Context类，应用程序环境的信息（即上下文，环境），是一个抽象类。
public abstract class Context {}
通读该类后发现几乎都是我们平时需要用context调用的抽象方法，比如getResources()，getPackageManager()，getContentResolver()，getApplicationContext()，obtainStyledAttributes()，getSharedPreferences(),startActivity(),checkPermission(),各种服务类型的声明的静态常量等等。还有调用Toast、Dialog需要传入。那么谁来实现这个功能的呢，既然上面Context是一个抽象类，那么肯定有他的实现类。
ContextImpl，Context所有抽象功能的实现类：
class ContextImpl extends Context {
private Context mOuterContext;
......
}
Context的继承类ContextWrapper部分代码，代理Context的实现：
/**
* 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;
}
/**
* Set the base context for this ContextWrapper. All calls will then be
* delegated to the base context. Throws
* IllegalStateException if a base context has already been set.
*
* @param base The new base context for this wrapper.
*/
protected void attachBaseContext(Context base) {
if (mBase != null) {
throw new IllegalStateException("Base context already set");
}
mBase = base;
}
}
ContextThemeWrapper类代码：
/**
* A context wrapper that allows you to modify or replace the theme of the
* wrapped context.
*/
public class ContextThemeWrapper extends ContextWrapper {
private int mThemeResource;
private Resources.Theme mTheme;
private LayoutInflater mInflater;
private Configuration mOverrideConfiguration;
private Resources mResources;
ContextThemeWrapper有setTheme()方法：
@Override
public void setTheme(int resid) {
if (mThemeResource != resid) {
mThemeResource = resid;
initializeTheme();
}
}
Activity类代码，因为Activity带窗口，需要设置主题，所以需继承于ContextThemeWrapper：
public class Activity extends ContextThemeWrapper
implements LayoutInflater.Factory2,
Window.Callback, KeyEvent.Callback,
OnCreateContextMenuListener, ComponentCallbacks2,
Window.OnWindowDismissedCallback, WindowControllerCallback,
AutofillManager.AutofillClient {
private static final String TAG = "Activity";
private static final boolean DEBUG_LIFECYCLE = false;
/** Standard activity result: operation canceled. */
public static final int RESULT_CANCELED = 0;
/** Standard activity result: operation succeeded. */
public static final int RESULT_OK = -1;
/** Start of user-defined activity results. */
public static final int RESULT_FIRST_USER = 1;
}
Activity也复写该方法并最终给window设置主题
@Override
public void setTheme(int resid) {
super.setTheme(resid);
mWindow.setTheme(resid);
}
Application类代码：
public class Application extends ContextWrapper implements ComponentCallbacks2 {
private ArrayList<ComponentCallbacks> mComponentCallbacks =
new ArrayList<ComponentCallbacks>();
private ArrayList<ActivityLifecycleCallbacks> mActivityLifecycleCallbacks =
new ArrayList<ActivityLifecycleCallbacks>();
private ArrayList<OnProvideAssistDataListener> mAssistCallbacks = null;
/** @hide */
public LoadedApk mLoadedApk;
public interface ActivityLifecycleCallbacks {
void onActivityCreated(Activity activity, Bundle savedInstanceState);
void onActivityStarted(Activity activity);
void onActivityResumed(Activity activity);
void onActivityPaused(Activity activity);
void onActivityStopped(Activity activity);
void onActivitySaveInstanceState(Activity activity, Bundle outState);
void onActivityDestroyed(Activity activity);
}
}
Service类代码：
public abstract class Service extends ContextWrapper implements ComponentCallbacks2 {
private static final String TAG = "Service";
…
}
继承关系图：
表述为：
ContextImpl和ContextWrapper继承自Context，ContextThemeWrapper、Application和Service继承自ContextWrapper，Activity继承自ContextThemeWrapper。ContextWrapper和ContextThemeWrapper都是Context的包装类，它们都含有Context类型的mBase对象，mBase具体指向的是ContextImpl，这样通过ContextWrapper和ContextThemeWrapper也可以使用Context的方法。ContextThemeWrapper中包含setTheme()和getTheme()等一些列主题相关的方法，所以需要设置主题的Activity需继承ContextThemeWrapper，让mWindow.setTheme()，而不需要主题的Service则继承ContextWrapper。
那么问题又来了
-
一个应用有多少个Context？
Application、Activity、Service都间接继承与Context，一个应用有且只有一个Application对象，那么一个应用Context总数为：
Context 数 = Application 数(1) + Activity 数(Activity以及所有子类) + Service 数(Service以及所有子类);
-
引用是该使用getApplicationcontext还是用Activity才不会引起内存泄漏？
当一个Context销毁的时候，却因为被引用导致销毁失败，导致内存泄露。如一个单例模式的Manager类需要传入一个Context，而这个Manager的对象又需要在Activity中使用，如果我们传递Activity的Context就可能造成内存泄漏，Application的Context对象是随着进程存在的，所以应该传递Application的Context。所以最终注意：尽量不要让生命周期长于Activity的对象引用Activity。
-
getApplication()与getApplicationContext()的爱恨情仇
getApplication()方法属于Activity和Service，获取到的是Activity中的mApplication对象；getApplicationContext()方法属于ContextWrapper，获取到的是ContextWrapper中的Context类对象mBase。
在Activity和Service中可调用getApplication()获取context，而其他以外的组件获取context就只能调用getApplicationContext()。
来自Activity：
/** Return the application that owns this activity. */
public final Application getApplication() {
return mApplication;
}
来自ContextWrapper：
@Override
public Context getApplicationContext() {
return mBase.getApplicationContext();
}
打印它们的内存地址发现是相同的，所以getApplication()和getApplicationContext()获取到的是同一对象。
Application mApplication = getApplication();
Context mBase = getApplicationContext();
Log.e("TAG","application " + mApplication);
Log.e("TAG","context " + mBase);
关于Context的总结就到这里了，该篇文章会不断更新不断扩充详细，对Context有个更完整认识。
