c# - 你必须调用 Xamarin.Forms.Init();使用前

Question

在我的 app.xaml.cs 中创建一个新页面。

 public App()
  {
      InitializeComponent();
      MainPage = new NavigationPage(new WrapLayoutPage());
  }

这个页面调用了一个静态类，它使用 DependencyService 来执行一些任务。

抛出错误的行:

var tmpTable = SqLiteHelper.GetItem<TableX>("someId");

SqLiteHelper:

public static class SqLiteHelper
{
    private static readonly SQLiteConnection DatabaseConnection = DependencyService.Get<ISqLite>().GetConnection();
    private static readonly object Locker = new object();

    public static DbObjectV3 GetItem<T>(Guid inId) where T : DbObjectV3, new()
    {
        lock (Locker)
        {
            var tmpItem = DatabaseConnection.Table<T>().FirstOrDefault(inItem => inItem.Id == inId);
            tmpItem.IsNewObject = false;
            return tmpItem;
        }
    }
}

这会抛出一个带有 InnerException 的 TypeInitializationException:

You MUST call Xamarin.Forms.Init(); prior to using it

它在某种程度上与静态帮助程序类相关，因为在该调用之前，我可以毫无问题地使用 DependencyService!

作为主启动器，我正在使用启动画面。在这个类中，我做了一些依赖于 DependencyService 的启动工作。

启动画面:

 [Activity(Theme = "@style/MyTheme.Splash", NoHistory = true, MainLauncher = true)]
    public class SplashScreen : Activity
    {
        static readonly string TAG = "X:" + typeof(SplashScreen).Name;

        public override void OnCreate(Bundle savedInstanceState, PersistableBundle persistentState)
        {
            base.OnCreate(savedInstanceState, persistentState);
            Log.Debug(TAG, "SplashActivity.OnCreate");
        }
}

我的主要 Activity :

 [Activity(Label = "FrameworkForms", Icon = "@drawable/icon", ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation, Theme = "@style/MainActivityTheme", MainLauncher = false)]
    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
            Xamarin.Forms.Forms.Init(this, bundle);
            App.ScreenWidth = (double)(Resources.DisplayMetrics.WidthPixels / Resources.DisplayMetrics.Density);
            LoadApplication(new App());
        }
    }

现在，将 SplashScreen 中的 Activity 更改为 FormsAppCompatActivity 后，出现另一个错误。

Call Forms.Init() before Hide Keyboard

这是什么东西？

Answer 1

这很不幸。我在 SplashScreen 中使用了错误的 OnCreate() 方法。

我将 SplashScreen 更改为:

 [Activity(Theme = "@style/MyTheme.Splash", NoHistory = true, MainLauncher = true)]
    public class SplashScreen : Activity
    {
        static readonly string TAG = "X:" + typeof(SplashScreen).Name;

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            Xamarin.Forms.Forms.Init(this, savedInstanceState);
            Log.Debug(TAG, "SplashActivity.OnCreate");
        }


        protected override void OnResume()
        {
            base.OnResume();

            Task tmpStartupWork = new Task(() =>
            {
                Log.Debug(TAG, "Performing some startup work that takes a bit of time.");
                StartUpTasks.InitializeDatabaseCreation();
                Log.Debug(TAG, "Working in the background - important stuff.");
            });

            tmpStartupWork.ContinueWith(inT =>
            {
                Log.Debug(TAG, "Work is finished - start MainActivity.");
                StartActivity(new Intent(Application.Context, typeof(MainActivity)));
            }, TaskScheduler.FromCurrentSynchronizationContext());

            tmpStartupWork.Start();
        }
    }

不幸的是，关于创建启动画面的 Xamarin 文档使用了带有 2 个参数的 OnCreate() 方法!