Xamarin.Forms:Forms.Context已过时 [英] Xamarin.Forms: Forms.Context is obsolete

问题描述

Xamarin.Forms 2.5中新的过时警告确实让我感到困惑. 例如，我应该在Dependency Services中使用什么上下文来调用GetSystemService()?

我应该将xamarin表单初始化所针对的活动的上下文存储在静态字段中吗?

我应该重写android Application类并使用其上下文吗?

我应该在活动创建时调用GetSystemService并将其保存在某个地方吗?

解决方案

我在几个Dependency Services

上遇到了相同的问题

最简单的解决方案

在许多情况下，单个活动应用程序

Xamarin.Forms.Forms.Context

可以替换为

Android.App.Application.Context

---

更详细的背景信息

Android.App.Application.Context返回与应用程序"生命周期相关联的当前进程的全局应用程序上下文"，并与活动"上下文相关.

使用Application上下文的典型示例是启动活动，例如

Android.App.Application.Context.StartActivity(myIntent);

一般的经验法则是使用当前的活动上下文，除非您需要 保存对对象的上下文的引用，该对象超出了您的生命范围 活动.在这种情况下，请使用应用程序上下文

为什么Forms.Context过时了?

Xmarin.Forms 2.5 引入了新的表单嵌入"功能，该功能可以将表单页面嵌入Xamarin.iOS/Xamarin.Android应用中.但是，由于Xamarin.Android应用程序可以使用多个Activity，因此似乎存在Xamarin.Android用户调用Forms.Context并进而获得对MainActivity的引用的危险，这有可能导致问题.

解决方法

现在在渲染器"中，您可以获取对视图上下文的引用，该引用将传递到构造函数中.

在任何其他类上，您都面临如何获取活动上下文的问题.在单个活动应用程序(在大多数情况下)中，Application.Context可以正常工作.

但是要在多个活动应用程序中获取当前的活动上下文，则需要保留对其的引用.最简单，最可靠的方法是通过实现Application.IActivityLifecycleCallbacks接口的class.

主要思想是在活动时保留上下文的引用 创建，启动或恢复.

[Application]
public partial class MainApplication : Application, Application.IActivityLifecycleCallbacks
{
   internal static Context ActivityContext { get; private set; }

   public MainApplication(IntPtr handle, JniHandleOwnership transfer) : base(handle, transfer) { }

   public override void OnCreate()
   {
      base.OnCreate();
      RegisterActivityLifecycleCallbacks(this);
   }    

   public override void OnTerminate()
   {
      base.OnTerminate();
      UnregisterActivityLifecycleCallbacks(this);
   }  

   public void OnActivityCreated(Activity activity, Bundle savedInstanceState)
   {
      ActivityContext = activity;
   }   

   public void OnActivityResumed(Activity activity)
   {
      ActivityContext = activity;
   }

   public void OnActivityStarted(Activity activity)
   {
      ActivityContext = activity;
   }

   public void OnActivityDestroyed(Activity activity) { }  
   public void OnActivityPaused(Activity activity) { }
   public void OnActivitySaveInstanceState(Activity activity, Bundle outState) { }
   public void OnActivityStopped(Activity activity) { }
}

使用上述方法，单个活动应用程序和多个活动应用程序现在可以始终访问当前/本地活动上下文.例如，而不是依赖全局上下文

Android.App.Application.Context 

// or previously

Xamarin.Forms.Forms.Context

现在可以替换为

MainApplication.ActivityContext

---

依赖服务中的示例调用

if (MainApplication.ActivityContext!= null)
{
    versionNumber = MainApplication.ActivityContext
                       .PackageManager
                       .GetPackageInfo(MainApplication.ActivityContext.PackageName, 0)
                       .VersionName;
}

---

其他资源

Android.App.Application.IActivityLifecycleCallbacks

registerActivityLifecycleCallbacks

The new obsolete warning in Xamarin.Forms 2.5 really puzzled me. What context should I be using in Dependency Services, for example, to call GetSystemService()?

Should I store in a static field the context of activity the xamarin forms were initialized against?

Should I override the android Application class and use its Context?

Should I call GetSystemService at activity create and save it somewhere?

解决方案

I was having the same issue with several Dependency Services

The simplest solution

In a lot of cases for Single Activity Applications

Xamarin.Forms.Forms.Context

Can be replaced with

Android.App.Application.Context

---

The Background in more detail

Android.App.Application.Context returns the global Application Context of the current process tied to the lifecycle of the Application, as apposed to an Activity context.

A typical example of using the Application context is for starting an Activity e.g.

Android.App.Application.Context.StartActivity(myIntent);

The general rule of thumb is to use the current Activity Context, unless you need to save a reference to a context from an object that lives beyond your Activity. In which case use the Application context

Why did Forms.Context go obsolete?

Xmarin.Forms 2.5 introduced a new "Forms embedding" feature, which can embed Forms pages into Xamarin.iOS / Xamarin.Android apps. However, since Xamarin.Android apps can use multiple Activities, seemingly there was a danger of Xamarin.Android users calling Forms.Context and in turn getting a reference to the MainActivity, which has the potential cause problems.

The work around

Inside a Renderer you now get a reference to the view’s context which is passed into the constructor.

With any other class you are faced with the issue of how to get the Activity Context. In a single Activity application (in most cases) the Application.Context will work just fine.

However to get the current Activity Context in a Multiple Activity Application you will need to hold a reference to it. The easiest and most reliable way to do this is via a class that implements the Application.IActivityLifecycleCallbacks Interface.

The main idea is to keep a reference of the Context when an Activity is created, started, or resumed.

[Application]
public partial class MainApplication : Application, Application.IActivityLifecycleCallbacks
{
   internal static Context ActivityContext { get; private set; }

   public MainApplication(IntPtr handle, JniHandleOwnership transfer) : base(handle, transfer) { }

   public override void OnCreate()
   {
      base.OnCreate();
      RegisterActivityLifecycleCallbacks(this);
   }    

   public override void OnTerminate()
   {
      base.OnTerminate();
      UnregisterActivityLifecycleCallbacks(this);
   }  

   public void OnActivityCreated(Activity activity, Bundle savedInstanceState)
   {
      ActivityContext = activity;
   }   

   public void OnActivityResumed(Activity activity)
   {
      ActivityContext = activity;
   }

   public void OnActivityStarted(Activity activity)
   {
      ActivityContext = activity;
   }

   public void OnActivityDestroyed(Activity activity) { }  
   public void OnActivityPaused(Activity activity) { }
   public void OnActivitySaveInstanceState(Activity activity, Bundle outState) { }
   public void OnActivityStopped(Activity activity) { }
}

With the above approach, single Activity Applications and multiple Activity Applications can now always have access to the Current/Local Activity Context. e.g instead of relying on the global context

Android.App.Application.Context 

// or previously

Xamarin.Forms.Forms.Context

Can now be replaced with

MainApplication.ActivityContext

---

Example call in a Dependency Service

if (MainApplication.ActivityContext!= null)
{
    versionNumber = MainApplication.ActivityContext
                       .PackageManager
                       .GetPackageInfo(MainApplication.ActivityContext.PackageName, 0)
                       .VersionName;
}

---

Additional Resources

Android.App.Application.IActivityLifecycleCallbacks

registerActivityLifecycleCallbacks

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