Android开发中Context类的作用以及Context的详细用法

Android中Context的作用以及Context的详细用法

本文我们一起来探讨一下关于Android中Context的作用以及Context的详细用法，这对我们学习Android的资源访问有很大的帮助，文章中也贴出了一些关于Android Context使用的示例代码，非常不错，以下是原文：

Context基本概念

Context是什么?

1) Context是一个抽象类，其通用实现在ContextImpl类中。

2) Context：是一个访问application环境全局信息的接口，通过它可以访问application的资源和相关的类，其主要功能如下：

启动Activity
启动和停止Service
发送广播消息(Intent)
注册广播消息(Intent)接收者
可以访问APK中各种资源(如Resources和AssetManager等)
可以访问Package的相关信息
APK的各种权限管理

从以上分析可以看出，Context就是一个对APK包无所不知的大管家，大家需要什么，直接问它就可以了。

Context与View的关系

View与Context(或Activity)的关系类似于明星与经纪人的关系，所以创建View时，必须明确指定其Context(即经纪人或大管家)，否则View就成不了明星。

Context家族关系

Context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<String, SharedPreferencesImpl> sSharedPrefs =
 new HashMap<String, SharedPreferencesImpl>();
 
 /*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

它只是对Context类的一种封装，它的构造函数包含了一个真正的Context引用，即ContextImpl对象。

 /**
 * 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

该类内部包含了主题(Theme)相关的接口，即android:theme属性指定的。只有Activity需要主题，Service不需要主题，所以Service直接继承于ContextWrapper类。

 /**
 * 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;
 }
 }

何时创建Context

应用程序在以下几种情况下创建Context实例：

1) 创建Application 对象时， 而且整个App共一个Application对象

2) 创建Service对象时

3) 创建Activity对象时

因此应用程序App共有的Context数目公式为：

总Context实例个数 = Service个数 + Activity个数 + 1（Application对应的Context实例）

ActivityThread消息处理函数与本节相关的内容如下：

 public void handleMessage(Message msg) {
 if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
 switch (msg.what) {
 case LAUNCH_ACTIVITY: { // 创建Activity对象
 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
 ActivityClientRecord r = (ActivityClientRecord)msg.obj;
 
 r.packageInfo = getPackageInfoNoCheck(
 r.activityInfo.applicationInfo, r.compatInfo);
 handleLaunchActivity(r, null);
 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
 } break;
 
 case BIND_APPLICATION: // 创建Application对象
 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
 AppBindData data = (AppBindData)msg.obj;
 handleBindApplication(data);
 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
 break;
 
 case CREATE_SERVICE: // 创建Service对象
 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceCreate");
 handleCreateService((CreateServiceData)msg.obj);
 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
 break;
 
 case BIND_SERVICE:  // Bind Service对象
 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
 handleBindService((BindServiceData)msg.obj);
 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
 break;
 }
 }

创建Application对象时创建Context实例

每个应用程序在第一次启动时，都会首先创建一个Application对象。从startActivity流程可知，创建Application的时机在handleBindApplication()方法中，该函数位于 ActivityThread.java类中 ，相关代码如下：

 // ActivityThread.java
 private void handleBindApplication(AppBindData data) {
 try {
 // If the app is being launched for full backup or restore, bring it up in
 // a restricted environment with the base application class.
 Application app = data.info.makeApplication(data.restrictedBackupMode, null);
 mInitialApplication = app;
 ...
 } finally {
 StrictMode.setThreadPolicy(savedPolicy);
 }
 }
 
 // LoadedApk.java
 public Application makeApplication(boolean forceDefaultAppClass,
 Instrumentation instrumentation) {
 if (mApplication != null) {
 return mApplication;
 }
 
 Application app = null;
 
 String appClass = mApplicationInfo.className;
 if (forceDefaultAppClass || (appClass == null)) {
 appClass = "android.app.Application";
 }
 
 try {
 java.lang.ClassLoader cl = getClassLoader();
 ContextImpl appContext = new ContextImpl(); // 创建ContextImpl实例
 appContext.init(this, null, mActivityThread);
 app = mActivityThread.mInstrumentation.newApplication(
 cl, appClass, appContext);
 appContext.setOuterContext(app); // 将Application实例传递给Context实例
 } catch (Exception e) {
 ...
 }
 mActivityThread.mAllApplications.add(app);
 mApplication = app;
 
 return app;
 }
 private Context createBaseContextForActivity(ActivityClientRecord r,
 final Activity activity) {
 ContextImpl appContext = new ContextImpl();  // 创建ContextImpl实例
 appContext.init(r.packageInfo, r.token, this);
 appContext.setOuterContext(activity);
 
