Android - 简单路径，使用arcTo会导致“UnsupportedOperationException？”

Question

我正在尝试在我创建的SurfaceView对象的onDraw方法中创建一个简单路径，并且每当我向路径添加arcTo命令时，布局编辑器都会给我“UnsupportedOperationException：null”。

我该如何解决这个问题？

谢谢，

赖安

public class myView extends SurfaceView
{
    private Path myPath;

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

    @Override
    protected void onDraw(Canvas canvas)
    {
        super.onDraw(canvas);

        //set bounds
        final int top = 0 +this.getPaddingTop();
        final int bottom = this.getMeasuredHeight() - this.getPaddingBottom();
        final int left = 0 + this.getPaddingLeft();
        final int right = this.getMeasuredWidth() - this.getPaddingRight();
                int someHeight;
                int someWidth;
                innerLeft = new RectF(left + someWidth, bottom - otherHeight,
                       left + someHeight + 10 * 2, bottom);

        myPath.moveTo(left, bottom);
        myPath.lineTo(left, top + 10);
        myPath.arcTo(innerLeft, 180, 90);
        myPath.close();
    }
}