Android中Context的作用以及Context的詳細用法
本文我們一起來探讨一下關于Android中Context的作用以及Context的詳細用法，這對我們學習Android的資源通路有很大的幫助，文章中也貼出了一些關于Android Context使用的示例代碼，非常不錯，以下是原文：
Context基本概念
Context是什麼?
1) Context是一個抽象類，其通用實作在ContextImpl類中。
2) Context：是一個通路application環境全局資訊的接口，通過它可以通路application的資源和相關的類，其主要功能如下：
啟動Activity
啟動和停止Service
發送廣播消息(Intent)
注冊廣播消息(Intent)接收者
可以通路APK中各種資源(如Resources和AssetManager等)
可以通路Package的相關資訊
APK的各種權限管理
從以上分析可以看出，Context就是一個對APK包無所不知的大管家，大家需要什麼，直接問它就可以了。
Context與View的關系
View與Context(或Activity)的關系類似于明星與經紀人的關系，是以建立View時，必須明确指定其Context(即經紀人或大管家)，否則View就成不了明星。
Context家族關系
Context關鍵函數
1 public abstract class Context {
2
3 // 擷取應用程式包的AssetManager執行個體
4 public abstract AssetManager getAssets();
5
6 // 擷取應用程式包的Resources執行個體
7 public abstract Resources getResources();
8
9 // 擷取PackageManager執行個體，以檢視全局package資訊
10 public abstract PackageManager getPackageManager();
11
12 // 擷取應用程式包的ContentResolver執行個體
13 public abstract ContentResolver getContentResolver();
14
15 // 它傳回目前程序的主線程的Looper，此線程分發調用給應用元件(activities, services等)
16 public abstract Looper getMainLooper();
17
18 // 傳回目前程序的單執行個體全局Application對象的Context
19 public abstract Context getApplicationContext();
20
21 // 從string表中擷取本地化的、格式化的字元序列
22 public final CharSequence getText(int resId) {
23 return getResources().getText(resId);
24 }
25
26 // 從string表中擷取本地化的字元串
27 public final String getString(int resId) {
28 return getResources().getString(resId);
29 }
30
31 public final String getString(int resId, Object... formatArgs) {
32 return getResources().getString(resId, formatArgs);
33 }
34
35 // 傳回一個可用于擷取包中類資訊的class loader
36 public abstract ClassLoader getClassLoader();
37
38 // 傳回應用程式包名
39 public abstract String getPackageName();
40
41 // 傳回應用程式資訊
42 public abstract ApplicationInfo getApplicationInfo();
43
44 // 根據檔案名擷取SharedPreferences
45 public abstract SharedPreferences getSharedPreferences(String name,
46 int mode);
47
48 // 其根目錄為: Environment.getExternalStorageDirectory()
49 /*
50 * @param type The type of files directory to return. May be null for
51 * the root of the files directory or one of
52 * the following Environment constants for a subdirectory:
53 * {@link android.os.Environment#DIRECTORY_MUSIC},
54 * {@link android.os.Environment#DIRECTORY_PODCASTS},
55 * {@link android.os.Environment#DIRECTORY_RINGTONES},
56 * {@link android.os.Environment#DIRECTORY_ALARMS},
57 * {@link android.os.Environment#DIRECTORY_NOTIFICATIONS},
58 * {@link android.os.Environment#DIRECTORY_PICTURES}, or
59 * {@link android.os.Environment#DIRECTORY_MOVIES}.
