Android中Context的作用是什么
这期内容当中小编将会给大家带来有关Android中Context的作用是什么，文章内容丰富且以专业的角度为大家分析和叙述，阅读完这篇文章希望大家可以有所收获。
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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 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
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***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; }创建Activity对象时创建Context实例
通过startActivity()或startActivityForResult()请求启动一个Activity时，如果系统检测需要新建一个Activity对象时，就会回调handleLaunchActivity()方法，该方法继而调用performLaunchActivity()方法，去创建一个Activity实例，并且回调onCreate()，onStart()方法等，函数都位于 ActivityThread.java类 ，相关代码如下：
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) { ... Activity a = performLaunchActivity(r, customIntent); // 到下一步 if (a != null) { r.createdConfig = new Configuration(mConfiguration); Bundle oldState = r.state; handleResumeActivity(r.token, false, r.isForward, !r.activity.mFinished && !r.startsNotResumed); ... } ... } private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) { ... Activity activity = null; try { java.lang.ClassLoader cl = r.packageInfo.getClassLoader(); activity = mInstrumentation.newActivity( cl, component.getClassName(), r.intent); StrictMode.incrementExpectedActivityCount(activity.getClass()); r.intent.setExtrasClassLoader(cl); if (r.state != null) { r.state.setClassLoader(cl); } } catch (Exception e) { ... } try { Application app = r.packageInfo.makeApplication(false, mInstrumentation); if (activity != null) { Context appContext = createBaseContextForActivity(r, activity); // 创建Context CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager()); Configuration config = new Configuration(mCompatConfiguration); if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity " + r.activityInfo.name + " with config " + config); activity.attach(appContext, this, getInstrumentation(), r.token, r.ident, app, r.intent, r.activityInfo, title, r.parent, r.embeddedID, r.lastNonConfigurationInstances, config); if (customIntent != null) { activity.mIntent = customIntent; } r.lastNonConfigurationInstances = null; activity.mStartedActivity = false; int theme = r.activityInfo.getThemeResource(); if (theme != 0) { activity.setTheme(theme); } mActivities.put(r.token, r); } catch (SuperNotCalledException e) { ... } catch (Exception e) { ... } return activity; }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); } } }上述就是小编为大家分享的Android中Context的作用是什么了，如果刚好有类似的疑惑，不妨参照上述分析进行理解。如果想知道更多相关知识，欢迎关注创新互联行业资讯频道。
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