 // For debugging purposes, if the activity's package name contains the value of
 // the "debug.use-second-display" system property as a substring, then show
 // its content on a secondary display if there is one.
 Context baseContext = appContext;
 String pkgName = SystemProperties.get("debug.second-display.pkg");
 if (pkgName != null && !pkgName.isEmpty()
 && r.packageInfo.mPackageName.contains(pkgName)) {
 DisplayManagerGlobal dm = DisplayManagerGlobal.getInstance();
 for (int displayId : dm.getDisplayIds()) {
 if (displayId != Display.DEFAULT_DISPLAY) {
 Display display = dm.getRealDisplay(displayId);
 baseContext = appContext.createDisplayContext(display);
 break;
 }
 }
 }
 return baseContext;
 }

创建Service对象时创建Context实例

通过startService或者bindService时，如果系统检测到需要新创建一个Service实例，就会回调handleCreateService()方法，完成相关数据操作。handleCreateService()函数位于 ActivityThread.java类，如下：

 private void handleCreateService(CreateServiceData data) {
 // If we are getting ready to gc after going to the background, well
 // we are back active so skip it.
 unscheduleGcIdler();
 
 LoadedApk packageInfo = getPackageInfoNoCheck(
 data.info.applicationInfo, data.compatInfo);
 Service service = null;
 try {
 java.lang.ClassLoader cl = packageInfo.getClassLoader();
 service = (Service) cl.loadClass(data.info.name).newInstance();
 } catch (Exception e) {
 if (!mInstrumentation.onException(service, e)) {
 throw new RuntimeException(
 "Unable to instantiate service " + data.info.name
 + ": " + e.toString(), e);
 }
 }
 
 try {
 if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);
 
 ContextImpl context = new ContextImpl(); // 创建ContextImpl实例
 context.init(packageInfo, null, this);
 
 Application app = packageInfo.makeApplication(false, mInstrumentation);
 context.setOuterContext(service);
 service.attach(context, this, data.info.name, data.token, app,
 ActivityManagerNative.getDefault());
 service.onCreate();
 mServices.put(data.token, service);
 try {
 ActivityManagerNative.getDefault().serviceDoneExecuting(
 data.token, 0, 0, 0);
 } catch (RemoteException e) {
 // nothing to do.
 }
 } catch (Exception e) {
 if (!mInstrumentation.onException(service, e)) {
 throw new RuntimeException(
 "Unable to create service " + data.info.name
 + ": " + e.toString(), e);
 }
 }
 }

 private void handleCreateService(CreateServiceData data) {
 // If we are getting ready to gc after going to the background, well
 // we are back active so skip it.
 unscheduleGcIdler();
 
 LoadedApk packageInfo = getPackageInfoNoCheck(
 data.info.applicationInfo, data.compatInfo);
 Service service = null;
 try {
 java.lang.ClassLoader cl = packageInfo.getClassLoader();
 service = (Service) cl.loadClass(data.info.name).newInstance();
 } catch (Exception e) {
 if (!mInstrumentation.onException(service, e)) {
 throw new RuntimeException(
 "Unable to instantiate service " + data.info.name
 + ": " + e.toString(), e);
 }
 }
 
 try {
 if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);
 
 ContextImpl context = new ContextImpl(); // 创建ContextImpl实例
 context.init(packageInfo, null, this);
 
 Application app = packageInfo.makeApplication(false, mInstrumentation);
 context.setOuterContext(service);
 service.attach(context, this, data.info.name, data.token, app,
 ActivityManagerNative.getDefault());
 service.onCreate();
 mServices.put(data.token, service);
 try {
 ActivityManagerNative.getDefault().serviceDoneExecuting(
 data.token, 0, 0, 0);
 } catch (RemoteException e) {
 // nothing to do.
 }
 } catch (Exception e) {
 if (!mInstrumentation.onException(service, e)) {
 throw new RuntimeException(
 "Unable to create service " + data.info.name
 + ": " + e.toString(), e);
 }
 }
 }

小结

通过对ContextImp的分析可知，其方法的大多数操作都是直接调用其属性mPackageInfo(该属性类型为PackageInfo)的相关方法而来。这说明ContextImp是一种轻量级类，而PackageInfo才是真正重量级的类。而一个App里的所有ContextImpl实例，都对应同一个packageInfo对象。