RTFSC - 认识 Android Context

认识 Android Context

Android L

Context 自身

Q：为何要了解Context？
A：为了在使用的时候不犯迷糊

context 意思有语境，上下文，背景，环境等等
Android程序需要一个完整的工程环境。
Context是维持Android程序中各组件能够正常工作的一个核心功能类。

Context 类是一个 app 全局环境的“接口”，由 Android 系统提供继承类（例如Activity）。
它能连接到应用的资源和类，也能使用应用级的操作，比如启动activity，广播和接收intent。

/**
 * 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.
 */
public abstract class Context {

这是一个抽象类，接下来看看主要的几个实现类

Context 的子类

先看我们熟悉的几个类：

Activity -> ContextThemeWrapper -> ContextWrapper -> Context

public class Activity extends ContextThemeWrapper

Application -> ContextWrapper -> Context

public abstract class Service extends ContextWrapper implements ComponentCallbacks2 {

Service - ContextWrapper -> Context

public class Application extends ContextWrapper implements ComponentCallbacks2 {

简单的继承关系示意

Context
├── ContextImpl
└── ContextWrapper
    ├── Application
    ├── ContextThemeWrapper
    │   └── Activity
    └── Service

ContextImpl

Context 是一个抽象类，子类 ContextImpl 实现了Context的方法；
为Activity和其他应用组件提供基本的context对象
ContextImpl.java (frameworks\base\core\java\android\app)

/**
 * Common implementation of Context API, which provides the base
 * context object for Activity and other application components.
 */
class ContextImpl extends Context {
    ......
    @Override
    public Context getApplicationContext() {
        return (mPackageInfo != null) ?
                mPackageInfo.getApplication() : mMainThread.getApplication();
    }
    ......
}

ContextWrapper

Wrapper 有封装的意思；ContextWrapper是Context的封装类

ContextWrapper.java (frameworks\base\core\java\android\content)

/**
 * Proxying implementation of Context that simply delegates all of its calls to
 * another Context. Can be subclassed to modify behavior without changing
 * the original Context.
 */
public class ContextWrapper extends Context {
    Context mBase;

    public ContextWrapper(Context base) {
        mBase = base;
    }
    ......
    @Override
    public Context getApplicationContext() {
        return mBase.getApplicationContext();
    }// 
    ......
}

构造方法中传入一个 ContextImpl Context 实例，就变成ContextImpl的装饰者模式

ContextThemeWrapper

允许在封装的context中修改主题（theme）
ContextThemeWrapper.java (frameworks\base\core\java\android\view)

/**
 * A ContextWrapper that allows you to modify the theme from what is in the 
 * wrapped context. 
 */
public class ContextThemeWrapper extends ContextWrapper {

其中提供了关于theme的方法，app开发中 android:theme 与此有关
相同的代码，相同的调用，使用不同的 theme 会有不同的效果

getApplicationContext() 和 getBaseContext()

public class ContextWrapper extends Context {
    Context mBase;
    ......
    @Override
    public Context getApplicationContext() {
        return mBase.getApplicationContext();
    }
    ......
    /**
 * @return the base context as set by the constructor or setBaseContext
 */
    public Context getBaseContext() {
        return mBase;// Don't use getBaseContext(), just use the Context you have.
    }
    ......
}

getApplicationContext() = android.app.Application@39d42b0e
getBaseContext() = android.app.ContextImpl@1f48c92f

getApplicationContext() 从application取得context（上下文）
getBaseContext() 从实现类ContextImpl那得来；
返回由构造函数指定或setBaseContext()设置的上下文

平时使用getApplicationContext()即可