Android中Context用法詳解學習
Android中Context用法詳解學習
本文我們一起來探討一下關於Android中Context的作用以及Context的詳細用法,這對我們學習Android的資源訪問有很大的幫助,文章中也貼出了一些關於Android Context使用的示例程式碼,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,其通用實現在ContextImpl類中。
2) Context:是一個訪問application環境全域性資訊的介面,通過它可以訪問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 傳送廣播訊息(Intent) 註冊廣播訊息(Intent)接收者 可以訪問APK中各種資源(如Resources和AssetManager等) 可以訪問Package的相關資訊 APK的各種許可權管理
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的關係
View與Context(或Activity)的關係類似於明星與經紀人的關係,所以建立View時,必須明確指定其Context(即經紀人或大管家),否則View就成不了明星。
Context家族關係
Context關鍵函式
public abstract class Context { // 獲取應用程式包的AssetManager例項 public abstract AssetManager getAssets(); // 獲取應用程式包的Resources例項 public abstract Resources getResources(); // 獲取PackageManager例項,以檢視全域性package資訊 public abstract PackageManager getPackageManager(); // 獲取應用程式包的ContentResolver例項 public abstract ContentResolver getContentResolver(); // 它返回當前程序的主執行緒的Looper,此執行緒分發呼叫給應用元件(activities, services等) public abstract Looper getMainLooper(); // 返回當前程序的單例項全域性Application物件的Context public abstract Context getApplicationContext(); // 從string表中獲取本地化的、格式化的字元序列 public final CharSequence getText(int resId) { return getResources().getText(resId); } // 從string表中獲取本地化的字串 public final String getString(int resId) { return getResources().getString(resId); } public final String getString(int resId, Object... formatArgs) { return getResources().getString(resId, formatArgs); } // 返回一個可用於獲取包中類資訊的class loader public abstract ClassLoader getClassLoader(); // 返回應用程式包名 public abstract String getPackageName(); // 返回應用程式資訊 public abstract ApplicationInfo getApplicationInfo(); // 根據檔名獲取SharedPreferences public abstract SharedPreferences getSharedPreferences(String name, int mode); // 其根目錄為: 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); // 返回應用程式obb檔案路徑 public abstract File getObbDir(); // 啟動一個新的activity public abstract void startActivity(Intent intent); // 啟動一個新的activity public void startActivityAsUser(Intent intent, UserHandle user) { throw new RuntimeException("Not implemented. Must override in a subclass."); } // 啟動一個新的activity // intent: 將被啟動的activity的描述資訊 // options: 描述activity將如何被啟動 public abstract void startActivity(Intent intent, Bundle options); // 啟動多個新的activity public abstract void startActivities(Intent[] intents); // 啟動多個新的activity public abstract void startActivities(Intent[] intents, Bundle options); // 廣播一個intent給所有感興趣的接收者,非同步機制 public abstract void sendBroadcast(Intent intent); // 廣播一個intent給所有感興趣的接收者,非同步機制 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); // 註冊一個BroadcastReceiver,且它將在主activity執行緒中執行 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); // 請求啟動一個application service public abstract ComponentName startService(Intent service); // 請求停止一個application service public abstract boolean stopService(Intent service); // 連線一個應用服務,它定義了application和service間的依賴關係 public abstract boolean bindService(Intent service, ServiceConnection conn, int flags); // 斷開一個應用服務,當服務重新開始時,將不再接收到呼叫, // 且服務允許隨時停止 public abstract void unbindService(ServiceConnection conn); // 返回系統級service控制代碼 /* * @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); // 返回一個新的與application name對應的Context物件 public abstract Context createPackageContext(String packageName, int flags) throws PackageManager.NameNotFoundException; // 返回基於當前Context物件的新物件,其資源與display相匹配 public abstract Context createDisplayContext(Display display); }
ContextImpl關鍵成員和函式
/** * 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; // 關鍵資料成員 private String mBasePackageName; private Resources mResources; /*package*/ ActivityThread mMainThread; // 主執行緒 @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
它只是對Context類的一種封裝,它的建構函式包含了一個真正的Context引用,即ContextImpl物件。
/**
* 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; //該屬性指向一個ContextIml例項
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.
* 建立Application、Service、Activity,會呼叫該方法給mBase屬性賦值
*/
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
該類內部包含了主題(Theme)相關的介面,即android:theme屬性指定的。只有Activity需要主題,Service不需要主題,所以Service直接繼承於ContextWrapper類。
/**
* 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;
}
}
何時建立Context
應用程式在以下幾種情況下建立Context例項:
1) 建立Application 物件時, 而且整個App共一個Application物件
2) 建立Service物件時
3) 建立Activity物件時
因此應用程式App共有的Context數目公式為:
總Context例項個數 = Service個數 + Activity個數 + 1(Application對應的Context例項)
ActivityThread訊息處理函式與本節相關的內容如下:
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
case LAUNCH_ACTIVITY: { // 建立Activity物件
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: // 建立Application物件
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: // 建立Service物件
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物件
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
handleBindService((BindServiceData)msg.obj);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
}
}
建立Application物件時建立Context例項
每個應用程式在第一次啟動時,都會首先建立一個Application物件。從startActivity流程可知,建立Application的時機在handleBindApplication()方法中,該函式位於 ActivityThread.java類中 ,相關程式碼如下:
// 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(); // 建立ContextImpl例項
appContext.init(this, null, mActivityThread);
app = mActivityThread.mInstrumentation.newApplication(
cl, appClass, appContext);
appContext.setOuterContext(app); // 將Application例項傳遞給Context例項
} catch (Exception e) {
...
}
mActivityThread.mAllApplications.add(app);
mApplication = app;
return app;
}
private Context createBaseContextForActivity(ActivityClientRecord r,
final Activity activity) {
ContextImpl appContext = new ContextImpl(); // 建立ContextImpl例項
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;
}
建立Service物件時建立Context例項
通過startService或者bindService時,如果系統檢測到需要新建立一個Service例項,就會回撥handleCreateService()方法,完成相關資料操作。handleCreateService()函式位於 ActivityThread.java類,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(); // 建立ContextImpl例項
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);
}
}
}
小結
通過對ContextImp的分析可知,其方法的大多數操作都是直接呼叫其屬性mPackageInfo(該屬性型別為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例項,都對應同一個packageInfo物件。