60 */
61 public abstract File getExternalFilesDir(String type);
62
63 // 傳回應用程式obb檔案路徑
64 public abstract File getObbDir();
65
66 // 啟動一個新的activity
67 public abstract void startActivity(Intent intent);
68
69 // 啟動一個新的activity
70 public void startActivityAsUser(Intent intent, UserHandle user) {
71 throw new RuntimeException("Not implemented. Must override in a subclass.");
72 }
73
74 // 啟動一個新的activity
75 // intent: 将被啟動的activity的描述資訊
76 // options: 描述activity将如何被啟動
77 public abstract void startActivity(Intent intent, Bundle options);
78
79 // 啟動多個新的activity
80 public abstract void startActivities(Intent[] intents);
81
82 // 啟動多個新的activity
83 public abstract void startActivities(Intent[] intents, Bundle options);
84
85 // 廣播一個intent給所有感興趣的接收者，異步機制
86 public abstract void sendBroadcast(Intent intent);
87
88 // 廣播一個intent給所有感興趣的接收者，異步機制
89 public abstract void sendBroadcast(Intent intent,String receiverPermission);
90
91 public abstract void sendOrderedBroadcast(Intent intent,String receiverPermission);
92
93 public abstract void sendOrderedBroadcast(Intent intent,
94 String receiverPermission, BroadcastReceiver resultReceiver,
95 Handler scheduler, int initialCode, String initialData,
96 Bundle initialExtras);
97
98 public abstract void sendBroadcastAsUser(Intent intent, UserHandle user);
99
100 public abstract void sendBroadcastAsUser(Intent intent, UserHandle user,
101 String receiverPermission);
102
103 // 注冊一個BroadcastReceiver，且它将在主activity線程中運作
104 public abstract Intent registerReceiver(BroadcastReceiver receiver,
105 IntentFilter filter);
106
107 public abstract Intent registerReceiver(BroadcastReceiver receiver,
108 IntentFilter filter, String broadcastPermission, Handler scheduler);
109
110 public abstract void unregisterReceiver(BroadcastReceiver receiver);
111
112 // 請求啟動一個application service
113 public abstract ComponentName startService(Intent service);
114
115 // 請求停止一個application service
116 public abstract boolean stopService(Intent service);
117
118 // 連接配接一個應用服務，它定義了application和service間的依賴關系
119 public abstract boolean bindService(Intent service, ServiceConnection conn,
120 int flags);
121
122 // 斷開一個應用服務，當服務重新開始時，将不再接收到調用，
123 // 且服務允許随時停止
124 public abstract void unbindService(ServiceConnection conn);
125
126 // 傳回系統級service句柄
127 /*
128 * @see #WINDOW_SERVICE
129 * @see android.view.WindowManager
130 * @see #LAYOUT_INFLATER_SERVICE
131 * @see android.view.LayoutInflater
132 * @see #ACTIVITY_SERVICE
133 * @see android.app.ActivityManager
134 * @see #POWER_SERVICE
135 * @see android.os.PowerManager
136 * @see #ALARM_SERVICE
137 * @see android.app.AlarmManager
138 * @see #NOTIFICATION_SERVICE
139 * @see android.app.NotificationManager
140 * @see #KEYGUARD_SERVICE
141 * @see android.app.KeyguardManager
142 * @see #LOCATION_SERVICE
143 * @see android.location.LocationManager
144 * @see #SEARCH_SERVICE
145 * @see android.app.SearchManager
146 * @see #SENSOR_SERVICE
147 * @see android.hardware.SensorManager
148 * @see #STORAGE_SERVICE
149 * @see android.os.storage.StorageManager
150 * @see #VIBRATOR_SERVICE
151 * @see android.os.Vibrator
152 * @see #CONNECTIVITY_SERVICE
153 * @see android.net.ConnectivityManager
154 * @see #WIFI_SERVICE
155 * @see android.net.wifi.WifiManager
156 * @see #AUDIO_SERVICE
157 * @see android.media.AudioManager
158 * @see #MEDIA_ROUTER_SERVICE
159 * @see android.media.MediaRouter
160 * @see #TELEPHONY_SERVICE
161 * @see android.telephony.TelephonyManager
162 * @see #INPUT_METHOD_SERVICE
163 * @see android.view.inputmethod.InputMethodManager
164 * @see #UI_MODE_SERVICE
165 * @see android.app.UiModeManager
166 * @see #DOWNLOAD_SERVICE
167 * @see android.app.DownloadManager
168 */
169 public abstract Object getSystemService(String name);
170
171 public abstract int checkPermission(String permission, int pid, int uid);
172
173 // 傳回一個新的與application name對應的Context對象
174 public abstract Context createPackageContext(String packageName,
175 int flags) throws PackageManager.NameNotFoundException;
176
177 // 傳回基于目前Context對象的新對象，其資源與display相比對
178 public abstract Context createDisplayContext(Display display);
179 } ContextImpl關鍵成員和函數
1 /**
2 * Common implementation of Context API, which provides the base
3 * context object for Activity and other application components.
