Forms.context 已过时。从 2.5 版开始，上下文已过时，请改用本地上下文

Question

我遇到了这个问题： Forms.context 已过时。从 2.5 版开始，上下文已过时，请改用本地上下文。

我正在尝试使用 Azure Active Directory 登录，代码如下。 请帮忙。

using Xamarin.Forms;
using myMobile.Service;
using Microsoft.IdentityModel.Clients.ActiveDirectory;
using System.Threading.Tasks;

[assembly: Dependency(typeof(myMobile.Droid.Authenticator))]
namespace myMobile.Droid
{
    class Authenticator: IAuthenticator 
    {
        public async Task<AuthenticationResult> Authenticate(string tenantUrl, string graphResourceUri, string ApplicationID, string returnUri)
        {
            try
            {
                var authContext = new AuthenticationContext(tenantUrl);
                if (authContext.TokenCache.ReadItems().Any())
                    authContext = new AuthenticationContext(authContext.TokenCache.ReadItems().FirstOrDefault().Authority);

  var authResult = await authContext.AcquireTokenAsync(graphResourceUri, ApplicationID, new Uri(returnUri), new PlatformParameters((Activity)Forms.Context));

                return authResult;
            }
            catch (Exception)       
            {
                return null;
            }
        }
    }
}


// err encountered on this line :(Activity)Forms.Context)

 Forms.context is obsolete.Context is obsolete as of version2.5,please use a local context instead.

var authResult = await authContext.AcquireTokenAsync(graphResourceUri, ApplicationID, new Uri(returnUri), new PlatformParameters((Activity)Forms.Context));

//--- 更新：

 //-------- Login Page:

private async void Login()
{           
     try
     {
       var data = await DependencyService.Get<IAuthenticator>()
                  .Authenticate(AzureSettings.tenanturl, AzureSettings.GraphResourceUri, AzureSettings.ApplicationID, AzureSettings.ReturnUri);

  AzureSettings.AuthenticationResult = data;

                //NavigateTopage(data);

            }
            catch (Exception)
            { }
        }
   }

//--------- in Shared Project :

//-- interface: IAuthenticator


using Microsoft.IdentityModel.Clients.ActiveDirectory;
using System.Threading.Tasks;

namespace myMobile.Service
{
    public interface IAuthenticator
    {
        Task<AuthenticationResult> Authenticate(string tenantUrl, string graphResourceUri, string ApplicationID, string returnUri);
    }
}



//-------- in Android Project: add

1) Class : Authenticator.cs

using Android.App;
using Android.Content;
using Xamarin.Forms;
using myMobile.Service;
using Microsoft.IdentityModel.Clients.ActiveDirectory;
using System.Threading.Tasks;

[assembly: Dependency(typeof(myMobile.Droid.Authenticator))]
namespace myMobile.Droid
{
    class Authenticator: IAuthenticator 
    {
        private readonly Context _context;

        public static void Init(Context context)
        {
            _context = context;  //--error 
        }


        public async Task<AuthenticationResult> Authenticate(string tenantUrl, string graphResourceUri, string ApplicationID, string returnUri)
        {
            try
            {
                var authContext = new AuthenticationContext(tenantUrl);
                if (authContext.TokenCache.ReadItems().Any())
                    authContext = new AuthenticationContext(authContext.TokenCache.ReadItems().FirstOrDefault().Authority);

var authResult = await authContext.AcquireTokenAsync(graphResourceUri, ApplicationID, new Uri(returnUri), new PlatformParameters((Activity) _context));

                return authResult;
            }
            catch (Exception)       
            {
                return null;
            }
        }
    }
}

error : 

An Object reference is required for non-static field,method or property.Authenticator._context



//------- Class: MainActivity



namespace myMobile.Droid
{
    [Activity(Label = "myMobile", Icon = "@mipmap/icon", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
    public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
    {
        protected override void OnCreate(Bundle bundle)
        {
            TabLayoutResource = Resource.Layout.Tabbar;
            ToolbarResource = Resource.Layout.Toolbar;

               DependencyService.Get<IAuthenticator>().Init(this);  //<-- Error

            base.OnCreate(bundle);

            global::Xamarin.Forms.Forms.Init(this, bundle);
            LoadApplication(new App());
        }
    }
}

Error Message:

IUAthenticator does not contain a definition for Init and no extension method accepting
a first argument of  type IAuthenticator

Answer 1

您现在必须实现一个自定义构造函数，该构造函数将 Context 放入一个局部变量中并使用它而不是 this，例如 new PlatformParameters((Activity)Forms.Context)。

对于自定义渲染器，您可以使用下面的解决方案。这样做：

public MyControlRenderer : ControlRenderer
{
    private readonly Context _context;

    public MyControlRenderer(Context context) : base(context)
    {
        _context = context;
    }
}

对于像您这样的依赖服务，您必须找到一种方法来提供Context。由于 Xamarin.Forms 使用单个活动，因此您可以使用某种 init 方法。

将此添加到您的代码中：

public class MyService : IMyService
{
    private static Context _context;

    public static void Init(Context context)
    {
        _context = context;
    }
}

现在从您的MainActivity 拨打Init，之后您应该会很好。也这样做：DependencyService.Get<IMyService>().Init(this);

对于因多项活动而遇到此问题的其他人，请参阅此处的文档：https://www.davidbritch.com/2017/11/xamarinforms-25-and-local-context-on.html 这就是本文的灵感来源。