关于Context的理解

Context

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翻译成中文大概是：“上下文，背景”的意思。

这是一个抽象类，这个类的具体左右是什么呢？

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Api文档是这样描述的：

Interface to global information about an application environment. This is an abstract class whose implementation is provided by the Android system. It allows access to application-specific resources and classes, as well as up-calls for application-level operations such as launching activities, broadcasting and receiving intents, etc.

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下面摘自http://stackoverflow.com/questions/3572463/what-is-context-in-android的描述

As the name suggests, its the context of current state of the application/object. It lets newly created objects understand what has been going on. Typically you call it to get information regarding another part of your program (activity, package/application)

You can get the context by invoking?getApplicationContext(),?getContext(),getBaseContext()?or?this?(when in the activity class).

Typical uses of context:

• Creating New objects: Creating new views, adapters, listeners:

  TextView tv = new TextView(getContext());ListAdapter adapter = new SimpleCursorAdapter(getApplicationContext(), ...);

• Accessing Standard Common Resources: Services like LAYOUT_INFLATER_SERVICE, SharedPreferences:

  context.getSystemService(LAYOUT_INFLATER_SERVICE);

  this.getResources().get***

   getApplicationContext().getSharedPreferences(*name*, *mode*);

  (AlarmManager)getSystemService(Context.ALARM_SERVICE);//闹钟相关

  getApplicationContext().getContentResolver().query(uri, ...);

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  主要的作用有3个：

  1.创建对象

  2.获取公共资源文件,获取系统的某些管理器，比如控制电源，网络，等等

  3.调用系统组件

  Context是一个抽象类，我们先看下他的继承方式：

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  java.lang.Object????android.content.ContextKnown Direct SubclassesKnown Indirect Subclassespublic class ContextWrapper{? ? Context mContext? ? public ContextWrapper(Context ctx){? ? ? ? mContext = ctx? ? }? ? public Resourced getResources(){? ? ? ? return mContext.getResources()? ? }? ? //... all other Methods are implementing the same AdapterPattern}

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  这下明白了吧？原来我们平时经常在Activity中使用的this.getResources().***是在这个地方被实现的！

  以上摘自：http://stackoverflow.com/questions/9004739/which-class-is-implementing-contextwrappers-methods-in-android

  最后，如果需要使用Context的时候，但是这时候没有Context怎么办？比如不在Activity类中。参考前面

  You can get the context by invoking?getApplicationContext(),?getContext(),getBaseContext()?or?this?(when in the activity class).

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