4 */
5 class ContextImpl extends Context {
6 private final static String TAG = "ContextImpl";
7 private final static boolean DEBUG = false;
8
9 private static final HashMap<String, SharedPreferencesImpl> sSharedPrefs =
10 new HashMap<String, SharedPreferencesImpl>();
11
12 /*package*/ LoadedApk mPackageInfo; // 關鍵資料成員
13 private String mBasePackageName;
14 private Resources mResources;
15 /*package*/ ActivityThread mMainThread; // 主線程
16
17 @Override
18 public AssetManager getAssets() {
19 return getResources().getAssets();
20 }
21
22 @Override
23 public Looper getMainLooper() {
24 return mMainThread.getLooper();
25 }
26
27 @Override
28 public Object getSystemService(String name) {
29 ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
30 return fetcher == null ? null : fetcher.getService(this);
31 }
32
33 @Override
34 public void startActivity(Intent intent, Bundle options) {
35 warnIfCallingFromSystemProcess();
36 if ((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
37 throw new AndroidRuntimeException(
38 "Calling startActivity() from outside of an Activity "
39 + " context requires the FLAG_ACTIVITY_NEW_TASK flag."
40 + " Is this really what you want?");
41 }
42 mMainThread.getInstrumentation().execStartActivity(
43 getOuterContext(), mMainThread.getApplicationThread(), null,
44 (Activity)null, intent, -1, options);
45 }
46 } ContextWrapper
它隻是對Context類的一種封裝，它的構造函數包含了一個真正的Context引用，即ContextImpl對象。
1 /**
2 * Proxying implementation of Context that simply delegates all of its calls to
3 * another Context. Can be subclassed to modify behavior without changing
4 * the original Context.
5 */
6 public class ContextWrapper extends Context {
7 Context mBase; //該屬性指向一個ContextIml執行個體
8
9 public ContextWrapper(Context base) {
10 mBase = base;
11 }
12
13 /**
14 * Set the base context for this ContextWrapper. All calls will then be
15 * delegated to the base context. Throws
16 * IllegalStateException if a base context has already been set.
17 *
18 * @param base The new base context for this wrapper.
19 * 建立Application、Service、Activity，會調用該方法給mBase屬性指派
20 */
21 protected void attachBaseContext(Context base) {
22 if (mBase != null) {
23 throw new IllegalStateException("Base context already set");
24 }
25 mBase = base;
26 }
27
28 @Override
29 public Looper getMainLooper() {
30 return mBase.getMainLooper();
31 }
32
33 @Override
34 public Object getSystemService(String name) {
35 return mBase.getSystemService(name);
36 }
37
38 @Override
39 public void startActivity(Intent intent) {
40 mBase.startActivity(intent);
41 }
42 } ContextThemeWrapper
該類内部包含了主題(Theme)相關的接口，即android:theme屬性指定的。隻有Activity需要主題，Service不需要主題，是以Service直接繼承于ContextWrapper類。
1 /**
2 * A ContextWrapper that allows you to modify the theme from what is in the
3 * wrapped context.
