一定要用相同的Context 对同一个receiver进行registerReceiver与unregisterReceiver吗？

大家好，又见面了，我是你们的朋友全栈君。如果您正在找激活码,请点击查看最新教程,关注关注公众号 “全栈程序员社区” 获取激活教程,可能之前旧版本教程已经失效.最新Idea2022.1教程亲测有效,一键激活。

Jetbrains全系列IDE使用 1年只要46元售后保障童叟无欺

最近在开发一些功能，突然想到在动态注册和反注册receiver的时候一定要用相同的context吗？我不敢肯定咨询了同事，得到的答案是不行的，然而为了进一步佐证他的观点我自己尝试了一下。也就是查了一下相关代码。

众所周知android的应用程序在ActivityThread的public static void main(String[] args)中开始调用我们写的代码，Application 、Activity、Service等等。

我们都知道在动态注册和反注册receiver的时候需要一个context，然而无论Application还是Activity都是直接或间接继承自ContextWrapper，而ContextWrapper继承自Context。

在ContextWrapper中

public class ContextWrapper extends Context { 
   
    Context mBase;

    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. */
    protected void attachBaseContext(Context base) {
        if (mBase != null) {
            throw new IllegalStateException("Base context already set");
        }
        mBase = base;
    }

其中mBase是关键，因为

    @Override
    public Intent registerReceiver(
        BroadcastReceiver receiver, IntentFilter filter) {
        return mBase.registerReceiver(receiver, filter);
    }


    @Override
    public void unregisterReceiver(BroadcastReceiver receiver) {
        mBase.unregisterReceiver(receiver);
    }

都是通过mBase来实现的。ok现在关键点就是看在给Activity和Application的mBase赋值时候是不是使用同一个mBase就可以了。

ok我们先看Application的创建过程

public static void main(String[] args) {
        SamplingProfilerIntegration.start();

        // CloseGuard defaults to true and can be quite spammy. We
        // disable it here, but selectively enable it later (via
        // StrictMode) on debug builds, but using DropBox, not logs.
        CloseGuard.setEnabled(false);

        Environment.initForCurrentUser();

        // Set the reporter for event logging in libcore
        EventLogger.setReporter(new EventLoggingReporter());

        Security.addProvider(new AndroidKeyStoreProvider());

        // Make sure TrustedCertificateStore looks in the right place for CA certificates
        final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
        TrustedCertificateStore.setDefaultUserDirectory(configDir);

        Process.setArgV0("<pre-initialized>");

        Looper.prepareMainLooper();

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

其中
ActivityThread thread = new ActivityThread();
thread.attach(false);

    private void attach(boolean system) {
        sCurrentActivityThread = this;
        mSystemThread = system;
        if (!system) {
            ViewRootImpl.addFirstDrawHandler(new Runnable() {
                @Override
                public void run() {
                    ensureJitEnabled();
                }
            });
            android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
                                                    UserHandle.myUserId());
            RuntimeInit.setApplicationObject(mAppThread.asBinder());
            final IActivityManager mgr = ActivityManagerNative.getDefault();
            try {
                mgr.attachApplication(mAppThread);
            } catch (RemoteException ex) {
                // Ignore
            }
            // Watch for getting close to heap limit.
            BinderInternal.addGcWatcher(new Runnable() {
                @Override public void run() {
                    if (!mSomeActivitiesChanged) {
                        return;
                    }
                    Runtime runtime = Runtime.getRuntime();
                    long dalvikMax = runtime.maxMemory();
                    long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
                    if (dalvikUsed > ((3*dalvikMax)/4)) {
                        if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
                                + " total=" + (runtime.totalMemory()/1024)
                                + " used=" + (dalvikUsed/1024));
                        mSomeActivitiesChanged = false;
                        try {
                            mgr.releaseSomeActivities(mAppThread);
                        } catch (RemoteException e) {
                        }
                    }
                }
            });
        } else {
            // Don't set application object here -- if the system crashes,
            // we can't display an alert, we just want to die die die.
            android.ddm.DdmHandleAppName.setAppName("system_process",
                    UserHandle.myUserId());
            try {
                mInstrumentation = new Instrumentation();
                ContextImpl context = ContextImpl.createAppContext(
                        this, getSystemContext().mPackageInfo);
                mInitialApplication = context.mPackageInfo.makeApplication(true, null);
                mInitialApplication.onCreate(); 
            } catch (Exception e) {
                throw new RuntimeException(
                        "Unable to instantiate Application():" + e.toString(), e);
            }
        }

