无法从列表中绘制点

Question

我正在使用 Xamarin.Android，我想从 List 中绘制圆圈，该圆圈由 TouchEvent 填充。这是我的代码：

public class Ceiling : View
{
    private List<PointF> _points;
    private Canvas _canvas;
    private Paint paint;

    protected Ceiling(IntPtr javaReference, JniHandleOwnership transfer) : base(javaReference, transfer)
    {
    }

    public Ceiling(Context context) : base(context)
    {
        _points = new List<PointF>();
        paint = new Paint
        {
            Color = Color.Red
        };
        paint.SetStyle(Paint.Style.Fill);

        Touch += OnTouch;
    }



    public Ceiling(Context context, IAttributeSet attrs) : base(context, attrs)
    {
    }

    public Ceiling(Context context, IAttributeSet attrs, int defStyleAttr) : base(context, attrs, defStyleAttr)
    {
    }

    public Ceiling(Context context, IAttributeSet attrs, int defStyleAttr, int defStyleRes) : base(context, attrs, defStyleAttr, defStyleRes)
    {
    }

    protected override void OnDraw(Canvas canvas)
    {
        base.OnDraw(canvas);
        if (_canvas == null)
        {
            _canvas = canvas;
            Background = new PaintDrawable(Color.AliceBlue);
            canvas.SetViewport(Width, Height);
        }

        base.OnDraw(canvas);
        if (_points.Count > 0)
        {
            foreach (var point in _points)
            canvas.DrawCircle(point.X, point.Y, 10, paint);
        }
    }

    private void OnTouch(object sender, TouchEventArgs e)
    {
        switch (e.Event.Action)
        {
            case MotionEventActions.Down:
            {
                var point = new PointF(e.Event.GetX(), e.Event.GetY());
                if (!_points.Contains(point))
                {
                    _points.Add(point);
                        _canvas.DrawCircle(point.X, point.Y, 10, paint);
                }
                break;
            }
        }

    }
}

只有在_points已经定义的情况下才有效。我怎样才能使这项工作？

Answer 1

您有多个构造函数，其中只有一个构造函数初始化私有变量。您确定调用的是构造函数吗（我假设这是从布局初始化的）？

在任何情况下，您最好将初始化移动到一个单独的方法中，这样您就可以从所有构造函数中调用它：

    protected Ceiling(IntPtr javaReference, JniHandleOwnership transfer) : base(javaReference, transfer)
    {
        // need to call init here as well?
    }

    public Ceiling(Context context) : base(context)
    {
        Init();
    }

    public Ceiling(Context context, IAttributeSet attrs) : base(context, attrs)
    {
        Init();
    }

    public Ceiling(Context context, IAttributeSet attrs, int defStyleAttr) : base(context, attrs, defStyleAttr)
    {
        Init();
    }

    public Ceiling(Context context, IAttributeSet attrs, int defStyleAttr, int defStyleRes) : base(context, attrs, defStyleAttr, defStyleRes)
    {
        Init();
    }

    private void Init()
    {
        _points = new List<PointF>();
        this.paint = new Paint
        {
            Color = Color.Red
        };
        this.paint.SetStyle(Paint.Style.Fill);

        Touch += OnTouch;
    }

无法从列表中绘制点

Can't draw points from List

c#

android

xamarin.android

xamarin