4 */
5 public class ContextThemeWrapper extends ContextWrapper {
6 private Context mBase;
7 private int mThemeResource;
8 private Resources.Theme mTheme;
9 private LayoutInflater mInflater;
10 private Configuration mOverrideConfiguration;
11 private Resources mResources;
12
13 public ContextThemeWrapper() {
14 super(null);
15 }
16
17 public ContextThemeWrapper(Context base, int themeres) {
18 super(base);
19 mBase = base;
20 mThemeResource = themeres;
21 }
22
23 @Override protected void attachBaseContext(Context newBase) {
24 super.attachBaseContext(newBase);
25 mBase = newBase;
26 }
27
28 @Override public void setTheme(int resid) {
29 mThemeResource = resid;
30 initializeTheme();
31 }
32
33 @Override public Resources.Theme getTheme() {
34 if (mTheme != null) {
35 return mTheme;
36 }
37
38 mThemeResource = Resources.selectDefaultTheme(mThemeResource,
39 getApplicationInfo().targetSdkVersion);
40 initializeTheme();
41
42 return mTheme;
43 }
44 } 何時建立Context
應用程式在以下幾種情況下建立Context執行個體：
1) 建立Application 對象時， 而且整個App共一個Application對象
2) 建立Service對象時
3) 建立Activity對象時
是以應用程式App共有的Context數目公式為：
總Context執行個體個數 = Service個數 + Activity個數 + 1（Application對應的Context執行個體）
ActivityThread消息處理函數與本節相關的内容如下：
1 public void handleMessage(Message msg) {
2 if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
3 switch (msg.what) {
4 case LAUNCH_ACTIVITY: { // 建立Activity對象
5 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
6 ActivityClientRecord r = (ActivityClientRecord)msg.obj;
7
8 r.packageInfo = getPackageInfoNoCheck(
9 r.activityInfo.applicationInfo, r.compatInfo);
10 handleLaunchActivity(r, null);
11 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
12 } break;
13
14 case BIND_APPLICATION: // 建立Application對象
15 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
16 AppBindData data = (AppBindData)msg.obj;
17 handleBindApplication(data);
18 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
19 break;
20
21 case CREATE_SERVICE: // 建立Service對象
22 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceCreate");
23 handleCreateService((CreateServiceData)msg.obj);
24 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
25 break;
26
27 case BIND_SERVICE: // Bind Service對象
28 Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
29 handleBindService((BindServiceData)msg.obj);
30 Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
31 break;
32 }
33 } 建立Application對象時建立Context執行個體
每個應用程式在第一次啟動時，都會首先建立一個Application對象。從startActivity流程可知，建立Application的時機在handleBindApplication()方法中，該函數位于 ActivityThread.java類中 ，相關代碼如下：
1 // ActivityThread.java
2 private void handleBindApplication(AppBindData data) {
3 try {
4 // If the app is being launched for full backup or restore, bring it up in
5 // a restricted environment with the base application class.
6 Application app = data.info.makeApplication(data.restrictedBackupMode, null);
7 mInitialApplication = app;
8 ...
9 } finally {
10 StrictMode.setThreadPolicy(savedPolicy);
11 }
12 }
13
14 // LoadedApk.java
15 public Application makeApplication(boolean forceDefaultAppClass,
16 Instrumentation instrumentation) {
17 if (mApplication != null) {
18 return mApplication;
19 }
20
21 Application app = null;
22
23 String appClass = mApplicationInfo.className;
24 if (forceDefaultAppClass || (appClass == null)) {
25 appClass = "android.app.Application";
26 }
27
28 try {
29 java.lang.ClassLoader cl = getClassLoader();
30 ContextImpl appContext = new ContextImpl(); // 建立ContextImpl執行個體
31 appContext.init(this, null, mActivityThread);
32 app = mActivityThread.mInstrumentation.newApplication(
33 cl, appClass, appContext);
34 appContext.setOuterContext(app); // 将Application執行個體傳遞給Context執行個體
35 } catch (Exception e) {
36 ...
37 }
38 mActivityThread.mAllApplications.add(app);
39 mApplication = app;
40
41 return app;
42 }
43 private Context createBaseContextForActivity(ActivityClientRecord r,
44 final Activity activity) {
45 ContextImpl appContext = new ContextImpl(); // 建立ContextImpl執行個體
46 appContext.init(r.packageInfo, r.token, this);
47 appContext.setOuterContext(activity);
48
49 // For debugging purposes, if the activity\'s package name contains the value of
50 // the "debug.use-second-display" system property as a substring, then show
51 // its content on a secondary display if there is one.
