android context

++++Context
    ++++ContextIml api的通用实现 
    ++++ContextWrapper
        ++++ContextThemeWrapper
            ++++Activity
        ++++Service
        ++++Application
        

注意

• 长期存在的对象不要持有activity的context引用
• 凡是跟UI相关的，都应该使用Activity做为Context来处理
• application的context不能用于：
  • Dialog
  • start Activity 这需要创建一个新的task
  • Layout Inflation 会使用系统默认的主题样式

关键函数

public abstract class Context {

// 获取应用程序包的AssetManager实例
public abstract AssetManager getAssets();

// 获取应用程序包的Resources实例
public abstract Resources getResources();

// 获取PackageManager实例，以查看全局package信息
public abstract PackageManager getPackageManager();

// 获取应用程序包的ContentResolver实例
public abstract ContentResolver getContentResolver();

// 它返回当前进程的主线程的Looper，此线程分发调用给应用组件(activities, services等)
public abstract Looper getMainLooper();

// 返回当前进程的单实例全局Application对象的Context
public abstract Context getApplicationContext();

// 从string表中获取本地化的、格式化的字符序列
public final CharSequence getText(int resId) {
return getResources().getText(resId);
}

// 从string表中获取本地化的字符串
public final String getString(int resId) {
return getResources().getString(resId);
}

public final String getString(int resId, Object... formatArgs) {
return getResources().getString(resId, formatArgs);
}

// 返回一个可用于获取包中类信息的class loader
public abstract ClassLoader getClassLoader();

// 返回应用程序包名
public abstract String getPackageName();

// 返回应用程序信息
public abstract ApplicationInfo getApplicationInfo();

// 根据文件名获取SharedPreferences
public abstract SharedPreferences getSharedPreferences(String name,
int mode);

// 其根目录为: Environment.getExternalStorageDirectory()
/*
* @param type The type of files directory to return.  May be null for
* the root of the files directory or one of
* the following Environment constants for a subdirectory:
* {@link android.os.Environment#DIRECTORY_MUSIC},
* {@link android.os.Environment#DIRECTORY_PODCASTS},
* {@link android.os.Environment#DIRECTORY_RINGTONES},
* {@link android.os.Environment#DIRECTORY_ALARMS},
* {@link android.os.Environment#DIRECTORY_NOTIFICATIONS},
* {@link android.os.Environment#DIRECTORY_PICTURES}, or
* {@link android.os.Environment#DIRECTORY_MOVIES}.
*/
public abstract File getExternalFilesDir(String type);

// 返回应用程序obb文件路径
public abstract File getObbDir();

// 启动一个新的activity
public abstract void startActivity(Intent intent);

// 启动一个新的activity
public void startActivityAsUser(Intent intent, UserHandle user) {
throw new RuntimeException("Not implemented. Must override in a subclass.");
}

// 启动一个新的activity
// intent: 将被启动的activity的描述信息
// options: 描述activity将如何被启动
public abstract void startActivity(Intent intent, Bundle options);

// 启动多个新的activity
public abstract void startActivities(Intent[] intents);

// 启动多个新的activity
public abstract void startActivities(Intent[] intents, Bundle options);

// 广播一个intent给所有感兴趣的接收者，异步机制
public abstract void sendBroadcast(Intent intent);

// 广播一个intent给所有感兴趣的接收者，异步机制
public abstract void sendBroadcast(Intent intent,String receiverPermission);

public abstract void sendOrderedBroadcast(Intent intent,String receiverPermission);

public abstract void sendOrderedBroadcast(Intent intent,
String receiverPermission, BroadcastReceiver resultReceiver,
Handler scheduler, int initialCode, String initialData,
Bundle initialExtras);

public abstract void sendBroadcastAsUser(Intent intent, UserHandle user);

public abstract void sendBroadcastAsUser(Intent intent, UserHandle user,
String receiverPermission);

// 注册一个BroadcastReceiver，且它将在主activity线程中运行
public abstract Intent registerReceiver(BroadcastReceiver receiver,
IntentFilter filter);

public abstract Intent registerReceiver(BroadcastReceiver receiver,
IntentFilter filter, String broadcastPermission, Handler scheduler);

public abstract void unregisterReceiver(BroadcastReceiver receiver);

// 请求启动一个application service
public abstract ComponentName startService(Intent service);

// 请求停止一个application service
public abstract boolean stopService(Intent service);

// 连接一个应用服务，它定义了application和service间的依赖关系
public abstract boolean bindService(Intent service, ServiceConnection conn,
int flags);

// 断开一个应用服务，当服务重新开始时，将不再接收到调用，
// 且服务允许随时停止
public abstract void unbindService(ServiceConnection conn);

