基于Andoird 4.2.2的同步框架源代码学习——同步发起端

关键组件：

• ContentResolver
• ContentService
• SyncManager
• SyncManager.ActiveSyncContext
• SyncManager.SyncOperation
• SyncManager.SyncHandler

ContentResolver

外部的应用程序通过调用ContentResolve.requestSync()静态方法发起同步：

    /**
     * @param account which account should be synced
     * @param authority which authority should be synced
     * @param extras any extras to pass to the SyncAdapter.
     */
    public static void requestSync(Account account, String authority, Bundle extras) {
        validateSyncExtrasBundle(extras);
        try {
            getContentService().requestSync(account, authority, extras);
        } catch (RemoteException e) {
        }
    }

方法接收三个参数：

- account：需要同步的帐号

- authority：需要进行同步的authority

- extras：需要传递给sync adapter的附加数据

在这里，getContentService()方法返回系统服务ContentService的代理对象，然后通过它远程调用ContentService.requestSync()。

ContentService

ContentService是Android的系统服务，它提供一系列数据同步及数据访问等相关的操作。它的行为在IContentService.aidl中描述。

这里，通过远程调用ContentService.requestSync()方法来启动针对指定帐号（account）的指定内容（authority）的同步：

    public void requestSync(Account account, String authority, Bundle extras) {
        ...
        try {
            SyncManager syncManager = getSyncManager();
            if (syncManager != null) {
                syncManager.scheduleSync(account, userId, authority, extras, 0 /* no delay */,
                        false /* onlyThoseWithUnkownSyncableState */);
            }
        }
        ...
    }

在这个方法中，会获取一个SyncManager类的实例。顾名思义，SyncManager管理与同步相关的处理。

SyncManager

    public void scheduleSync(Account requestedAccount, int userId, String requestedAuthority,
            Bundle extras, long delay, boolean onlyThoseWithUnkownSyncableState) {
        ...
        final boolean backgroundDataUsageAllowed = !mBootCompleted ||
                getConnectivityManager().getBackgroundDataSetting();
        ...
        
        // 产生一个同步帐户列表。对于手动同步，列表中仅有一个AccountUser元素，它封装了需要同步的帐号以及对应的应用程序（userId）
        AccountAndUser[] accounts;
        if (requestedAccount != null && userId != UserHandle.USER_ALL) {
            accounts = new AccountAndUser[] { new AccountAndUser(requestedAccount, userId) };
        }
        ...
        for (AccountAndUser account : accounts) {
            // 在这里，会扫描系统中所有提供了sync adapter的service：根据intent filter
            // 然后从得到service info中取得各自的authority。service info从对应服务的meta-data标签中指定的sync adapter描述文件中解析出来。
            final HashSet syncableAuthorities = new HashSet();
            for (RegisteredServicesCache.ServiceInfo syncAdapter :
                    mSyncAdapters.getAllServices(account.userId)) {
                syncableAuthorities.add(syncAdapter.type.authority);
            }

            ...

            for (String authority : syncableAuthorities) {
                // 检查帐户是否能够同步
                int isSyncable = mSyncStorageEngine.getIsSyncable(account.account, account.userId,
                        authority);
                if (isSyncable == 0) {
                    continue;
                }
                final RegisteredServicesCache.ServiceInfo syncAdapterInfo;
                syncAdapterInfo = mSyncAdapters.getServiceInfo(
                        SyncAdapterType.newKey(authority, account.account.type), account.userId);
                ...

                if (isSyncable < 0) {
                    Bundle newExtras = new Bundle();
                    newExtras.putBoolean(ContentResolver.SYNC_EXTRAS_INITIALIZE, true);
                    ...
                    // 部署同步操作
                    scheduleSyncOperation(
                            new SyncOperation(account.account, account.userId, source, authority,
                                    newExtras, 0, backoffTime, delayUntil, allowParallelSyncs));
                }
                ...
            }
        }
    }

这里，首先从系统中筛选出符合限定条件的service的信息，然后发起对应的同步。

首先为每一个同步操作生成一个SyncOperation实例，它封装了同步操作需要的全部信息：

public class SyncOperation implements Comparable {
    public final Account account;
    public final int userId;
    public int syncSource;
    public String authority;
    public final boolean allowParallelSyncs;
    public Bundle extras;
    public final String key;
    public long earliestRunTime;
    public boolean expedited;
    public SyncStorageEngine.PendingOperation pendingOperation;
    public Long backoff;
    public long delayUntil;
    public long effectiveRunTime;

然后调用scheduleSyncOperation方法：

    public void scheduleSyncOperation(SyncOperation syncOperation) {
        boolean queueChanged;
        synchronized (mSyncQueue) {
            queueChanged = mSyncQueue.add(syncOperation);
        }

        if (queueChanged) {
            ...
            sendCheckAlarmsMessage();
        }
        ...
    }

首先将SyncOperation实例插入队列mSyncQueue然后向SyncManager中定义的SyncHandler发送消息，通知其队列发生变化：

    private void sendCheckAlarmsMessage() {
        ...
        mSyncHandler.removeMessages(SyncHandler.MESSAGE_CHECK_ALARMS);
        mSyncHandler.sendEmptyMessage(SyncHandler.MESSAGE_CHECK_ALARMS);
    }

随后，SyncHandler处理这个消息：

       public void handleMessage(Message msg) {
            ...
            try {
                ...
                switch (msg.what) {
                    ...
                    case SyncHandler.MESSAGE_CHECK_ALARMS:
                        ...
                        nextPendingSyncTime = maybeStartNextSyncLocked();
                        break;
                }
            }
            ...
        }

这里，maybeStartNextSyncLocked()方法经过一系列的检查，确认执行同步的全部条件已经达到之后，对SyncOperation进行分发：

        private long maybeStartNextSyncLocked() {
                ...
                dispatchSyncOperation(candidate);
            }

            return nextReadyToRunTime;
        }

接下来，将绑定到提供sync adapter的应用程序中对应的service：

        private boolean dispatchSyncOperation(SyncOperation op) {
            ...
            // connect to the sync adapter
            SyncAdapterType syncAdapterType = SyncAdapterType.newKey(op.authority, op.account.type);
            final RegisteredServicesCache.ServiceInfo syncAdapterInfo;
            syncAdapterInfo = mSyncAdapters.getServiceInfo(syncAdapterType, op.userId);
            ...

            ActiveSyncContext activeSyncContext =
                    new ActiveSyncContext(op, insertStartSyncEvent(op), syncAdapterInfo.uid);
            activeSyncContext.mSyncInfo = mSyncStorageEngine.addActiveSync(activeSyncContext);
            mActiveSyncContexts.add(activeSyncContext);
            ...
            if (!activeSyncContext.bindToSyncAdapter(syncAdapterInfo, op.userId)) {
                Log.e(TAG, "Bind attempt failed to " + syncAdapterInfo);
                closeActiveSyncContext(activeSyncContext);
                return false;
            }

            return true;
        }

与前面的AccountManager非常的雷同，这里通过ActiveSyncContext类来完成service的绑定：

    class ActiveSyncContext extends ISyncContext.Stub
            implements ServiceConnection, IBinder.DeathRecipient {
        ...
        public void onServiceConnected(ComponentName name, IBinder service) {
            Message msg = mSyncHandler.obtainMessage();
            msg.what = SyncHandler.MESSAGE_SERVICE_CONNECTED;
            msg.obj = new ServiceConnectionData(this, ISyncAdapter.Stub.asInterface(service));
            mSyncHandler.sendMessage(msg);
        }
        ...
        boolean bindToSyncAdapter(RegisteredServicesCache.ServiceInfo info, int userId) {
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                Log.d(TAG, "bindToSyncAdapter: " + info.componentName + ", connection " + this);
            }
            Intent intent = new Intent();
            intent.setAction("android.content.SyncAdapter");
            intent.setComponent(info.componentName);
            intent.putExtra(Intent.EXTRA_CLIENT_LABEL,
                    com.android.internal.R.string.sync_binding_label);
            intent.putExtra(Intent.EXTRA_CLIENT_INTENT, PendingIntent.getActivityAsUser(
                    mContext, 0, new Intent(Settings.ACTION_SYNC_SETTINGS), 0,
                    null, new UserHandle(userId)));
            mBound = true;
            final boolean bindResult = mContext.bindService(intent, this,
                    Context.BIND_AUTO_CREATE | Context.BIND_NOT_FOREGROUND
                    | Context.BIND_ALLOW_OOM_MANAGEMENT,
                    mSyncOperation.userId);
            if (!bindResult) {
                mBound = false;
            }
            return bindResult;
        }
        ...
    }

其中，bindToSyncAdapter()中创建相应的Intent，发起绑定。

然后，因为本类实现了ServiceConnection接口，所以当绑定成功时，将回调本类的onServiceConnected()方法。在这个回调中，向SyncHandler发送一条MESSAGE_SERVICE_CONNECTED消息。

紧接着，轮到SyncHandler来处理消息：

                    case SyncHandler.MESSAGE_SERVICE_CONNECTED: {
                        ServiceConnectionData msgData = (ServiceConnectionData)msg.obj;
                        if (Log.isLoggable(TAG, Log.VERBOSE)) {
                            Log.d(TAG, "handleSyncHandlerMessage: MESSAGE_SERVICE_CONNECTED: "
                                    + msgData.activeSyncContext);
                        }
                        // check that this isn't an old message
                        if (isSyncStillActive(msgData.activeSyncContext)) {
                            runBoundToSyncAdapter(msgData.activeSyncContext, msgData.syncAdapter);
                        }
                        break;
                    }

这里主要就是调用了runBoundToSyncAdapter()方法：

        private void runBoundToSyncAdapter(final ActiveSyncContext activeSyncContext,
              ISyncAdapter syncAdapter) {
            activeSyncContext.mSyncAdapter = syncAdapter;
            final SyncOperation syncOperation = activeSyncContext.mSyncOperation;
            try {
                ...
                syncAdapter.startSync(activeSyncContext, syncOperation.authority,
                        syncOperation.account, syncOperation.extras);
            }
            ...
        }

这里，对传入syncAdapter实例（实际上是AbstractThreadedSyncAdpter.ISyncAdapterImpl服务的代理对象）调用startSync()方法。这样，通过IPC即可调用对应的应用程序执行同步了。详见本系列上一篇文章。