Android PathShape无法绘制路径（Canvas / Java / Android Studio）

Question

我只是想在Android Studio中绘制一个基本的三角形，但是运行该程序时却什么也没显示。这是我的第一个Android程序，所以我真的不知道自己在做什么。怎么了？

    public class CustomDrawableView extends View {
    private ShapeDrawable mDrawable;

    public CustomDrawableView(Context context, Display display) {
        super(context);

        Point size = new Point();
        display.getSize(size);

        int screenWidth = size.x;
        int screenHeight = size.y;
        int pathWidth = screenWidth / 6;
        int startWidth = screenWidth / 10;
        Path path = new Path();
        path.moveTo(startWidth, 0);
        path.moveTo(startWidth, screenHeight / 5);
        path.moveTo(screenWidth / 5, screenHeight / 5);
        path.close();
        mDrawable = new ShapeDrawable(new PathShape(path, screenWidth, screenHeight));
        mDrawable.setDither(true);
        int color = Color.parseColor("#ff74AC23");
        mDrawable.getPaint().setColor(color);
        mDrawable.setBounds(0, 0 , screenWidth, screenHeight);
    }

    protected void onDraw(Canvas canvas) {
        mDrawable.draw(canvas);
    }
   }



 public class MainActivity extends AppCompatActivity {
    CustomDrawableView mCustomDrawableView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Display display = getWindowManager().getDefaultDisplay();
        mCustomDrawableView = new CustomDrawableView(this, display);
        setContentView(mCustomDrawableView);
    }
   }