Change the color of the ScrollBar in Xamarin.Forms

Question

I need advice. I'm trying to change the color of the ScrollBar in Xamarin Forms. I created ScrollBarColorEffect but unfortunately it doesn't work it reports this error.

Someone would know where the mistake was.

XF ScrollBarColorEffect.cs

public class ScrollBarColorEffect : RoutingEffect
{
    public ScrollBarColorEffect() : base($"MPlay.{nameof(ScrollBarColorEffect)}"){}
    
    public Color ScrollBarColor { get; set; }
}

Xamarin Android ScrollBarColorEffect.cs

[assembly: ResolutionGroupName("MPlay")]
[assembly: ExportEffect(typeof(MPlay.Droid.ScrollBarColorEffect), 
nameof(MPlay.Droid.ScrollBarColorEffect))]
namespace MPlay.Droid
{

public class ScrollBarColorEffect : PlatformEffect
{
    protected override void OnAttached()
    {
        UpdateUI();
    }

    protected override void OnDetached()
    {
    }
  
    void UpdateUI()
    {
        Xamarin.Forms.ScrollView _scrollView = Element as Xamarin.Forms.ScrollView;
        if (Element != null && Control is AndroidX.Core.Widget.NestedScrollView scrollView)
        {
            Java.Lang.Reflect.Field mScrollCacheField = Class.FromType(typeof(Android.Views.View)).GetDeclaredField("mScrollCache");
            mScrollCacheField.Accessible = true;

            var mScrollCache = mScrollCacheField.Get(scrollView);
            var scrollBarField = mScrollCache.Class.GetDeclaredField("scrollBar");
            scrollBarField.Accessible = true;
            var scrollBar = scrollBarField.Get(mScrollCache);
            if (scrollBar != null)
            {
                var method = scrollBar.Class.GetDeclaredMethod("setVerticalThumbDrawable", Class.FromType(typeof(Drawable)));
                method.Accessible = true;

                var layers = new Drawable[1];
                var shapeDrawable = new ShapeDrawable(new RectShape());
                var scrollBarColor = Color.Default;

                var effect = _scrollView.Effects.FirstOrDefault(e => e is Core.Effects.ScrollBarColorEffect) as Core.Effects.ScrollBarColorEffect;
                //var effect = (Core.Effects.ScrollBarColorEffect)Element.Effects.FirstOrDefault(e => e is Core.Effects.ScrollBarColorEffect);
                if (effect != null)
                {
                    scrollBarColor = effect.ScrollBarColor;
                }

                shapeDrawable.Paint.Color = scrollBarColor.ToAndroid();
                shapeDrawable.SetIntrinsicWidth(5);

                layers[0] = shapeDrawable;
                method.Invoke(scrollBar, layers);
            }
        }
    }
}

XAML XF

 <ScrollView>
        <ScrollView.Effects>
            <helpers:ScrollBarColorEffect ScrollBarColor="Red"/>
        </ScrollView.Effects>
 </ScrollView>

I tried to create my own render, see here: https://stackoverflow.com/a/65127686/6473719

Unfortunately, I get the same error

Answer 1

The code you used should work before API 29. On Android 10(API 29), there are non-SDK interface restrictions.

The method setVerticalThumbDrawable you used is in the Non-SDK interfaces that are now blocked in Android 10.

Non-SDK interfaces that are now blocked in Android 10: https://developer.android.google.cn/about/versions/10/non-sdk-q#q-list-changes

In the Android developer, it suggest to use setVerticalScrollbarThumbDrawable instead. https://developer.android.google.cn/reference/android/view/View?hl=zh-cn#setVerticalScrollbarThumbDrawable(android.graphics.drawable.Drawable)

Answer 2

    protected override void OnElementChanged(VisualElementChangedEventArgs e)
    {
        base.OnElementChanged(e);
        this.VerticalScrollbarThumbDrawable = Context.GetDrawable(Resource.Drawable.scrollbar_style);
    }