[Android 9中xamarin.forms中的设备方向更改时，我得到NullReferenceException

Question

在我使用Xamarin.forms开发的android应用程序中，每当我更改设备方向时，都会收到NullReferenceException异常，并且该应用程序在Android 9中崩溃。但是在以前的android版本中，该应用程序运行良好。没有崩溃。

这是MainActivity.cs

namespace RSMapp.Droid
{
    [Activity(Label = "FIRE", Icon = "@drawable/launcherIcon", Theme = "@style/splashscreen", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation, ScreenOrientation = ScreenOrientation.Portrait)]
    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
    {
        internal static MainActivity Instance { get; private set; }

        protected async override void OnCreate(Bundle savedInstanceState)
        {

            TryToGetPermissions();

            TabLayoutResource = Resource.Layout.Tabbar;
            ToolbarResource = Resource.Layout.Toolbar;

            base.Window.RequestFeature(WindowFeatures.ActionBar);
            base.SetTheme(Resource.Style.MainTheme);
            UserDialogs.Init(this);
            base.OnCreate(savedInstanceState);
            Instance = this;
            global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
            Xamarin.Essentials.Platform.Init(this, savedInstanceState);
            CrossCurrentActivity.Current.Init(this, savedInstanceState);
            Plugin.InputKit.Platforms.Droid.Config.Init(this, savedInstanceState);
            UltimateXFSettup.Initialize(this);
            LoadApplication(new App());

        }


}

}

我尝试删除ScreenOrientation = ScreenOrientation.Portrait，但是存在问题。

将方向肖像更改为横向后，在“输出”窗口中获取此信息

10-18 11:52:37.366 I/MultiWindowDecorSupport(19021): updateCaptionType >> DecorView@3acb272[MainActivity], isFloating: false, isApplication: true, hasWindowDecorCaption: false, hasWindowControllerCallback: true
10-18 11:52:37.366 D/MultiWindowDecorSupport(19021): setCaptionType = 0, DecorView = DecorView@3acb272[MainActivity]
10-18 11:52:37.373 D/ViewRootImpl@403bf69[MainActivity](19021): MSG_RESIZED_REPORT: frame=[0,0][2048,1536] ci=[0,48][0,0] vi=[0,48][0,0] or=2
10-18 11:52:37.399 D/ViewRootImpl@403bf69[MainActivity](19021): Relayout returned: old=[0,0][2048,1536] new=[0,0][2048,1536] result=0x23 surface={true 490213163008} changed=false
10-18 11:52:37.401 D/OpenGLRenderer(19021): eglDestroySurface = 0x7222da2700, 0x7222fb4000
10-18 11:52:37.402 D/OpenGLRenderer(19021): eglCreateWindowSurface = 0x7222da2700, 0x7222fb4010
Unhandled Exception:

System.NullReferenceException: Object reference not set to an instance of an object.

此代码中存在问题。

public override Resources Resources
{
    get
    {
        if ((int)Build.VERSION.SdkInt >= 25)
        {
            Configuration config = new Configuration();
            config.SetToDefaults();
            Context context = CreateConfigurationContext(config);
            Resources resources = context.Resources;
            return resources;
        }
        else
        {
            Resources res = base.Resources;
            Configuration config = new Configuration();
            config.SetToDefaults();
            res.UpdateConfiguration(config, res.DisplayMetrics);
            return res;
        }

    }
}

我正在使用它来省略用户更改为应用程序的系统级字体大小。这给出了Android 9中的错误

Answer 1

删除ScreenOrientation = ScreenOrientation.Portrait

然后将其添加到您的MainActivity中：

public override void OnConfigurationChanged(Configuration newConfig)
    {
        base.OnConfigurationChanged(newConfig);
    }