// 返回系统级service句柄
/*
* @see #WINDOW_SERVICE
* @see android.view.WindowManager
* @see #LAYOUT_INFLATER_SERVICE
* @see android.view.LayoutInflater
* @see #ACTIVITY_SERVICE
* @see android.app.ActivityManager
* @see #POWER_SERVICE
* @see android.os.PowerManager
* @see #ALARM_SERVICE
* @see android.app.AlarmManager
* @see #NOTIFICATION_SERVICE
* @see android.app.NotificationManager
* @see #KEYGUARD_SERVICE
* @see android.app.KeyguardManager
* @see #LOCATION_SERVICE
* @see android.location.LocationManager
* @see #SEARCH_SERVICE
* @see android.app.SearchManager
* @see #SENSOR_SERVICE
* @see android.hardware.SensorManager
* @see #STORAGE_SERVICE
* @see android.os.storage.StorageManager
* @see #VIBRATOR_SERVICE
* @see android.os.Vibrator
* @see #CONNECTIVITY_SERVICE
* @see android.net.ConnectivityManager
* @see #WIFI_SERVICE
* @see android.net.wifi.WifiManager
* @see #AUDIO_SERVICE
* @see android.media.AudioManager
* @see #MEDIA_ROUTER_SERVICE
* @see android.media.MediaRouter
* @see #TELEPHONY_SERVICE
* @see android.telephony.TelephonyManager
* @see #INPUT_METHOD_SERVICE
* @see android.view.inputmethod.InputMethodManager
* @see #UI_MODE_SERVICE
* @see android.app.UiModeManager
* @see #DOWNLOAD_SERVICE
* @see android.app.DownloadManager
*/
public abstract Object getSystemService(String name);

public abstract int checkPermission(String permission, int pid, int uid);

// 返回一个新的与application name对应的Context对象
public abstract Context createPackageContext(String packageName,
int flags) throws PackageManager.NameNotFoundException;

// 返回基于当前Context对象的新对象，其资源与display相匹配
public abstract Context createDisplayContext(Display display);
}

ContextImpl关键成员和函数

/**
* Common implementation of Context API, which provides the base
* context object for Activity and other application components.
*/
class ContextImpl extends Context {
private final static String TAG = "ContextImpl";
private final static boolean DEBUG = false;

private static final HashMap sSharedPrefs =
new HashMap();

/*package*/ LoadedApk mPackageInfo; // 关键数据成员
private String mBasePackageName;
private Resources mResources;
/*package*/ ActivityThread mMainThread; // 主线程

@Override
public AssetManager getAssets() {
return getResources().getAssets();
}

@Override
public Looper getMainLooper() {
return mMainThread.getLooper();
}

@Override
public Object getSystemService(String name) {
ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
return fetcher == null ? null : fetcher.getService(this);
}

@Override
public void startActivity(Intent intent, Bundle options) {
warnIfCallingFromSystemProcess();
if ((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
throw new AndroidRuntimeException(
"Calling startActivity() from outside of an Activity "
+ " context requires the FLAG_ACTIVITY_NEW_TASK flag."
+ " Is this really what you want?");
}
mMainThread.getInstrumentation().execStartActivity(
getOuterContext(), mMainThread.getApplicationThread(), null,
(Activity)null, intent, -1, options);
}
}

ContextWrapper

/**
* 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; //该属性指向一个ContextIml实例

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.
* 创建Application、Service、Activity，会调用该方法给mBase属性赋值
*/
protected void attachBaseContext(Context base) {
if (mBase != null) {
throw new IllegalStateException("Base context already set");
}
mBase = base;
}

@Override
public Looper getMainLooper() {
return mBase.getMainLooper();
}

@Override
public Object getSystemService(String name) {
return mBase.getSystemService(name);
}

@Override
public void startActivity(Intent intent) {
mBase.startActivity(intent);
}
}

ContextThemeWrapper 包含主题(Theme)相关的接口,android:theme

/**
* A ContextWrapper that allows you to modify the theme from what is in the
* wrapped context.
*/
public class ContextThemeWrapper extends ContextWrapper {
private Context mBase;
private int mThemeResource;
private Resources.Theme mTheme;
private LayoutInflater mInflater;
private Configuration mOverrideConfiguration;
private Resources mResources;

public ContextThemeWrapper() {
super(null);
}

public ContextThemeWrapper(Context base, int themeres) {
super(base);
mBase = base;
mThemeResource = themeres;
}

@Override protected void attachBaseContext(Context newBase) {
super.attachBaseContext(newBase);
mBase = newBase;
}

@Override public void setTheme(int resid) {
mThemeResource = resid;
initializeTheme();
}

@Override public Resources.Theme getTheme() {
if (mTheme != null) {
return mTheme;
}

mThemeResource = Resources.selectDefaultTheme(mThemeResource,
getApplicationInfo().targetSdkVersion);
initializeTheme();

return mTheme;
}
}