Xamarin.Forms:Forms.Context 已过时

本文介绍了Xamarin.Forms:Forms.Context 已过时的处理方法,对大家解决问题具有一定的参考价值

问题描述

Xamarin.Forms 2.5 中新的过时警告真的让我感到困惑.例如,我应该在依赖服务中使用什么上下文来调用 GetSystemService()?

我是否应该将 xamarin 表单针对其初始化的活动上下文存储在静态字段中?

我应该覆盖 android Application 类并使用它的 Context 吗?

我应该在活动创建时调用 GetSystemService 并将其保存在某处吗?

解决方案

我在使用多个 Dependency Services

时遇到了同样的问题

最简单的解决方案

在很多情况下单一活动应用程序

Xamarin.Forms.Forms.Context

可以替换为

Android.App.Application.Context

<小时>

更详细的背景

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

使用应用程序上下文的典型示例是启动 Activity,例如

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

<块引用>

一般的经验法则是使用当前的活动上下文,除非你需要从超出您的范围的对象保存对上下文的引用活动.在这种情况下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,可以将表单页面嵌入到 Xamarin.iOS/Xamarin.Android 应用中.但是,由于 Xamarin.Android 应用程序可以使用多个活动,因此 Xamarin.Android 用户调用 Forms.Context 并反过来获取对 MainActivity,有潜在的问题.

周围的工作

在渲染器中,您现在可以获得对传递给构造函数的视图上下文的引用.

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

但是,要在多活动应用程序中获取当前活动上下文,您需要持有对它的引用.最简单、最可靠的方法是通过一个实现 Application.IActivityLifecycleCallbacks 接口的 class.

<块引用>

主要思想是在一个 Activity 时保持对 Context 的引用已创建、启动或恢复.

[应用程序]公共部分类 MainApplication : Application, Application.IActivityLifecycleCallbacks{内部静态上下文 ActivityContext { 获取;私人订制;}public MainApplication(IntPtr handle, JniHandleOwnership transfer) : base(handle, transfer) { }公共覆盖无效 OnCreate(){base.OnCreate();RegisterActivityLifecycleCallbacks(this);}公共覆盖无效 OnTerminate(){base.OnTerminate();UnregisterActivityLifecycleCallbacks(this);}公共无效 OnActivityCreated(活动活动,捆绑已保存的实例状态){ActivityContext = 活动;}public void OnActivityResumed(活动活动){ActivityContext = 活动;}public void OnActivityStarted(活动活动){ActivityContext = 活动;}公共无效 OnActivityDestroyed(活动活动){}公共无效OnActivityPaused(活动活动){}public void OnActivitySaveInstanceState(Activity activity, Bundle outState) { }公共无效 OnActivityStopped(活动活动){}}

通过上述方法,单个活动应用程序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,而不是依赖于全局上下文

Android.App.Application.Context//或之前Xamarin.Forms.Forms.Context

现在可以替换为

MainApplication.ActivityContext

<小时>

依赖服务中的示例调用

if (MainApplication.ActivityContext!= null){versionNumber = MainApplication.ActivityContext.包管理器.GetPackageInfo(MainApplication.ActivityContext.PackageName, 0).版本名称;}

<小时>

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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