Android中Context用法详解

Android中Context用法详解

 

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> sSharedP
12174
refs =
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

<span style="white-space:pre"> </span>@Override
<span style="white-space:pre"> </span>public AssetManager getAssets() {
<span style="white-space:pre"> </span>return getResources().getAssets();
<span style="white-space:pre"> </span>}

<span style="white-space:pre"> </span>@Override
<span style="white-space:pre"> </span>public Looper getMainLooper() {
<span style="white-space:pre"> </span>return mMainThread.getLooper();
<span style="white-space:pre"> </span>}

<span style="white-space:pre"> </span>@Override
<span style="white-space:pre"> </span>public Object getSystemService(String name) {
<span style="white-space:pre"> </span>ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
<span style="white-space:pre"> </span>return fetcher == null ? null : fetcher.getService(this);
<span style="white-space:pre"> </span>}

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

}

}

}

 

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。