如何在画布上绘画(android)

public class MyDrawingView extends View {
    private Paint paint;

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

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

    private void init() {
        paint = new Paint();
        paint.setColor(Color.RED);
        paint.setStrokeWidth(5);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        
        // 绘制一个矩形
        canvas.drawRect(100, 100, 300, 300, paint);
        
        // 绘制一个圆形
        canvas.drawCircle(500, 200, 100, paint);
        
        // 绘制一条直线
        canvas.drawLine(700, 100, 900, 300, paint);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        // 处理触摸事件，实现绘图的交互效果
        return super.onTouchEvent(event);
    }
}

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        // 获取布局中的自定义View
        MyDrawingView drawingView = findViewById(R.id.drawing_view);
    }
}

<com.example.myapp.MyDrawingView
    android:id="@+id/drawing_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />