關鍵組件：
ContentResolver
ContentService
SyncManager
SyncManager.ActiveSyncContext
SyncManager.SyncOperation
SyncManager.SyncHandler
ContentResolver
外部的應用程序通過調用ContentResolve.requestSync()靜態方法發起同步：

[java]
/**
?* @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) {?
??? }?
}?

??? /**
???? * @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) {
??????? }
??? }
方法接收三個參數：

- account：需要同步的帳號

- authority：需要進行同步的authority

- extras：需要傳遞給sync adapter的附加數據

在這里，getContentService()方法返回系統服務ContentService的代理對象，然后通過它遠程調用ContentService.requestSync()。

ContentService
ContentService是Android的系統服務，它提供一系列數據同步及數據訪問等相關的操作。它的行為在IContentService.aidl中描述。

這里，通過遠程調用ContentService.requestSync()方法來啟動針對指定帳號（account）的指定內容（authority）的同步：

[java]
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 */);?
??????? }?
??? }?
??? ...?
}?

??? 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 */);
??????????? }
??????? }
??????? ...
??? }
在這個方法中，會獲取一個SyncManager類的實例。顧名思義，SyncManager管理與同步相關的處理。

SyncManager

[java]
public void scheduleSync(Account requestedAccount, int userId, String requestedAuthority,?
??????? Bundle extras, long delay, boolean onlyThoseWithUnkownSyncableState) {?
??? ...?
??? final boolean backgroundDataUsageAllowed = !mBootCompleted ||?
??????????? getConnectivityManager().getBackgroundDataSetting();?
??? ...?
?????
??? // 產生一個同步帳戶列表。對于手動同步，列表中僅有一個AccountUser元素，它封裝了需要同步的帳號以及對應的應用程序（userId）??
??? AccountAndUser[] accounts;?
??? if (requestedAccount != null && userId != UserHandle.USER_ALL) {?
??????? accounts = new AccountAndUser[] { new AccountAndUser(requestedAccount, userId) };?
??? }?
??? ...?
??? for (AccountAndUser account : accounts) {?
??????? // 在這里，會掃描系統中所有提供了sync adapter的service：根據intent filter??
??????? // 然后從得到service info中取得各自的authority。service info從對應服務的meta-data標簽中指定的sync adapter描述文件中解析出來。??
??????? final HashSet<String> syncableAuthorities = new HashSet<String>();?
??????? for (RegisteredServicesCache.ServiceInfo<SyncAdapterType> syncAdapter :?
??????????????? mSyncAdapters.getAllServices(account.userId)) {?
??????????? syncableAuthorities.add(syncAdapter.type.authority);?
??????? }?
?
??????? ...?
?
??????? for (String authority : syncableAuthorities) {?
??????????? // 檢查帳戶是否能夠同步??
??????????? int isSyncable = mSyncStorageEngine.getIsSyncable(account.account, account.userId,?
??????????????????? authority);?
??????????? if (isSyncable == 0) {?
??????????????? continue;?
??????????? }?
??????????? final RegisteredServicesCache.ServiceInfo<SyncAdapterType> 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));?
??????????? }?
??????????? ...?
??????? }?
??? }?
}?

??? public void scheduleSync(Account requestedAccount, int userId, String requestedAuthority,
??????????? Bundle extras, long delay, boolean onlyThoseWithUnkownSyncableState) {
??????? ...
??????? final boolean backgroundDataUsageAllowed = !mBootCompleted ||
??????????????? getConnectivityManager().getBackgroundDataSetting();
??????? ...
???????
??????? // 產生一個同步帳戶列表。對于手動同步，列表中僅有一個AccountUser元素，它封裝了需要同步的帳號以及對應的應用程序（userId）
??????? AccountAndUser[] accounts;
??????? if (requestedAccount != null && userId != UserHandle.USER_ALL) {
??????????? accounts = new AccountAndUser[] { new AccountAndUser(requestedAccount, userId) };
??????? }
??????? ...
??????? for (AccountAndUser account : accounts) {
??????????? // 在這里，會掃描系統中所有提供了sync adapter的service：根據intent filter
??????????? // 然后從得到service info中取得各自的authority。service info從對應服務的meta-data標簽中指定的sync adapter描述文件中解析出來。
??????????? final HashSet<String> syncableAuthorities = new HashSet<String>();
??????????? for (RegisteredServicesCache.ServiceInfo<SyncAdapterType> syncAdapter :
??????????????????? mSyncAdapters.getAllServices(account.userId)) {
??????????????? syncableAuthorities.add(syncAdapter.type.authority);
??????????? }

??????????? ...

