关于Context的理解

Context

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？

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.

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

  Context.getSystemService(Context.CONNECTIVITY_SERVICE);//网络服务相关

• Accessing Components Implicitly: Regarding content providers, broadcasts, intent

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

 

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：

 

java.lang.Object
↳	android.content.Context

Known Direct Subclasses ContextWrapper,  MockContext

Known Indirect Subclasses AbstractInputMethodService,  AccessibilityService,  AccountAuthenticatorActivity,  Activity,  ActivityGroup,  AliasActivity, Application,  BackupAgent,  BackupAgentHelper,  BasicDream,  ContextThemeWrapper, and  16 others.

可以看出，常见的Activity，Application等都是继承自Context的子类ContextWrapper。

这里不得不提下：由于Context是抽象类，没有具体的实现，他有两个实现类，其中一个就是ContextWrapper，一个包装类。这个包装类就替Context实现具体的操作，比如获取公共的资源

 

public class ContextWrapper{
    Context mContext

    public ContextWrapper(Context ctx){
        mContext = ctx
    }

    public Resourced getResources(){
        return mContext.getResources()
    }

    //... all other Methods are implementing the same AdapterPattern
}

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).