什么是 Android 上的“上下文"?

What is #39;Context#39; on Android?(什么是 Android 上的“上下文?)

问题描述

In Android programming, what exactly is a Context class and what is it used for?

I read about it on the developer site, but I am unable to understand it clearly.

Putting it simply:

As the name suggests, it's the context of the 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 and package/application).

You can get the context by invoking getApplicationContext(), getContext(), getBaseContext() or this (when in a class that extends from Context, such as the Application, Activity, Service and IntentService classes).

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

• Accessing components implicitly: Regarding content providers, broadcasts, intent

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

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