Xamarin.Forms 设置自定义视图的大小

Question

How do I set the size, in xaml, of a custom view or, more specifically, a custom BoxView? Currently, the view takes up the entire device size instead of the requested 20x20.

Here's my xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:Beltpack"
             xmlns:controls="clr-namespace:Beltpack.Views;assembly=Beltpack"
             x:Class="Beltpack.MainPage">

  <controls:bpButton Height="20" Width="20" HeightRequest="20" WidthRequest="20" />

</ContentPage>

---

My button deriving from BoxView (currently doesn't do anything)

using Xamarin.Forms;

namespace Beltpack.Views {
    public class bpButton : BoxView {

        public bpButton() { }

    }

}

---

And my custom renderer

using Android.Graphics;
using Xamarin.Forms.Platform.Android;
using Xamarin.Forms;
using Beltpack.Views;
using Beltpack.Droid;

[assembly: ExportRenderer(typeof(bpButton), typeof(bpButtonRenderer))]

namespace Beltpack.Droid {
    public class bpButtonRenderer : BoxRenderer {

        public bpButtonRenderer() {}

        protected override void OnDraw(Canvas canvas) {
            bpButton btn = (bpButton)this.Element;

            Paint paint = new Paint();
            paint.Color = Android.Graphics.Color.Aqua;

            canvas.DrawCircle((int)(btn.WidthRequest * .5), (int)(btn.HeightRequest * .5), (int)(btn.WidthRequest * .5), paint);
        }        
    }
}

Answer 1

In addition to setting WidthRequest and HeightRequest , you must also set your HorizontalOptions and VerticalOptions settings.

Why? I have no clue, but that has been the solution suggested elsewhere and has consistently worked for me.

It looks like you want it in the center, so you could simply write

<controls:bpButton 
    Height="20"
    Width="20" 
    HeightRequest="20"
    WidthRequest="20"
    HorizontalOptions="Center"
    VerticalOptions="Center" />