BoxView 的自定義渲染器

自定義渲染器幫助允許新增新屬性並在本機平臺中以不同方式呈現它們，這是通過共享程式碼無法實現的。在這個例子中，我們將半徑和陰影新增到 boxview。

首先，我們應該在 PCL 專案中建立自定義控制元件，它將宣告一些必需的可繫結屬性：

namespace Mobile.Controls
{
    public class ExtendedBoxView : BoxView
    {
        /// <summary>
        /// Respresents the background color of the button.
        /// </summary>
        public static readonly BindableProperty BorderRadiusProperty = BindableProperty.Create<ExtendedBoxView, double>(p => p.BorderRadius, 0);

        public double BorderRadius
        {
            get { return (double)GetValue(BorderRadiusProperty); }
            set { SetValue(BorderRadiusProperty, value); }
        }

        public static readonly BindableProperty StrokeProperty =
            BindableProperty.Create<ExtendedBoxView, Color>(p => p.Stroke, Color.Transparent);

        public Color Stroke
        {
            get { return (Color)GetValue(StrokeProperty); }
            set { SetValue(StrokeProperty, value); }
        }

        public static readonly BindableProperty StrokeThicknessProperty =
            BindableProperty.Create<ExtendedBoxView, double>(p => p.StrokeThickness, 0);

        public double StrokeThickness
        {
            get { return (double)GetValue(StrokeThicknessProperty); }
            set { SetValue(StrokeThicknessProperty, value); }
        }
    }
}

下一步將為每個平臺建立一個渲染器。

iOS 版：

[assembly: ExportRenderer(typeof(ExtendedBoxView), typeof(ExtendedBoxViewRenderer))]
namespace Mobile.iOS.Renderers
{
 public class ExtendedBoxViewRenderer : VisualElementRenderer<BoxView>
 {
    public ExtendedBoxViewRenderer()
    {    
    }

    protected override void OnElementChanged(ElementChangedEventArgs<BoxView> e)
    {
        base.OnElementChanged(e);
        if (Element == null)
            return;

        Layer.MasksToBounds = true;
        Layer.CornerRadius = (float)((ExtendedBoxView)this.Element).BorderRadius / 2.0f;
    }

     
    protected override void OnElementPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        base.OnElementPropertyChanged(sender, e);
        if (e.PropertyName == ExtendedBoxView.BorderRadiusProperty.PropertyName)
        {
            SetNeedsDisplay();
        }
    }

    public override void Draw(CGRect rect)
    {
        ExtendedBoxView roundedBoxView = (ExtendedBoxView)this.Element;
        using (var context = UIGraphics.GetCurrentContext())
        {
            context.SetFillColor(roundedBoxView.Color.ToCGColor());
            context.SetStrokeColor(roundedBoxView.Stroke.ToCGColor());
            context.SetLineWidth((float)roundedBoxView.StrokeThickness);

            var rCorner = this.Bounds.Inset((int)roundedBoxView.StrokeThickness / 2, (int)roundedBoxView.StrokeThickness / 2);

            nfloat radius = (nfloat)roundedBoxView.BorderRadius;
            radius = (nfloat)Math.Max(0, Math.Min(radius, Math.Max(rCorner.Height / 2, rCorner.Width / 2)));

            var path = CGPath.FromRoundedRect(rCorner, radius, radius);
            context.AddPath(path);
            context.DrawPath(CGPathDrawingMode.FillStroke);
        }
    }
    
 }
}

你可以再次在 draw 方法中自定義。

Android 也一樣：

[assembly: ExportRenderer(typeof(ExtendedBoxView), typeof(ExtendedBoxViewRenderer))]
namespace Mobile.Droid
{
    /// <summary>
    /// 
    /// </summary>
    public class ExtendedBoxViewRenderer : VisualElementRenderer<BoxView>
    {
        /// <summary>
        /// 
        /// </summary>
        public ExtendedBoxViewRenderer()
        {    
        }

        /// <summary>
        /// 
        /// </summary>
        /// <param name="e"></param>
        protected override void OnElementChanged(ElementChangedEventArgs<BoxView> e)
        {
            base.OnElementChanged(e);

            SetWillNotDraw(false);

            Invalidate();
        }

        /// <summary>
        /// 
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        protected override void OnElementPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);

            if (e.PropertyName == ExtendedBoxView.BorderRadiusProperty.PropertyName)
            {
                Invalidate();
            }
        }

        /// <summary>
        /// 
        /// </summary>
        /// <param name="canvas"></param>
        public override void Draw(Canvas canvas)
        {
            var box = Element as ExtendedBoxView;
            base.Draw(canvas);
            Paint myPaint = new Paint();

            myPaint.SetStyle(Paint.Style.Stroke);
            myPaint.StrokeWidth = (float)box.StrokeThickness;
            myPaint.SetARGB(convertTo255ScaleColor(box.Color.A), convertTo255ScaleColor(box.Color.R), convertTo255ScaleColor(box.Color.G), convertTo255ScaleColor(box.Color.B));
            myPaint.SetShadowLayer(20, 0, 5, Android.Graphics.Color.Argb(100, 0, 0, 0));

            SetLayerType(Android.Views.LayerType.Software, myPaint);

            var number = (float)box.StrokeThickness / 2;
            RectF rectF = new RectF(
                        number, // left
                        number, // top
                        canvas.Width - number, // right
                        canvas.Height - number // bottom
                );

            var radius = (float)box.BorderRadius;
            canvas.DrawRoundRect(rectF, radius, radius, myPaint);
        }

        /// <summary>
        ///
        /// </summary>
        /// <param name="color"></param>
        /// <returns></returns>
        private int convertTo255ScaleColor(double color)
        {
            return (int) Math.Ceiling(color * 255);
        }
}

}

XAML：

我們首先使用我們之前定義的名稱空間引用我們的控制元件。

xmlns:Controls="clr-namespace:Mobile.Controls"

然後我們使用 Control 如下並使用開頭定義的屬性：

<Controls:ExtendedBoxView 
    x:Name="search_boxview"
    Color="#444"
    BorderRadius="5"
    HorizontalOptions="CenterAndExpand" 
    />