你必须调用Xamarin.Forms.Init(); 在使用之前

Question

在我的app.xaml.cs中,我创建了一个新页面.

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

此页面调用静态类,该类使用DependencyService执行某些任务.

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TypeInitializationExceptionInnerException:

你必须调用Xamarin.Forms.Init(); 在使用之前

它以某种方式与静态助手类相关,因为在该调用之前,我可以使用DependencyService而不会出现任何问题!

作为主要发射器,我正在使用闪屏.在这个类中,我做了一些启动工作,它依赖于DependencyService.

闪屏:

 [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");
        }
}

我的主要活动:

 [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,我收到另一个错误.

在隐藏键盘之前调用Forms.Init()

这是什么东西？

Answer 1

这是很不幸的。我OnCreate()在SplashScreen中使用了错误的方法。

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，Xamarin上有关创建初始屏幕的文档使用了带有2个参数的OnCreate（）方法！