registerContentObserver回调两次，ContentObserver回调两次

背景

项目中用到了ContentProvider存取数据，同时想动态监听数据变化。使用getContentResolver().registerContentObserver监听。

存数据代码

Settings.Secure.putInt(getContentResolver(), SECURE_KEY_VOLUME_UP, 1);getContentResolver().notifyChange(Settings.Secure.getUriFor(SECURE_KEY_VOLUME_UP), null);

代码很简单，Settings.Secure其实就是封装的ContentProvider用法，put值，同时notify告知所有监听者。

监听代码

context.getContentResolver().registerContentObserver(uri, false, volumeUpObserver);private ContentObserver volumeUpObserver = new ContentObserver(null) {        @Override        public void onChange(boolean selfChange) {            super.onChange(selfChange);            int value = Settings.Secure.getInt(context.getContentResolver(), SECURE_KEY_VOLUME_UP, -1);            Log.d(GabbroAssistantOemService.TAG, SECURE_KEY_VOLUME_UP + " : " + value);        }};

结果

打印日志发现会有两条onChange改变的日志。

解决此问题

一、首先用法是没有问题的，很标准。于是网上搜有没有同类问题。发现了一会类似的问题：
https://blog.csdn.net/serapme/article/details/7404233

二、于是好奇为啥会回调两次，初步怀疑是getContentResolver内部未知原因存了两个observer。
1、先看下ContentResolver注册代码

    public final void registerContentObserver(@NonNull Uri uri, boolean notifyForDescendants,            @NonNull ContentObserver observer) {        Preconditions.checkNotNull(uri, "uri");        Preconditions.checkNotNull(observer, "observer");        registerContentObserver(                ContentProvider.getUriWithoutUserId(uri),                notifyForDescendants,                observer,                ContentProvider.getUserIdFromUri(uri, mContext.getUserId()));    }    /** @hide - designated user version */    @UnsupportedAppUsage    public final void registerContentObserver(Uri uri, boolean notifyForDescendents,            ContentObserver observer, @UserIdInt int userHandle) {        try {            getContentService().registerContentObserver(uri, notifyForDescendents,                    observer.getContentObserver(), userHandle, mTargetSdkVersion);        } catch (RemoteException e) {            throw e.rethrowFromSystemServer();        }    }

2、最终是调用了ContentService.registerContentObserver，代码在ContentService.java中

    @Override    public void registerContentObserver(Uri uri, boolean notifyForDescendants,            IContentObserver observer, int userHandle, int targetSdkVersion) {        if (observer == null || uri == null) {            throw new IllegalArgumentException("You must pass a valid uri and observer");        }......        synchronized (mRootNode) {            mRootNode.addObserverLocked(uri, observer, notifyForDescendants, mRootNode,                    uid, pid, userHandle);            if (false) Log.v(TAG, "Registered observer " + observer + " at " + uri +                    " with notifyForDescendants " + notifyForDescendants);        }    }public static final class ObserverNode {    private ArrayList<ObserverEntry> mObservers = new ArrayList<ObserverEntry>();private void addObserverLocked(Uri uri, int index, IContentObserver observer,                                       boolean notifyForDescendants, Object observersLock,                                       int uid, int pid, int userHandle) {            // If this is the leaf node add the observer            if (index == countUriSegments(uri)) {                mObservers.add(new ObserverEntry(observer, notifyForDescendants, observersLock,                        uid, pid, userHandle, uri));                return;            }            // Look to see if the proper child already exists            String segment = getUriSegment(uri, index);            if (segment == null) {                throw new IllegalArgumentException("Invalid Uri (" + uri + ") used for observer");            }            int N = mChildren.size();            for (int i = 0; i < N; i++) {                ObserverNode node = mChildren.get(i);                if (node.mName.equals(segment)) {                    node.addObserverLocked(uri, index + 1, observer, notifyForDescendants,                            observersLock, uid, pid, userHandle);                    return;                }            }            // No child found, create one            ObserverNode node = new ObserverNode(segment);            mChildren.add(node);            node.addObserverLocked(uri, index + 1, observer, notifyForDescendants,                    observersLock, uid, pid, userHandle);        }   }

代码很明显，存到了mObservers，是个list，中间并没有什么逻辑添加两次。
3、register没问题，那会不会是notify有问题，于是看了notify代码，代码在ContentResolver.java

    public void notifyChange(@NonNull Uri uri, ContentObserver observer, boolean syncToNetwork,            @UserIdInt int userHandle) {        try {            getContentService().notifyChange(                    uri, observer == null ? null : observer.getContentObserver(),                    observer != null && observer.deliverSelfNotifications(),                    syncToNetwork ? NOTIFY_SYNC_TO_NETWORK : 0,                    userHandle, mTargetSdkVersion, mContext.getPackageName());        } catch (RemoteException e) {            throw e.rethrowFromSystemServer();        }    }

