var upArrow = Resources.GetDrawable(Resource.Drawable.abc_ic_ab_back_material);
upArrow.SetColorFilter(Resources.GetColor(Android.Resource.Color.White),PorterDuff.Mode.SrcIn);
SupportActionBar.SetHomeAsUpIndicator(upArrow);

上面的代码不会改变箭头颜色. upArrow ColorFilter值为null.这应该是什么原因？我不是问如何改变可绘制的颜色.我的问题是为什么上面的代码无法设置滤色器？以下是MainActivity代码.

public class MvxFormsApplicationActivity : FormsAppCompatActivity
{
    protected override void OnCreate(Bundle bundle)
    {
        TabLayoutResource = Resource.Layout.Tabbar;
        ToolbarResource = Resource.Layout.Toolbar;
        base.OnCreate(bundle);

        Xamarin.Forms.Forms.Init(this,bundle); ;
        //Plugins.NewictLib.Forms.Android.Renderers.GifImageViewRenderer.Init ();
        DLToolkit.Forms.Controls.FlowListView.Init ();
        UserDialogs.Init ((Activity) Xamarin.Forms.Forms.Context);

        OxyPlot.Xamarin.Forms.Platform.Android.PlotViewRenderer.Init();

        //            var mvxFormsApp = new MvxFormsApp ();
        //            LoadApplication (mvxFormsApp);
        var myApp = new MyFormsApp();
        LoadApplication (myApp );
        AppCompatDelegate.CompatVectorFromResourcesEnabled = true;
        //the following needs to set so that the back button color can be changed
        var upArrow = Resources.GetDrawable(Resource.Drawable.abc_ic_ab_back_material);
        upArrow.SetColorFilter(Resources.GetColor(Android.Resource.Color.White),PorterDuff.Mode.SrcIn);
        SupportActionBar.SetHomeAsUpIndicator(upArrow);

               if (IsPlayServicesAvailable())
        {
            var intent = new Intent(this,typeof(RegistrationIntentService));
            StartService(intent);
        }
        //var presenter = Mvx.Resolve