在ContextImple中

 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();
            if (!mPackageName.equals("android")) {
                initializeJavaContextClassLoader();
            }
            ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
            app = mActivityThread.mInstrumentation.newApplication(
                    cl, appClass, appContext);
            appContext.setOuterContext(app);
        } catch (Exception e) {
            if (!mActivityThread.mInstrumentation.onException(app, e)) {
                throw new RuntimeException(
                    "Unable to instantiate application " + appClass
                    + ": " + e.toString(), e);
            }
        }
        mActivityThread.mAllApplications.add(app);
        mApplication = app;

        if (instrumentation != null) {
            try {
                instrumentation.callApplicationOnCreate(app);
            } catch (Exception e) {
                if (!instrumentation.onException(app, e)) {
                    throw new RuntimeException(
                        "Unable to create application " + app.getClass().getName()
                        + ": " + e.toString(), e);
                }
            }
        }

        // Rewrite the R 'constants' for all library apks.
        SparseArray<String> packageIdentifiers = getAssets(mActivityThread)
                .getAssignedPackageIdentifiers();
        final int N = packageIdentifiers.size();
        for (int i = 0; i < N; i++) {
            final int id = packageIdentifiers.keyAt(i);
            if (id == 0x01 || id == 0x7f) {
                continue;
            }

            rewriteRValues(getClassLoader(), packageIdentifiers.valueAt(i), id);
        }

        return app;
    }

其中

ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
app = mActivityThread.mInstrumentation.newApplication(cl, appClass, appContext);

这里创建ContextImple是关键，因为他在创建app的时候会传入进去。

    static ContextImpl createAppContext(ActivityThread mainThread, LoadedApk packageInfo) {
        if (packageInfo == null) throw new IllegalArgumentException("packageInfo");
        return new ContextImpl(null, mainThread,
                packageInfo, null, null, false, null, null);
    }

由此可以看出ContextImpl是new出来的。
而Application最后是

    static public Application newApplication(Class<?> clazz, Context context)
            throws InstantiationException, IllegalAccessException, 
            ClassNotFoundException {
        Application app = (Application)clazz.newInstance();
        app.attach(context);
        return app;
    }

通过java反射机制创建出来的。
由此可以猜测出Application和Activity的mBase不是使用同一个。

为了进一步证明观点。我来看看Activity的创建过程。在ActivityThread中

    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

        ActivityInfo aInfo = r.activityInfo;
        if (r.packageInfo == null) {
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);
        }

        ComponentName component = r.intent.getComponent();
        if (component == null) {
            component = r.intent.resolveActivity(
                mInitialApplication.getPackageManager());
            r.intent.setComponent(component);
        }

        if (r.activityInfo.targetActivity != null) {
            component = new ComponentName(r.activityInfo.packageName,
                    r.activityInfo.targetActivity);
        }

        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);
            r.intent.prepareToEnterProcess();
            if (r.state != null) {
                r.state.setClassLoader(cl);
            }
        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to instantiate activity " + component
                    + ": " + e.toString(), e);
            }
        }
try {
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);

            if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
            if (localLOGV) Slog.v(
                    TAG, r + ": app=" + app
                    + ", appName=" + app.getPackageName()
                    + ", pkg=" + r.packageInfo.getPackageName()
                    + ", comp=" + r.intent.getComponent().toShortString()
                    + ", dir=" + r.packageInfo.getAppDir());

            if (activity != null) {
                Context appContext = createBaseContextForActivity(r, activity);
                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,
                        r.voiceInteractor);