52 Context baseContext = appContext;
53 String pkgName = SystemProperties.get("debug.second-display.pkg");
54 if (pkgName != null && !pkgName.isEmpty()
55 && r.packageInfo.mPackageName.contains(pkgName)) {
56 DisplayManagerGlobal dm = DisplayManagerGlobal.getInstance();
57 for (int displayId : dm.getDisplayIds()) {
58 if (displayId != Display.DEFAULT_DISPLAY) {
59 Display display = dm.getRealDisplay(displayId);
60 baseContext = appContext.createDisplayContext(display);
61 break;
62 }
63 }
64 }
65 return baseContext;
66 } 建立Service對象時建立Context執行個體
通過startService或者bindService時，如果系統檢測到需要新建立一個Service執行個體，就會回調handleCreateService()方法，完成相關資料操作。handleCreateService()函數位于 ActivityThread.java類，如下：
1 private void handleCreateService(CreateServiceData data) {
2 // If we are getting ready to gc after going to the background, well
3 // we are back active so skip it.
4 unscheduleGcIdler();
5
6 LoadedApk packageInfo = getPackageInfoNoCheck(
7 data.info.applicationInfo, data.compatInfo);
8 Service service = null;
9 try {
10 java.lang.ClassLoader cl = packageInfo.getClassLoader();
11 service = (Service) cl.loadClass(data.info.name).newInstance();
12 } catch (Exception e) {
13 if (!mInstrumentation.onException(service, e)) {
14 throw new RuntimeException(
15 "Unable to instantiate service " + data.info.name
16 + ": " + e.toString(), e);
17 }
18 }
19
20 try {
21 if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);
22
23 ContextImpl context = new ContextImpl(); // 建立ContextImpl執行個體
24 context.init(packageInfo, null, this);
25
26 Application app = packageInfo.makeApplication(false, mInstrumentation);
27 context.setOuterContext(service);
28 service.attach(context, this, data.info.name, data.token, app,
29 ActivityManagerNative.getDefault());
30 service.onCreate();
31 mServices.put(data.token, service);
32 try {
33 ActivityManagerNative.getDefault().serviceDoneExecuting(
34 data.token, 0, 0, 0);
35 } catch (RemoteException e) {
36 // nothing to do.
37 }
38 } catch (Exception e) {
39 if (!mInstrumentation.onException(service, e)) {
40 throw new RuntimeException(
41 "Unable to create service " + data.info.name
42 + ": " + e.toString(), e);
43 }
44 }
45 } 1 private void handleCreateService(CreateServiceData data) {
2 // If we are getting ready to gc after going to the background, well
3 // we are back active so skip it.
4 unscheduleGcIdler();
5
6 LoadedApk packageInfo = getPackageInfoNoCheck(
7 data.info.applicationInfo, data.compatInfo);
8 Service service = null;
9 try {
10 java.lang.ClassLoader cl = packageInfo.getClassLoader();
11 service = (Service) cl.loadClass(data.info.name).newInstance();
12 } catch (Exception e) {
13 if (!mInstrumentation.onException(service, e)) {
14 throw new RuntimeException(
15 "Unable to instantiate service " + data.info.name
16 + ": " + e.toString(), e);
17 }
18 }
19
20 try {
21 if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);
22
23 ContextImpl context = new ContextImpl(); // 建立ContextImpl執行個體
24 context.init(packageInfo, null, this);
25
26 Application app = packageInfo.makeApplication(false, mInstrumentation);
27 context.setOuterContext(service);
28 service.attach(context, this, data.info.name, data.token, app,
29 ActivityManagerNative.getDefault());
30 service.onCreate();
31 mServices.put(data.token, service);
32 try {
33 ActivityManagerNative.getDefault().serviceDoneExecuting(
34 data.token, 0, 0, 0);
35 } catch (RemoteException e) {
36 // nothing to do.
37 }
38 } catch (Exception e) {
39 if (!mInstrumentation.onException(service, e)) {
40 throw new RuntimeException(
41 "Unable to create service " + data.info.name
42 + ": " + e.toString(), e);
43 }
44 }
45 } 小結
通過對ContextImp的分析可知，其方法的大多數操作都是直接調用其屬性mPackageInfo(該屬性類型為PackageInfo)的相關方法而來。這說明ContextImp是一種輕量級類，而PackageInfo才是真正重量級的類。而一個App裡的所有ContextImpl執行個體，都對應同一個packageInfo對象。