity="top"
            android:padding="5dp"
            android:scrollbars="vertical" />

---

Problem

The problem is that the grid variable is null inside the GridView class when I create an instance of GridView in MyActivity.java and pass in the grid in the setGrid setter function.

However, it works perfectly fine when I create the grid instance directly in the GridView constructor like this:

GridView.java (Working)

public class GridView extends View {
    public Grid grid;
    public GridView(Context context, AttributeSet attrs) {
        super(context, attrs);
        grid = new Grid(20, 20);
    }
}

---

What I think:

I think the problem might be because I am directly rendering the GridView class on the my_activity.xml file so it is rendering whatever is in that particular file instantly and it is not rendering the instance of that GridView that I have created in the MyActivity.java .

How am I supposed to render the instance of the GridView that I create in MyActivity.java on the my_activity.xml ?

Answer 1

<com.dev.myapp.GridView
    android:id="@+id/gridView"
    ... />

val gv = findViewById<GridView>(R.id.gridView)
gv.setGrid(grid)