其中

   private Context createBaseContextForActivity(ActivityClientRecord r,
            final Activity activity) {
        ContextImpl appContext = ContextImpl.createActivityContext(this, r.packageInfo, r.token);
        appContext.setOuterContext(activity);
        Context baseContext = appContext;

        final DisplayManagerGlobal dm = DisplayManagerGlobal.getInstance();
        try {
            IActivityContainer container =
                    ActivityManagerNative.getDefault().getEnclosingActivityContainer(r.token);
            final int displayId =
                    container == null ? Display.DEFAULT_DISPLAY : container.getDisplayId();
            if (displayId > Display.DEFAULT_DISPLAY) {
                Display display = dm.getRealDisplay(displayId, r.token);
                baseContext = appContext.createDisplayContext(display);
            }
        } cat

    static ContextImpl createActivityContext(ActivityThread mainThread,
            LoadedApk packageInfo, IBinder activityToken) {
        if (packageInfo == null) throw new IllegalArgumentException("packageInfo");
        if (activityToken == null) throw new IllegalArgumentException("activityInfo");
        return new ContextImpl(null, mainThread,
                packageInfo, activityToken, null, false, null, null);
    }

由此可以得出Activity和Application中的mBase是不同的！！！！！！虽然不同但是还不能完全证明注册的时候使用一个context反注册的时候使用另外一个context不可以。

然而他们的mBase都是ContextImpl，我们进一步看看ContextImpl中对于registerReceiver和unregisterReceiver的实现。

高潮在这里到来！！！

    @Override
    public void unregisterReceiver(BroadcastReceiver receiver) {
        if (mPackageInfo != null) {
            IIntentReceiver rd = mPackageInfo.forgetReceiverDispatcher(
                    getOuterContext(), receiver);
            try {
                ActivityManagerNative.getDefault().unregisterReceiver(rd);
            } catch (RemoteException e) {
            }
        } else {
            throw new RuntimeException("Not supported in system context");
        }
    }

 @Override
    public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter,
            String broadcastPermission, Handler scheduler) {
        return registerReceiverInternal(receiver, getUserId(),
                filter, broadcastPermission, scheduler, getOuterContext());
    }

    private Intent registerReceiverInternal(BroadcastReceiver receiver, int userId,
            IntentFilter filter, String broadcastPermission,
            Handler scheduler, Context context) {
        IIntentReceiver rd = null;
        if (receiver != null) {
            if (mPackageInfo != null && context != null) {
                if (scheduler == null) {
                    scheduler = mMainThread.getHandler();
                }
                rd = mPackageInfo.getReceiverDispatcher(
                    receiver, context, scheduler,
                    mMainThread.getInstrumentation(), true);
            } else {
                if (scheduler == null) {
                    scheduler = mMainThread.getHandler();
                }
                rd = new LoadedApk.ReceiverDispatcher(
                        receiver, context, scheduler, null, true).getIIntentReceiver();
            }
        }
        try {
            return ActivityManagerNative.getDefault().registerReceiver(
                    mMainThread.getApplicationThread(), mBasePackageName,
                    rd, filter, broadcastPermission, userId);
        } catch (RemoteException e) {
            return null;
        }

其中无论注册还是反注册都是通过 ActivityManagerNative.getDefault()的接口来实现的。
ok我们来看看这个接口

    /** * Retrieve the system's default/global activity manager. */
    static public IActivityManager getDefault() {
        return gDefault.get();
    }

    private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
        protected IActivityManager create() {
            IBinder b = ServiceManager.getService("activity");
            if (false) {
                Log.v("ActivityManager", "default service binder = " + b);
            }
            IActivityManager am = asInterface(b);
            if (false) {
                Log.v("ActivityManager", "default service = " + am);
            }
            return am;
        }
    };