4、再看下ContentService.notifyChange，代码在ContentService.java

    @Override    public void notifyChange(Uri uri, IContentObserver observer,            boolean observerWantsSelfNotifications, int flags, int userHandle,            int targetSdkVersion, String callingPackage) {......        // This makes it so that future permission checks will be in the context of this        // process rather than the caller's process. We will restore this before returning.        long identityToken = clearCallingIdentity();        try {            ArrayList<ObserverCall> calls = new ArrayList<ObserverCall>();            synchronized (mRootNode) {                mRootNode.collectObserversLocked(uri, 0, observer, observerWantsSelfNotifications,                        flags, userHandle, calls);            }            final int numCalls = calls.size();            for (int i=0; i<numCalls; i++) {                ObserverCall oc = calls.get(i);                try {                    oc.mObserver.onChange(oc.mSelfChange, uri, userHandle);                    if (DEBUG) Slog.d(TAG, "Notified " + oc.mObserver + " of " + "update at "                            + uri);                } catch (RemoteException ex) {                    synchronized (mRootNode) {                        Log.w(TAG, "Found dead observer, removing");                        IBinder binder = oc.mObserver.asBinder();                        final ArrayList<ObserverNode.ObserverEntry> list                                = oc.mNode.mObservers;                        int numList = list.size();                        for (int j=0; j<numList; j++) {                            ObserverNode.ObserverEntry oe = list.get(j);                            if (oe.observer.asBinder() == binder) {                                list.remove(j);                                j--;                                numList--;                            }                        }                    }                }            }            ......        } finally {            restoreCallingIdentity(identityToken);        }    }

代码也很明显，就是遍历mObserver.onChange了。同时代码里注意下mObserver.asBinder()，跨进程传递了observer，这里asBinder后的对象是客户端的IBinder，可以用来判断是否同一个对象。

三、研究了一通，发现似乎没什么问题，但是无论怎么搞就是onchange回调了两次。接口api中也没有明确指明这种漏洞。不过，突然想到，会不会Settings.Secure.putInt本身就做了notify！！！！
1、先看下Settings.puInt接口

        /**         * Convenience function for updating a single settings value as an         * integer. This will either create a new entry in the table if the         * given name does not exist, or modify the value of the existing row         * with that name.  Note that internally setting values are always         * stored as strings, so this function converts the given value to a         * string before storing it.         *         * @param cr The ContentResolver to access.         * @param name The name of the setting to modify.         * @param value The new value for the setting.         * @return true if the value was set, false on database errors         */        public static boolean putInt(ContentResolver cr, String name, int value) {            return putIntForUser(cr, name, value, cr.getUserId());        }

接口描述中没提到notify的事情
2、继续往代码深处看，还是在Settings.java类中

    private static class NameValueCache {        ......        public boolean putStringForUser(ContentResolver cr, String name, String value,                String tag, boolean makeDefault, final int userHandle) {            try {                Bundle arg = new Bundle();                arg.putString(Settings.NameValueTable.VALUE, value);                arg.putInt(CALL_METHOD_USER_KEY, userHandle);                if (tag != null) {                    arg.putString(CALL_METHOD_TAG_KEY, tag);                }                if (makeDefault) {                    arg.putBoolean(CALL_METHOD_MAKE_DEFAULT_KEY, true);                }                IContentProvider cp = mProviderHolder.getProvider(cr);                cp.call(cr.getPackageName(), mProviderHolder.mUri.getAuthority(),                        mCallSetCommand, name, arg);            } catch (RemoteException e) {                Log.w(TAG, "Can't set key " + name + " in " + mUri, e);                return false;            }            return true;        }   }

3、调用了IContentProvider.call()，代码在SettingsProvider.java

    private static int getRequestingUserId(Bundle args) {        final int callingUserId = UserHandle.getCallingUserId();        return (args != null) ? args.getInt(Settings.CALL_METHOD_USER_KEY, callingUserId)                : callingUserId;    }        @Override    public Bundle call(String method, String name, Bundle args) {        final int requestingUserId = getRequestingUserId(args);        switch (method) {            case Settings.CALL_METHOD_PUT_SECURE: {                String value = getSettingValue(args);                String tag = getSettingTag(args);                final boolean makeDefault = getSettingMakeDefault(args);                insertSecureSetting(name, value, tag, makeDefault, requestingUserId, false);                break;            }       }  }private boolean insertSecureSetting(String name, String value, String tag,            boolean makeDefault, int requestingUserId, boolean forceNotify) {        ......            if (forceNotify || success) {                notifyForSettingsChange(key, name);            }            return success;    }        private void notifyForSettingsChange(int key, String name) {            ......            // Always notify that our data changed            mHandler.obtainMessage(MyHandler.MSG_NOTIFY_DATA_CHANGED).sendToTarget();   }      private final class MyHandler extends Handler {            private static final int MSG_NOTIFY_URI_CHANGED = 1;            private static final int MSG_NOTIFY_DATA_CHANGED = 2;            public MyHandler(Looper looper) {                super(looper);            }            @Override            public void handleMessage(Message msg) {                switch (msg.what) {                    case MSG_NOTIFY_URI_CHANGED: {                        final int userId = msg.arg1;                        Uri uri = (Uri) msg.obj;                        try {                            getContext().getContentResolver().notifyChange(uri, null, true, userId);                        } catch (SecurityException e) {                            Slog.w(LOG_TAG, "Failed to notify for " + userId + ": " + uri, e);                        }                        if (DEBUG || true) {                            Slog.v(LOG_TAG, "Notifying for " + userId + ": " + uri);                        }                    } break;                    case MSG_NOTIFY_DATA_CHANGED: {                        mBackupManager.dataChanged();                    } break;                }            }        }

handler内部做了getContext().getContentResolver().notifyChange

结案

永远不要怀疑谷歌的源码有bug（除非你是专家），很明显，最终的结论是画蛇添足了。Settings.Secure.putInt后并不需要自己做getContentResolver().notifyChange。所以如果你碰到onchange回调两次的问题，可以追查下put的代码，看是否已经做了notify

背景

存数据代码

监听代码

结果

解决此问题

结案

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