??????????? for (String authority : syncableAuthorities) {
??????????????? // 檢查帳戶是否能夠同步
??????????????? int isSyncable = mSyncStorageEngine.getIsSyncable(account.account, account.userId,
??????????????????????? authority);
??????????????? if (isSyncable == 0) {
??????????????????? continue;
??????????????? }
??????????????? final RegisteredServicesCache.ServiceInfo<SyncAdapterType> 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));
??????????????? }
??????????????? ...
??????????? }
??????? }
??? }
這里，首先從系統中篩選出符合限定條件的service的信息，然后發起對應的同步。

首先為每一個同步操作生成一個SyncOperation實例，它封裝了同步操作需要的全部信息：

[java]
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;?

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;
然后調用scheduleSyncOperation方法：

[java]
public void scheduleSyncOperation(SyncOperation syncOperation) {?
??? boolean queueChanged;?
??? synchronized (mSyncQueue) {?
??????? queueChanged = mSyncQueue.add(syncOperation);?
??? }?
?
??? if (queueChanged) {?
??????? ...?
??????? sendCheckAlarmsMessage();?
??? }?
??? ...?
}?

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

??????? if (queueChanged) {
??????????? ...
??????????? sendCheckAlarmsMessage();
??????? }
??????? ...
??? }
首先將SyncOperation實例插入隊列mSyncQueue然后向SyncManager中定義的SyncHandler發送消息，通知其隊列發生變化：

[java]
private void sendCheckAlarmsMessage() {?
??? ...?
??? mSyncHandler.removeMessages(SyncHandler.MESSAGE_CHECK_ALARMS);?
??? mSyncHandler.sendEmptyMessage(SyncHandler.MESSAGE_CHECK_ALARMS);?
}?

??? private void sendCheckAlarmsMessage() {
??????? ...
??????? mSyncHandler.removeMessages(SyncHandler.MESSAGE_CHECK_ALARMS);
??????? mSyncHandler.sendEmptyMessage(SyncHandler.MESSAGE_CHECK_ALARMS);
??? }
隨后，SyncHandler處理這個消息：

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

?????? public void handleMessage(Message msg) {
??????????? ...
??????????? try {
??????????????? ...
??????????????? switch (msg.what) {
??????????????????? ...
??????????????????? case SyncHandler.MESSAGE_CHECK_ALARMS:
??????????????????????? ...
??????????????????????? nextPendingSyncTime = maybeStartNextSyncLocked();
??????????????????????? break;
??????????????? }
??????????? }
??????????? ...
??????? }
這里，maybeStartNextSyncLocked()方法經過一系列的檢查，確認執行同步的全部條件已經達到之后，對SyncOperation進行分發：

[java]
private long maybeStartNextSyncLocked() {?
??????? ...?
??????? dispatchSyncOperation(candidate);?
??? }?
?
??? return nextReadyToRunTime;?
}?

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

??????????? return nextReadyToRunTime;
??????? }
接下來，將綁定到提供sync adapter的應用程序中對應的service：

[java]
private boolean dispatchSyncOperation(SyncOperation op) {?
??? ...?
??? // connect to the sync adapter??
??? SyncAdapterType syncAdapterType = SyncAdapterType.newKey(op.authority, op.account.type);?
??? final RegisteredServicesCache.ServiceInfo<SyncAdapterType> 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;?
}?

??????? private boolean dispatchSyncOperation(SyncOperation op) {
??????????? ...
??????????? // connect to the sync adapter
??????????? SyncAdapterType syncAdapterType = SyncAdapterType.newKey(op.authority, op.account.type);
??????????? final RegisteredServicesCache.ServiceInfo<SyncAdapterType> 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;
??????? }

與前面的AccountManager非常的雷同，這里通過ActiveSyncContext類來完成service的綁定：

[java]
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;?
??? }?
??? ...?
}?

??? 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()中創建相應的Intent，發起綁定。

然后，因為本類實現了ServiceConnection接口，所以當綁定成功時，將回調本類的onServiceConnected()方法。在這個回調中，向SyncHandler發送一條MESSAGE_SERVICE_CONNECTED消息。

緊接著，輪到SyncHandler來處理消息：

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;?
}?

??????????????????? 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;
??????????????????? }
這里主要就是調用了runBoundToSyncAdapter()方法：

[java]
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);?
??? }?
??? ...?
}?